fix secrets no longer needed

Kept our version over remote (security hardening PR #12) for all conflicts:
- pom.xml: Spring Boot 4.0.6 vs 3.4.1, new test deps
- SecurityConfig: new csrf/package API for SB4
- GlobalExceptionHandler: full handler set with SLF4J logging
- AppUserController/ExpenseListController: clean delegation to GlobalExceptionHandler
- Test files: SB4 package paths, extra test coverage

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.claude/settings.local.json

@@ -0,0 +1,8 @@
+{
+  "permissions": {
+    "allow": [
+      "Bash(git add *)",
+      "Bash(git commit *)"
+    ]
+  }
+}

.gitea/workflows/build.yml

@@ -7,48 +7,51 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-java17
 
     steps:
       # 1. Checkout the code
       - name: Checkout code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
-      # 2. Set up Java and Maven
+      # 2. Build the Spring Boot application using the Maven wrapper (Java 17 pre-installed in runner image)
-      - name: Set up JDK (Eclipse Temurin)
-        uses: actions/setup-java@v3
-        with:
-          distribution: "temurin"
-          java-version: "17"
-          cache: maven
-
-      # 3. Verify Maven installation
-      - name: Install Maven
-        run: |
-          sudo apt-get update
-          sudo apt-get install -y maven
-          mvn -version
-
-      # 4. Build the Spring Boot application
       - name: Build Spring Boot Application
-        run: |
+        run: ./mvnw clean package -DskipTests
-          mvn clean package -DskipTests
 
-      # 5. Set up Docker
+      # 4. Set up Docker Buildx (enables layer caching)
-      - name: Set up Docker
+      - name: Set up Docker Buildx
-        run: |
+        uses: docker/setup-buildx-action@v3
-          docker --version
 
-      # 6. Build the Docker image
+      # 5. Login to Docker Hub to avoid pull rate limits
-      - name: Build and Package Docker Image
+      - name: Login to Docker Hub
-        run: |
+        uses: docker/login-action@v3
-          docker build -t tea.zendric.de/cedric/xpensely-server:latest .
+        with:
+          username: ${{ secrets.DOCKERHUB_USER }}
+          password: ${{ secrets.DOCKERHUB_TOKEN }}
 
-      # 7. Docker login
+      # 6. Docker login to Gitea registry
       - name: Login to Docker Registry
+        uses: docker/login-action@v3
+        with:
+          registry: tea.zendric.de
+          username: ${{ secrets.TEAUSER }}
+          password: ${{ secrets.TEAPASSWORD }}
+
+      # 7. Build and push Docker image with layer caching
+      - name: Build and Push Docker Image
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          push: true
+          tags: tea.zendric.de/cedric/xpensely-server:latest
+          cache-from: type=registry,ref=tea.zendric.de/cedric/xpensely-server:buildcache
+          cache-to: type=registry,ref=tea.zendric.de/cedric/xpensely-server:buildcache,mode=max
+
+      # 8. Trigger Dokploy to redeploy the dev server automatically via API
+      - name: Trigger Dokploy Redeploy
         run: |
-          echo "${{ secrets.TEAPASSWORD }}" | docker login tea.zendric.de -u ${{ secrets.TEAUSER }} --password-stdin
+          curl -X POST "https://dokploy.zendric.de/api/compose.deploy" \
-      # 8. Push Docker image
+            -H "Content-Type: application/json" \
-      - name: Push the Docker Image to registry
+            -H "x-api-key: ${{ secrets.DOKPLOY_API_TOKEN }}" \
-        run: |
+            -d "{\"composeId\": \"${{ secrets.DOKPLOY_COMPOSE_ID }}\"}" \
-          docker push tea.zendric.de/cedric/xpensely-server:latest
+            --fail

.gitignore

@@ -1,5 +1,6 @@
 HELP.md
 target/
+/docs/superpowers
 !.mvn/wrapper/maven-wrapper.jar
 !**/src/main/**/target/
 !**/src/test/**/target/

dev-docker-compose.yml

@@ -1,52 +0,0 @@
-version: "3.8"
-services:
-  xpensely-server:
-    build:
-      context: .
-      dockerfile: Dockerfile
-    image: xpensely-server:local
-    labels:
-      net.unraid.docker.icon: https://tea.zendric.de/Cedric/XpenselyServer/raw/branch/main/src/main/resources/static/xpensely_icon_white.png
-    container_name: xpensely-server
-    ports:
-      - 3636:8080
-    environment:
-      GOOGLE_CLIENT_ID: ${GOOGLE_CLIENT_ID}
-      GOOGLE_CLIENT_SECRET: ${GOOGLE_CLIENT_SECRET}
-      DB_PORT: 5432
-      DB_P_NAME: xpensely
-      DB_USERNAME: ${DB_USERNAME}
-      DB_PASSWORD: ${DB_PASSWORD}
-      SPRING_PROFILES_ACTIVE: test
-    depends_on:
-      postgresdb:
-        condition: service_healthy
-    networks:
-      - xpensely-network
-  postgresdb:
-    labels:
-      net.unraid.docker.icon: https://raw.githubusercontent.com/docker-library/docs/01c12653951b2fe592c1f93a13b4e289ada0e3a1/postgres/logo.png
-    image: postgres:14
-    container_name: postgresdb
-    ports:
-      - 5435:5432
-    environment:
-      POSTGRES_DB: xpensely
-      POSTGRES_USER: ${DB_USERNAME}
-      POSTGRES_PASSWORD: ${DB_PASSWORD}
-    networks:
-      - xpensely-network
-    volumes:
-      - db_data:/var/lib/postgresql/data
-    restart: unless-stopped
-    healthcheck:
-      test:
-        - CMD-SHELL
-        - pg_isready -U ${DB_USERNAME} -d xpensely
-      interval: 10s
-      timeout: 5s
-      retries: 5
-volumes:
-  db_data: null
-networks:
-  xpensely-network: null

docker-compose.yml

@@ -1,49 +1,41 @@
-version: "3.8"
 services:
   xpensely-server:
-    image: tea.zendric.de/cedric/xpensely-server:0
+    image: tea.zendric.de/cedric/xpensely-server:latest
-    labels:
+    pull_policy: always
-      net.unraid.docker.icon: https://tea.zendric.de/Cedric/XpenselyServer/raw/branch/main/src/main/resources/static/xpensely_icon_white.png
+    restart: always
-    container_name: xpensely-server
-    ports:
-      - 3636:8080
     environment:
-      GOOGLE_CLIENT_ID: ${GOOGLE_CLIENT_ID}
-      GOOGLE_CLIENT_SECRET: ${GOOGLE_CLIENT_SECRET}
       DB_PORT: 5432
-      DB_P_NAME: xpensely
+      DB_P_NAME: ${POSTGRES_DB}
-      DB_USERNAME: ${DB_USERNAME}
+      DB_USERNAME: ${POSTGRES_USER}
-      DB_PASSWORD: ${DB_PASSWORD}
+      DB_PASSWORD: ${POSTGRES_PASSWORD}
+
+      SPRING_PROFILES_ACTIVE: ${SPRING_PROFILES_ACTIVE}
     depends_on:
       postgresdb:
         condition: service_healthy
     networks:
       - xpensely-network
+
   postgresdb:
-    labels:
-      net.unraid.docker.icon: https://raw.githubusercontent.com/docker-library/docs/01c12653951b2fe592c1f93a13b4e289ada0e3a1/postgres/logo.png
     image: postgres:14
-    container_name: postgresdb
+    restart: unless-stopped
-    ports:
-      - 5435:5432
     environment:
-      POSTGRES_DB: xpensely
+      POSTGRES_DB: ${POSTGRES_DB}
-      POSTGRES_USER: ${DB_USERNAME}
+      POSTGRES_USER: ${POSTGRES_USER}
-      POSTGRES_PASSWORD: ${DB_PASSWORD}
+      POSTGRES_PASSWORD: ${POSTGRES_PASSWORD}
+    volumes:
+      - postgres_data:/var/lib/postgresql/data
     networks:
       - xpensely-network
-    volumes:
-      - db_data:/var/lib/postgresql/data
-      - /mnt/user/appdata/xpensely_backups:/backups
-    restart: unless-stopped
     healthcheck:
-      test:
+      test: ["CMD-SHELL", "pg_isready -U ${POSTGRES_USER} -d ${POSTGRES_DB}"]
-        - CMD-SHELL
-        - pg_isready -U ${DB_USERNAME} -d xpensely
       interval: 10s
       timeout: 5s
       retries: 5
+
 volumes:
-  db_data: null
+  postgres_data:
+
 networks:
-  xpensely-network: null
+  xpensely-network:
+    driver: bridge

dockerfile

@@ -1,4 +1,4 @@
-FROM openjdk:17-jdk-slim
+FROM eclipse-temurin:21-jdk
 
 COPY ./target/*.jar app.jar
 

docs/API.md

@@ -0,0 +1,732 @@
+# Xpensely Server — API Reference
+
+> Last updated: 2026-05-14 · Branch: `dev`
+
+## Table of Contents
+1. [Overview](#1-overview)
+2. [Authentication](#2-authentication)
+3. [Rate Limiting](#3-rate-limiting)
+4. [Endpoints](#4-endpoints)
+   - 4.1 [Home](#41-home)
+   - 4.2 [Users](#42-users)
+   - 4.3 [Expense Lists](#43-expense-lists)
+5. [Data Models](#5-data-models)
+6. [Error Handling](#6-error-handling)
+7. [Recent Changes — `feature/security-hardening`](#7-recent-changes)
+
+---
+
+## 1. Overview
+
+Xpensely Server is a Spring Boot REST API that manages shared expense lists for pairs of users. It uses Google OAuth2 JWT tokens for authentication. All protected endpoints require a valid Bearer token in the `Authorization` header.
+
+**Base URL (local dev):** `http://localhost:8080`
+
+**Content-Type:** `application/json` for all request and response bodies.
+
+**Public endpoints (no auth required):**
+| Method | Path | Description |
+|--------|------|-------------|
+| GET | `/` | Health check — returns `"Welcome"` |
+| GET | `/api/version` | Returns build version and timestamp |
+| POST | `/api/users/createUser` | Register a new user |
+| GET | `/api/users/byName` | Look up a user by username |
+
+All other endpoints require authentication (see [Section 2](#2-authentication)).
+
+## 2. Authentication
+
+The server uses **OAuth2 Resource Server** authentication with **Google ID JWT tokens**.
+
+### How it works
+
+1. The client authenticates with Google and receives a Google ID JWT.
+2. Every protected API request must include this token in the header:
+
+```
+Authorization: Bearer <google-id-jwt>
+```
+
+3. The server validates the JWT signature and extracts the `sub` claim (Google User ID).
+4. The `sub` value is used to look up the registered `AppUser` in the database via `AuthenticatedUserResolver`.
+5. If no `AppUser` exists for that Google ID, the request is rejected with **403 Forbidden**.
+
+### Test profile
+
+When the application runs under the `test` Spring profile (`-Dspring.profiles.active=test`), **all security is disabled** — every endpoint is accessible without a token. This is used for automated tests only.
+
+### User registration flow
+
+Before a user can call any protected endpoint they must first be registered:
+
+1. Authenticate with Google to obtain a Google ID JWT.
+2. Call `POST /api/users/createUser` with the JWT's `sub` value as `googleId` and a chosen `username`.
+3. All subsequent protected calls use the same JWT — the server resolves the caller automatically.
+
+## 3. Rate Limiting
+
+All requests pass through a `RateLimitFilter` (implemented with **Bucket4j**).
+
+| Setting | Value |
+|---------|-------|
+| Limit | 60 requests per minute |
+| Window | Rolling 1-minute bucket |
+| Key (authenticated) | JWT `sub` claim (Google User ID) |
+| Key (unauthenticated) | `X-Forwarded-For` header, falling back to remote IP |
+
+When the limit is exceeded the server responds with:
+
+```
+HTTP 429 Too Many Requests
+```
+
+No `Retry-After` header is currently returned. Clients should back off and retry after 60 seconds.
+
+> **Note:** Rate limiting applies in the `!test` profile only. Tests run without rate limiting.
+
+## 4. Endpoints
+
+### 4.1 Home
+
+#### `GET /`
+
+Health check. No authentication required.
+
+**Response:** `200 OK`
+```
+Welcome
+```
+
+---
+
+#### `GET /api/version`
+
+Returns the application version and build timestamp. No authentication required.
+
+**Response:** `200 OK`
+```json
+{
+  "version": "0.0.1-SNAPSHOT",
+  "builtAt": "2026-05-09T10:00:00Z"
+}
+```
+
+| Field | Type | Notes |
+|-------|------|-------|
+| `version` | String | Maven project version |
+| `builtAt` | String (ISO-8601) | UTC timestamp of the build |
+
+---
+
+### 4.2 Users
+
+Base path: `/api/users`
+
+---
+
+#### `POST /api/users/createUser` — Register a user
+
+**Auth required:** No
+
+**Request body:**
+```json
+{
+  "username": "alice",
+  "googleId": "118400012345678901234"
+}
+```
+
+| Field | Type | Constraints |
+|-------|------|-------------|
+| `username` | String | Required. 3–30 chars. Pattern: `^[a-zA-Z0-9_.\-]+$` |
+| `googleId` | String | Required. Non-blank. Must match the JWT `sub` from Google. |
+
+**Success response:** `201 Created` — returns the created [AppUser](#appuser) object.
+
+**Error responses:**
+| Status | Condition |
+|--------|-----------|
+| 400 | Validation failure (field errors returned as `{"fieldName": "message"}`) |
+| 409 | `username` already taken |
+
+---
+
+#### `GET /api/users` — Get user by ID
+
+**Auth required:** Yes
+
+**Query params:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `id` | Long | Yes | Database ID of the user |
+
+**Success response:** `200 OK` — returns [AppUser](#appuser).
+
+**Error responses:**
+| Status | Condition |
+|--------|-----------|
+| 403 | Authenticated user's ID does not match the requested `id` |
+| 404 | No user found for `id` |
+
+---
+
+#### `GET /api/users/byName` — Get user by username
+
+**Auth required:** No
+
+**Query params:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `username` | String | Yes | Exact username (case-sensitive) |
+
+**Success response:** `200 OK` — returns [AppUser](#appuser).
+
+**Error responses:**
+| Status | Condition |
+|--------|-----------|
+| 404 | No user found for `username` |
+
+---
+
+#### `GET /api/users/byGoogleId` — Get user by Google ID
+
+**Auth required:** Yes
+
+**Query params:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `id` | String | Yes | Google `sub` claim |
+
+**Success response:** `200 OK` — returns [AppUser](#appuser).
+
+**Error responses:**
+| Status | Condition |
+|--------|-----------|
+| 403 | Requested Google ID does not match the authenticated user's Google ID |
+| 404 | No user found for that Google ID |
+
+---
+
+#### `DELETE /api/users` — Delete a user
+
+**Auth required:** Yes
+
+**Query params:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `id` | Long | Yes | Database ID of the user to delete |
+
+**Success response:** `200 OK` — returns a plain string: `"User deleted: <username>"`.
+
+**Error responses:**
+| Status | Condition |
+|--------|-----------|
+| 403 | Authenticated user's ID does not match the requested `id` |
+| 404 | No user found for `id` |
+
+---
+
+### 4.3 Expense Lists
+
+Base path: `/api/expenselist`
+
+All endpoints in this group require authentication. The authenticated caller is resolved from the JWT and used for ownership checks.
+
+---
+
+#### `GET /api/expenselist/mine` — Get caller's expense lists
+
+Returns all expense lists where the caller is the owner **or** has been shared the list.
+
+**Auth required:** Yes
+
+**Request body:** None
+
+**Success responses:**
+| Status | Condition |
+|--------|-----------|
+| 200 OK | Returns array of [ExpenseList](#expenselist) |
+| 204 No Content | Caller has no expense lists |
+
+```json
+[
+  {
+    "id": 1,
+    "name": "Holiday Trip",
+    "inviteCode": null,
+    "owner": { "id": 1, "username": "alice" },
+    "sharedWith": null,
+    "expenses": [],
+    "customCategories": []
+  }
+]
+```
+
+---
+
+#### `GET /api/expenselist/byId` — Get expense list by ID
+
+**Auth required:** Yes
+
+**Query params:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `id` | Long | Yes | Expense list database ID |
+
+The caller must be the owner or the shared user of the list.
+
+**Success response:** `200 OK` — returns [ExpenseList](#expenselist).
+
+**Error responses:**
+| Status | Condition |
+|--------|-----------|
+| 403 | Caller is neither owner nor shared user |
+| 404 | No list found for `id` |
+
+---
+
+#### `POST /api/expenselist/create` — Create an expense list
+
+**Auth required:** Yes
+
+**Request body:**
+```json
+{
+  "name": "Road Trip 2026"
+}
+```
+
+| Field | Type | Constraints |
+|-------|------|-------------|
+| `name` | String | Required. Max 100 chars. |
+
+The authenticated caller becomes the `owner` of the new list. The list is initialised with the default Xpensely standard categories.
+
+**Success response:** `201 Created` — returns the created [ExpenseList](#expenselist).
+
+**Error responses:**
+| Status | Condition |
+|--------|-----------|
+| 400 | Validation failure |
+
+---
+
+#### `DELETE /api/expenselist/{id}` — Delete an expense list
+
+**Auth required:** Yes
+
+**Path params:**
+| Param | Type | Description |
+|-------|------|-------------|
+| `id` | Long | Expense list database ID |
+
+Only the **owner** may delete a list. Deleting a list cascades to all its expenses and custom categories.
+
+**Success response:** `204 No Content`
+
+**Error responses:**
+| Status | Condition |
+|--------|-----------|
+| 403 | Caller is not the owner |
+| 404 | No list found for `id` |
+
+---
+
+#### `POST /api/expenselist/{id}/add` — Add an expense to a list
+
+**Auth required:** Yes
+
+**Path params:**
+| Param | Type | Description |
+|-------|------|-------------|
+| `id` | Long | Expense list database ID |
+
+**Request body:**
+```json
+{
+  "title": "Dinner",
+  "owner": "alice",
+  "amount": 42.50,
+  "personalUseAmount": 21.25,
+  "otherPersonAmount": 21.25,
+  "date": "2026-05-09",
+  "category": "Food"
+}
+```
+
+| Field | Type | Constraints |
+|-------|------|-------------|
+| `title` | String | Required. Max 100 chars. |
+| `owner` | String | Required. Username of the person who paid. |
+| `amount` | Double | Required. Min 0.01. |
+| `personalUseAmount` | Double | Optional. Caller's share. |
+| `otherPersonAmount` | Double | Optional. Other person's share. |
+| `date` | String (ISO-8601) | Required. Format: `YYYY-MM-DD`. |
+| `category` | String | Required. Non-blank category name. |
+
+**Success response:** `201 Created` — returns the created [Expense](#expense).
+
+**Error responses:**
+| Status | Condition |
+|--------|-----------|
+| 400 | Validation failure |
+| 403 | Caller is not a member (owner or sharedWith) of the list |
+| 404 | List not found |
+
+---
+
+#### `PUT /api/expenselist/{id}/update` — Update an expense
+
+**Auth required:** Yes
+
+**Path params:**
+| Param | Type | Description |
+|-------|------|-------------|
+| `id` | Long | Expense list database ID |
+
+**Request body:**
+```json
+{
+  "id": 7,
+  "title": "Dinner (updated)",
+  "ownerName": "alice",
+  "amount": 50.00,
+  "personalUseAmount": 25.00,
+  "otherPersonAmount": 25.00,
+  "date": "2026-05-09",
+  "category": "Food"
+}
+```
+
+| Field | Type | Constraints |
+|-------|------|-------------|
+| `id` | Long | Required. ID of the expense to update. |
+| `title` | String | Required. Max 100 chars. |
+| `ownerName` | String | Required. Username of the payer. |
+| `amount` | Double | Required. Min 0.01. |
+| `personalUseAmount` | Double | Optional. |
+| `otherPersonAmount` | Double | Optional. |
+| `date` | String (ISO-8601) | Required. `YYYY-MM-DD`. |
+| `category` | String | Required. |
+
+Caller must be a member of the list. Expense must belong to the specified list.
+
+**Success response:** `200 OK` — returns the updated [Expense](#expense).
+
+**Error responses:**
+| Status | Condition |
+|--------|-----------|
+| 400 | Validation failure or expense does not belong to this list |
+| 403 | Caller is not a member of the list |
+| 404 | List or expense not found |
+
+---
+
+#### `DELETE /api/expenselist/{id}/delete` — Remove an expense from a list
+
+**Auth required:** Yes
+
+**Path params:**
+| Param | Type | Description |
+|-------|------|-------------|
+| `id` | Long | Expense list database ID |
+
+**Query params:**
+| Param | Type | Required | Description |
+|-------|------|----------|-------------|
+| `expenseId` | Long | Yes | ID of the expense to remove |
+
+Caller must be a member of the list.
+
+**Success response:** `204 No Content`
+
+**Error responses:**
+| Status | Condition |
+|--------|-----------|
+| 403 | Caller is not a member of the list |
+| 404 | List or expense not found |
+
+---
+
+#### `POST /api/expenselist/{listId}/invite` — Generate an invite code
+
+Generates (or refreshes) a 6-character uppercase invite code for the list, valid for **1 week**.
+
+**Auth required:** Yes
+
+**Path params:**
+| Param | Type | Description |
+|-------|------|-------------|
+| `listId` | Long | Expense list database ID |
+
+Caller must be the **owner** of the list.
+
+**Success response:** `200 OK` — returns the invite code as a plain string.
+
+```
+AB3X7Q
+```
+
+**Error responses:**
+| Status | Condition |
+|--------|-----------|
+| 403 | Caller is not the owner |
+| 404 | List not found |
+
+---
+
+#### `POST /api/expenselist/accept-invite` — Accept an invite
+
+Joins the caller to a shared expense list using an invite code.
+
+**Auth required:** Yes
+
+**Request body:**
+```json
+{
+  "inviteCode": "AB3X7Q"
+}
+```
+
+| Field | Type | Constraints |
+|-------|------|-------------|
+| `inviteCode` | String | Required. Exactly 6 characters. |
+
+**Success response:** `200 OK` — returns a plain string: `"User added to the list"`.
+
+**Error responses:**
+| Status | Condition |
+|--------|-----------|
+| 400 | Validation failure or caller is already the owner of the list |
+| 404 | Invite code not found or expired |
+| 226 IM Used | List already has a second member (`sharedWith` is not null) |
+
+---
+
+## 5. Data Models
+
+### AppUser
+
+Returned by all `/api/users` endpoints. Sensitive fields (`googleId`, `createdAt`) are hidden from API responses via `@JsonIgnore`.
+
+```json
+{
+  "id": 1,
+  "username": "alice"
+}
+```
+
+| Field | Type | Notes |
+|-------|------|-------|
+| `id` | Long | Auto-generated primary key |
+| `username` | String | Unique. 3–30 chars. |
+
+---
+
+### ExpenseList
+
+```json
+{
+  "id": 1,
+  "name": "Road Trip",
+  "inviteCode": "AB3X7Q",
+  "owner": { "id": 1, "username": "alice" },
+  "sharedWith": { "id": 2, "username": "bob" },
+  "xpenselyStandardCategories": {
+    "id": 1,
+    "categories": [
+      { "id": 1, "name": "Food", "colorCode": "#FF5733" }
+    ]
+  },
+  "customCategories": [],
+  "expenses": []
+}
+```
+
+| Field | Type | Notes |
+|-------|------|-------|
+| `id` | Long | Auto-generated primary key |
+| `name` | String | Display name of the list |
+| `inviteCode` | String \| null | 6-char code; `null` if not yet generated or expired |
+| `owner` | AppUser | User who created the list |
+| `sharedWith` | AppUser \| null | Second member of the list; `null` until an invite is accepted |
+| `xpenselyStandardCategories` | XpenselyStandardCategories | Default category set assigned at creation |
+| `customCategories` | XpenselyCustomCategory[] | User-defined categories, ordered A→Z |
+| `expenses` | Expense[] | All expenses, ordered by date ASC then id ASC |
+
+> `inviteCodeExpiration` is hidden from API responses (`@JsonIgnore`).
+
+---
+
+### Expense
+
+```json
+{
+  "id": 7,
+  "title": "Dinner",
+  "owner": { "id": 1, "username": "alice" },
+  "amount": 42.50,
+  "personalUseAmount": 21.25,
+  "otherPersonAmount": 21.25,
+  "category": "Food",
+  "date": "2026-05-09"
+}
+```
+
+| Field | Type | Notes |
+|-------|------|-------|
+| `id` | Long | Auto-generated |
+| `title` | String | Description of the expense |
+| `owner` | AppUser | Who paid |
+| `amount` | Double | Total amount. Min 0.01. |
+| `personalUseAmount` | Double \| null | Caller's share |
+| `otherPersonAmount` | Double \| null | Other member's share |
+| `category` | String | Category name (free text — must match a category on the list) |
+| `date` | String (ISO-8601) | `YYYY-MM-DD` |
+
+> The `expenseList` back-reference is excluded via `@JsonBackReference` to prevent circular serialisation.
+
+---
+
+### XpenselyStandardCategories
+
+A named group of standard categories assigned to every new expense list.
+
+```json
+{
+  "id": 1,
+  "categories": [
+    { "id": 1, "name": "Food", "colorCode": "#FF5733" },
+    { "id": 2, "name": "Transport", "colorCode": "#3498DB" }
+  ]
+}
+```
+
+---
+
+### XpenselyCustomCategory
+
+A user-defined category attached to a specific expense list.
+
+```json
+{
+  "id": 5,
+  "name": "Souvenirs",
+  "colorCode": "#9B59B6"
+}
+```
+
+| Field | Type | Notes |
+|-------|------|-------|
+| `id` | Long | Auto-generated |
+| `name` | String | Category label |
+| `colorCode` | String | 7-char hex string e.g. `#RRGGBB` |
+
+> The `expenseList` back-reference is excluded via `@JsonBackReference`.
+
+## 6. Error Handling
+
+All errors are returned as JSON. The `GlobalExceptionHandler` (`@RestControllerAdvice`) maps exceptions to HTTP status codes consistently across every endpoint.
+
+### Error response format
+
+Most errors:
+```json
+{
+  "error": "Human-readable message"
+}
+```
+
+Validation errors (`400`):
+```json
+{
+  "username": "size must be between 3 and 30",
+  "googleId": "must not be blank"
+}
+```
+
+### Status code reference
+
+| HTTP Status | Condition | Source |
+|-------------|-----------|--------|
+| 400 Bad Request | Input validation failed (missing/invalid fields) | `MethodArgumentNotValidException` via `@Valid` |
+| 400 Bad Request | Business rule violation (e.g. expense not in this list) | `IllegalArgumentException` |
+| 403 Forbidden | User not registered in the system | `AuthenticatedUserResolver` → `ResponseStatusException(FORBIDDEN)` |
+| 403 Forbidden | Ownership check failed (e.g. deleting someone else's list) | `ResponseStatusException(FORBIDDEN)` in controller |
+| 404 Not Found | Entity does not exist (user, list, expense) | `ResourceNotFoundException` |
+| 409 Conflict | Username already taken | `UsernameAlreadyExistsException` |
+| 429 Too Many Requests | Rate limit exceeded | `RateLimitFilter` (returned directly, not via exception handler) |
+| 500 Internal Server Error | Unexpected runtime or generic exception | `RuntimeException` / `Exception` — message is hidden from client |
+
+## 7. Recent Changes — `feature/security-hardening`
+
+This section summarises API-visible changes introduced on the `feature/security-hardening` branch (commits `3d456f2` → `b1324e3` → `8b96433` → `f0de751`). A developer or agent reading this document should be aware of these differences compared to the `main` branch.
+
+---
+
+### New: Centralised error handling (`GlobalExceptionHandler`)
+
+**File:** `src/main/java/.../controller/GlobalExceptionHandler.java`
+
+All error responses now follow the consistent JSON shape described in [Section 6](#6-error-handling). Previously, individual controllers returned ad-hoc responses, including HTTP 417 and raw stack trace output. Both have been removed. SLF4J logging has been added for server-side error visibility.
+
+---
+
+### New: `AuthenticatedUserResolver`
+
+**File:** `src/main/java/.../security/AuthenticatedUserResolver.java`
+
+A new component that extracts the Google ID from the JWT `sub` claim and resolves the corresponding `AppUser`. Injected into all protected controller methods. Returns `403 Forbidden` if the token is valid but the user has never called `POST /api/users/createUser`.
+
+---
+
+### New: `RateLimitFilter` — 60 req/min
+
+**File:** `src/main/java/.../security/RateLimitFilter.java`
+
+Rate limiting was not present on `main`. See [Section 3](#3-rate-limiting) for full details.
+
+---
+
+### New: `CreateExpenseListRequest` DTO with validation
+
+**File:** `src/main/java/.../model/CreateExpenseListRequest.java`
+
+`POST /api/expenselist/create` previously accepted a raw `ExpenseList` body. It now requires a `CreateExpenseListRequest` with a single validated `name` field (`@NotBlank`, `@Size(max=100)`). Clients must update their request body shape.
+
+---
+
+### Updated: Validation constraints tightened on existing DTOs
+
+| DTO | Field | Change |
+|-----|-------|--------|
+| `AppUserCreateRequest` | `username` | Added `@Pattern(regexp="^[a-zA-Z0-9_.\-]+$")` — special characters (other than `_`, `.`, `-`) now rejected |
+| `InviteRequest` | `inviteCode` | Added `@Size(min=6, max=6)` — non-6-char codes now rejected at binding |
+| `ExpenseInput` | `amount` | Added `@DecimalMin("0.01")` — zero and negative amounts now rejected |
+| `ExpenseChangeRequest` | `amount` | Added `@DecimalMin("0.01")` — zero and negative amounts now rejected |
+
+---
+
+### Updated: `SecurityConfig` — test-profile isolation
+
+**File:** `src/main/java/.../security/SecurityConfig.java`
+
+Two separate `SecurityFilterChain` beans now exist:
+- `@Profile("test")` — all endpoints permitted, CSRF disabled (for automated tests only).
+- `@Profile("!test")` — full JWT/OAuth2 enforcement (production behaviour).
+
+Previously a single config was used for both, which made tests fragile.
+
+---
+
+### Fixed: `ExpenseListService` — JPQL and exception consistency
+
+- Single `@Param` used in all JPQL queries (multi-param variant was broken).
+- `ResourceNotFoundException` is now thrown consistently instead of returning `null` or a generic `RuntimeException`.
+- `addExpenseToList` delegates directly to `ExpenseList.addExpense()` rather than looping.
+
+---
+
+### Fixed: `UserService` — null returns replaced with exceptions
+
+All not-found paths in `UserService` now throw `ResourceNotFoundException` instead of returning `null`. Callers that previously needed to null-check responses will now receive a `404` response.

pom.xml

@@ -1,110 +1,142 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
-	xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 https://maven.apache.org/xsd/maven-4.0.0.xsd">
-	<modelVersion>4.0.0</modelVersion>
+    <modelVersion>4.0.0</modelVersion>
-	<parent>
+    <parent>
-		<groupId>org.springframework.boot</groupId>
+        <groupId>org.springframework.boot</groupId>
-		<artifactId>spring-boot-starter-parent</artifactId>
+        <artifactId>spring-boot-starter-parent</artifactId>
-		<version>3.4.1</version>
+        <version>4.0.6</version>
-		<relativePath/> <!-- lookup parent from repository -->
+        <relativePath/> <!-- lookup parent from repository -->
-	</parent>
+    </parent>
-	<groupId>de.zendric.app</groupId>
+    <groupId>de.zendric.app</groupId>
-	<artifactId>XpenselyServer</artifactId>
+    <artifactId>XpenselyServer</artifactId>
-	<version>1.0.0</version>
+    <version>1.0.0</version>
-	<name>XpenselyServer</name>
+    <name>XpenselyServer</name>
-	<description>XpenselyServer used to handle the Xpensely App</description>
+    <description>XpenselyServer used to handle the Xpensely App</description>
-	<url/>
+    <url/>
-	<licenses>
+    <licenses>
-		<license/>
+        <license/>
-	</licenses>
+    </licenses>
-	<developers>
+    <developers>
-		<developer/>
+        <developer/>
-	</developers>
+    </developers>
-	<scm>
+    <scm>
-		<connection/>
+        <connection/>
-		<developerConnection/>
+        <developerConnection/>
-		<tag/>
+        <tag/>
-		<url/>
+        <url/>
-	</scm>
+    </scm>
-	<properties>
+    <properties>
-		<java.version>17</java.version>
+        <java.version>21</java.version>
-	</properties>
+        <lombok.version>1.18.46</lombok.version>
-	<dependencies>
+    </properties>
-		<dependency>
+    <dependencies>
-			<groupId>org.springframework.boot</groupId>
+        <dependency>
-			<artifactId>spring-boot-starter-data-jpa</artifactId>
+            <groupId>org.springframework.boot</groupId>
-		</dependency>
+            <artifactId>spring-boot-starter-data-jpa</artifactId>
-		<dependency>
+        </dependency>
-			<groupId>org.springframework.boot</groupId>
+        <dependency>
-			<artifactId>spring-boot-starter-security</artifactId>
+            <groupId>org.springframework.boot</groupId>
-		</dependency>
+            <artifactId>spring-boot-starter-security</artifactId>
-		<dependency>
+        </dependency>
-        	<groupId>org.springframework.boot</groupId>
+        <dependency>
-        	<artifactId>spring-boot-starter-oauth2-resource-server</artifactId>
+            <groupId>org.springframework.boot</groupId>
-    	</dependency>
+            <artifactId>spring-boot-starter-validation</artifactId>
-			<dependency>
+        </dependency>
-			<groupId>org.springframework.boot</groupId>
+        <dependency>
-			<artifactId>spring-boot-starter-oauth2-client</artifactId>
+            <groupId>com.bucket4j</groupId>
-		</dependency>
+            <artifactId>bucket4j-core</artifactId>
-		<dependency>
+            <version>8.10.1</version>
-			<groupId>org.springframework.boot</groupId>
+        </dependency>
-			<artifactId>spring-boot-starter-web</artifactId>
+        <dependency>
-		</dependency>
+            <groupId>org.springframework.boot</groupId>
-		<dependency>
+            <artifactId>spring-boot-starter-oauth2-resource-server</artifactId>
-			<groupId>org.springframework.boot</groupId>
+        </dependency>
-			<artifactId>spring-boot-devtools</artifactId>
+        <dependency>
-			<scope>runtime</scope>
+            <groupId>org.springframework.boot</groupId>
-			<optional>true</optional>
+            <artifactId>spring-boot-starter-oauth2-client</artifactId>
-		</dependency>
+        </dependency>
-		<dependency>
+        <dependency>
-			<groupId>org.postgresql</groupId>
+            <groupId>org.springframework.boot</groupId>
-			<artifactId>postgresql</artifactId>
+            <artifactId>spring-boot-starter-web</artifactId>
-			<scope>runtime</scope>
+        </dependency>
-		</dependency>
+        <dependency>
-		<dependency>
+            <groupId>org.springframework.boot</groupId>
-			<groupId>org.projectlombok</groupId>
+            <artifactId>spring-boot-devtools</artifactId>
-			<artifactId>lombok</artifactId>
+            <scope>runtime</scope>
-			<optional>true</optional>
+            <optional>true</optional>
-		</dependency>
+        </dependency>
-		<dependency>
+        <dependency>
-			<groupId>org.springframework.boot</groupId>
+            <groupId>org.postgresql</groupId>
-			<artifactId>spring-boot-starter-test</artifactId>
+            <artifactId>postgresql</artifactId>
-			<scope>test</scope>
+            <scope>runtime</scope>
-		</dependency>
+        </dependency>
-		<dependency>
+        <dependency>
-			<groupId>org.springframework.security</groupId>
+            <groupId>org.projectlombok</groupId>
-			<artifactId>spring-security-test</artifactId>
+            <artifactId>lombok</artifactId>
-			<scope>test</scope>
+            <optional>true</optional>
-		</dependency>
+        </dependency>
-	</dependencies>
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-test</artifactId>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-webmvc-test</artifactId>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-data-jpa-test</artifactId>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>com.h2database</groupId>
+            <artifactId>h2</artifactId>
+            <scope>test</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.springframework.security</groupId>
+            <artifactId>spring-security-test</artifactId>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
 
-	<build>
+    <build>
-		<plugins>
+        <plugins>
-			<plugin>
+            <plugin>
-				<groupId>org.apache.maven.plugins</groupId>
+                <groupId>org.apache.maven.plugins</groupId>
-				<artifactId>maven-compiler-plugin</artifactId>
+                <artifactId>maven-compiler-plugin</artifactId>
-				<configuration>
+                <configuration>
-					<annotationProcessorPaths>
+                    <annotationProcessorPaths>
-						<path>
+                        <path>
-							<groupId>org.projectlombok</groupId>
+                            <groupId>org.projectlombok</groupId>
-							<artifactId>lombok</artifactId>
+                            <artifactId>lombok</artifactId>
-						</path>
+                        </path>
-					</annotationProcessorPaths>
+                    </annotationProcessorPaths>
-				</configuration>
+                </configuration>
-			</plugin>
+            </plugin>
-			<plugin>
+            <plugin>
-				<groupId>org.springframework.boot</groupId>
+                <groupId>org.springframework.boot</groupId>
-				<artifactId>spring-boot-maven-plugin</artifactId>
+                <artifactId>spring-boot-maven-plugin</artifactId>
-				<configuration>
+                <configuration>
-					<excludes>
+                    <excludes>
-						<exclude>
+                        <exclude>
-							<groupId>org.projectlombok</groupId>
+                            <groupId>org.projectlombok</groupId>
-							<artifactId>lombok</artifactId>
+                            <artifactId>lombok</artifactId>
-						</exclude>
+                        </exclude>
-					</excludes>
+                    </excludes>
-				</configuration>
+                </configuration>
-			</plugin>
+                <executions>
-		</plugins>
+                    <execution>
-	</build>
+                        <goals>
+                            <goal>build-info</goal>
+                        </goals>
+                    </execution>
+                </executions>
+            </plugin>
+        </plugins>
+    </build>
 
 </project>

src/main/java/de/zendric/app/xpensely_server/controller/AppUserController.java

@@ -1,35 +1,34 @@
 package de.zendric.app.xpensely_server.controller;
 
-import org.springframework.beans.factory.annotation.Autowired;
+import jakarta.validation.Valid;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
-import org.springframework.web.bind.annotation.DeleteMapping;
+import org.springframework.security.core.Authentication;
-import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.*;
-import org.springframework.web.bind.annotation.PostMapping;
+import org.springframework.web.server.ResponseStatusException;
-import org.springframework.web.bind.annotation.RequestBody;
-import org.springframework.web.bind.annotation.RequestMapping;
-import org.springframework.web.bind.annotation.RequestParam;
-import org.springframework.web.bind.annotation.RestController;
 
 import de.zendric.app.xpensely_server.model.AppUser;
 import de.zendric.app.xpensely_server.model.AppUserCreateRequest;
-import de.zendric.app.xpensely_server.model.Exception.UsernameAlreadyExistsException;
+import de.zendric.app.xpensely_server.security.AuthenticatedUserResolver;
 import de.zendric.app.xpensely_server.services.UserService;
 
 @RestController
 @RequestMapping("/api/users")
 public class AppUserController {
 
-    private UserService userService;
+    private final UserService userService;
+    private final AuthenticatedUserResolver authenticatedUserResolver;
 
-    @Autowired
+    public AppUserController(UserService userService, AuthenticatedUserResolver authenticatedUserResolver) {
-    public AppUserController(UserService userService) {
         this.userService = userService;
+        this.authenticatedUserResolver = authenticatedUserResolver;
     }
 
     @GetMapping
-    public AppUser getUser(@RequestParam Long id) {
+    public ResponseEntity<AppUser> getUser(@RequestParam Long id, Authentication authentication) {
-        return userService.getUser(id);
+        AppUser self = authenticatedUserResolver.resolveCurrentUser(authentication);
+        assertSelf(self, id);
+        return ResponseEntity.ok(userService.getUser(id));
     }
 
     @GetMapping("/byName")
@@ -38,36 +37,30 @@ public class AppUserController {
     }
 
     @GetMapping("/byGoogleId")
-    public ResponseEntity<AppUser> getUserByGoogleId(@RequestParam String id) {
+    public ResponseEntity<AppUser> getUserByGoogleId(@RequestParam String id, Authentication authentication) {
-        try {
+        AppUser self = authenticatedUserResolver.resolveCurrentUser(authentication);
-            AppUser userByGoogleId = userService.getUserByGoogleId(id);
+        if (!self.getGoogleId().equals(id))
-            return new ResponseEntity<>(userByGoogleId, HttpStatus.OK);
+            throw new ResponseStatusException(HttpStatus.FORBIDDEN);
-
+        return ResponseEntity.ok(self);
-        } catch (IllegalArgumentException e) {
-            return new ResponseEntity<>(null, HttpStatus.BAD_REQUEST);
-        } catch (Exception e) {
-            return new ResponseEntity<>(null, HttpStatus.INTERNAL_SERVER_ERROR);
-        }
     }
 
     @PostMapping("/createUser")
-    public ResponseEntity<AppUser> createUser(@RequestBody AppUserCreateRequest userRequest) {
+    public ResponseEntity<AppUser> createUser(@RequestBody @Valid AppUserCreateRequest userRequest) {
-        try {
+        AppUser convertedUser = userRequest.convertToAppUser();
-            AppUser convertedUser = userRequest.convertToAppUser();
+        AppUser nUser = userService.createUser(convertedUser);
-
+        return new ResponseEntity<>(nUser, HttpStatus.CREATED);
-            AppUser nUser = userService.createUser(convertedUser);
-            return new ResponseEntity<>(nUser, HttpStatus.CREATED);
-        } catch (UsernameAlreadyExistsException e) {
-            return new ResponseEntity<>(null, HttpStatus.CONFLICT);
-        } catch (Exception e) {
-            return new ResponseEntity<>(null, HttpStatus.BAD_REQUEST);
-        }
-
     }
 
     @DeleteMapping
-    public String deleteUser(@RequestParam Long id) {
+    public ResponseEntity<String> deleteUser(@RequestParam Long id, Authentication authentication) {
+        AppUser self = authenticatedUserResolver.resolveCurrentUser(authentication);
+        assertSelf(self, id);
         AppUser user = userService.deleteUserById(id);
-        return "User deleted : " + user.getUsername();
+        return ResponseEntity.ok("User deleted: " + user.getUsername());
+    }
+
+    private void assertSelf(AppUser authenticated, Long requestedId) {
+        if (!authenticated.getId().equals(requestedId))
+            throw new ResponseStatusException(HttpStatus.FORBIDDEN);
     }
 }

src/main/java/de/zendric/app/xpensely_server/controller/ExpenseListController.java

@@ -1,196 +1,149 @@
 package de.zendric.app.xpensely_server.controller;
 
 import java.time.LocalDateTime;
-import java.util.ArrayList;
 import java.util.List;
 import java.util.Optional;
 
-import org.springframework.beans.factory.annotation.Autowired;
+import jakarta.validation.Valid;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
 import org.springframework.http.HttpStatus;
 import org.springframework.http.ResponseEntity;
-import org.springframework.web.bind.annotation.DeleteMapping;
+import org.springframework.security.core.Authentication;
-import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.*;
-import org.springframework.web.bind.annotation.PathVariable;
+import org.springframework.web.server.ResponseStatusException;
-import org.springframework.web.bind.annotation.PostMapping;
-import org.springframework.web.bind.annotation.PutMapping;
-import org.springframework.web.bind.annotation.RequestBody;
-import org.springframework.web.bind.annotation.RequestMapping;
-import org.springframework.web.bind.annotation.RequestParam;
-import org.springframework.web.bind.annotation.RestController;
 
-import de.zendric.app.xpensely_server.model.AppUser;
+import de.zendric.app.xpensely_server.model.*;
-import de.zendric.app.xpensely_server.model.Expense;
+import de.zendric.app.xpensely_server.security.AuthenticatedUserResolver;
-import de.zendric.app.xpensely_server.model.ExpenseChangeRequest;
-import de.zendric.app.xpensely_server.model.ExpenseInput;
-import de.zendric.app.xpensely_server.model.ExpenseList;
-import de.zendric.app.xpensely_server.model.InviteRequest;
-import de.zendric.app.xpensely_server.model.XpenselyStandardCategories;
 import de.zendric.app.xpensely_server.services.CategoryService;
 import de.zendric.app.xpensely_server.services.ExpenseListService;
 import de.zendric.app.xpensely_server.services.UserService;
 
 @RestController
 @RequestMapping("/api/expenselist")
-class ExpenseListController {
+public class ExpenseListController {
 
-    private ExpenseListService expenseListService;
+    private static final Logger log = LoggerFactory.getLogger(ExpenseListController.class);
-    private UserService userService;
+
-    private CategoryService categoryService;
+    private final ExpenseListService expenseListService;
+    private final UserService userService;
+    private final CategoryService categoryService;
+    private final AuthenticatedUserResolver authenticatedUserResolver;
 
-    @Autowired
     public ExpenseListController(ExpenseListService expenseListService, UserService userService,
-            CategoryService categoryService) {
+            CategoryService categoryService, AuthenticatedUserResolver authenticatedUserResolver) {
         this.expenseListService = expenseListService;
         this.userService = userService;
         this.categoryService = categoryService;
+        this.authenticatedUserResolver = authenticatedUserResolver;
     }
 
-    @GetMapping("/all")
+    @GetMapping("/mine")
-    public ResponseEntity<List<ExpenseList>> getAll() {
+    public ResponseEntity<List<ExpenseList>> getMine(Authentication authentication) {
-        try {
+        AppUser user = authenticatedUserResolver.resolveCurrentUser(authentication);
-            List<ExpenseList> items = new ArrayList<>();
+        List<ExpenseList> items = expenseListService.findByUserId(user.getId());
-
+        if (items.isEmpty())
-            expenseListService.findAll().forEach(items::add);
+            return new ResponseEntity<>(HttpStatus.NO_CONTENT);
-
+        return new ResponseEntity<>(items, HttpStatus.OK);
-            if (items.isEmpty())
-                return new ResponseEntity<>(HttpStatus.NO_CONTENT);
-
-            return new ResponseEntity<>(items, HttpStatus.OK);
-        } catch (Exception e) {
-            return new ResponseEntity<>(null, HttpStatus.INTERNAL_SERVER_ERROR);
-        }
-    }
-
-    @GetMapping("/byUser")
-    public ResponseEntity<List<ExpenseList>> getByUser(@RequestParam Long userId) {
-        try {
-            List<ExpenseList> items = expenseListService.findByUserId(userId);
-
-            if (items.isEmpty())
-                return new ResponseEntity<>(HttpStatus.NO_CONTENT);
-
-            return new ResponseEntity<>(items, HttpStatus.OK);
-        } catch (Exception e) {
-            return new ResponseEntity<>(null, HttpStatus.INTERNAL_SERVER_ERROR);
-        }
-    }
-
-    @GetMapping("/byUsername")
-    public ResponseEntity<List<ExpenseList>> getByUser(@RequestParam String username) {
-        try {
-            List<ExpenseList> items = expenseListService.findByUsername(username);
-
-            if (items.isEmpty())
-                return new ResponseEntity<>(HttpStatus.NO_CONTENT);
-
-            return new ResponseEntity<>(items, HttpStatus.OK);
-        } catch (Exception e) {
-            return new ResponseEntity<>(null, HttpStatus.INTERNAL_SERVER_ERROR);
-        }
     }
 
     @GetMapping("/byId")
-    public ResponseEntity<ExpenseList> getById(@RequestParam Long id) {
+    public ResponseEntity<ExpenseList> getById(@RequestParam Long id, Authentication authentication) {
+        AppUser user = authenticatedUserResolver.resolveCurrentUser(authentication);
         Optional<ExpenseList> existingItemOptional = expenseListService.findById(id);
-
+        if (existingItemOptional.isEmpty())
-        if (existingItemOptional.isPresent()) {
-            return new ResponseEntity<>(existingItemOptional.get(), HttpStatus.OK);
-        } else {
             return new ResponseEntity<>(HttpStatus.NOT_FOUND);
-        }
+        assertMember(user, existingItemOptional.get());
+        return new ResponseEntity<>(existingItemOptional.get(), HttpStatus.OK);
     }
 
     @PostMapping("/create")
-    // TODO add handling of categories by using DTO
+    public ResponseEntity<ExpenseList> create(@RequestBody @Valid CreateExpenseListRequest request,
-    public ResponseEntity<ExpenseList> create(@RequestBody ExpenseList expenseList) {
+            Authentication authentication) {
-        try {
+        AppUser authenticatedUser = authenticatedUserResolver.resolveCurrentUser(authentication);
-            if (expenseList.getOwner() != null) {
+        ExpenseList expenseList = new ExpenseList();
-                AppUser existingOwner = userService.getUser(expenseList.getOwner().getId());
+        expenseList.setName(request.getName());
-                if (existingOwner == null) {
+        expenseList.setOwner(authenticatedUser);
-                    throw new IllegalArgumentException("Owner does not exist.");
+        XpenselyStandardCategories standardCategories = categoryService.getDefaultCategories();
-                }
+        expenseList.setXpenselyStandardCategories(standardCategories);
-                expenseList.setOwner(existingOwner);
+        expenseList.setSharedWith(null);
-                XpenselyStandardCategories standardCategories = categoryService.getDefaultCategories();
+        ExpenseList savedItem = expenseListService.createList(expenseList);
-                expenseList.setXpenselyStandardCategories(standardCategories);
+        log.debug("Created expense list '{}' for user {}", savedItem.getName(), authenticatedUser.getId());
-            } else {
+        return new ResponseEntity<>(savedItem, HttpStatus.CREATED);
-                throw new IllegalArgumentException("Owner is required.");
-            }
-
-            expenseList.setSharedWith(null);
-
-            ExpenseList savedItem = expenseListService.createList(expenseList);
-            return new ResponseEntity<>(savedItem, HttpStatus.CREATED);
-        } catch (Exception e) {
-            e.printStackTrace();
-            return new ResponseEntity<>(null, HttpStatus.EXPECTATION_FAILED);
-        }
     }
 
     @DeleteMapping("{id}")
-    public ResponseEntity<HttpStatus> delete(@PathVariable("id") Long id) {
+    public ResponseEntity<HttpStatus> delete(@PathVariable("id") Long id, Authentication authentication) {
-        try {
+        AppUser user = authenticatedUserResolver.resolveCurrentUser(authentication);
-            expenseListService.deleteById(id);
+        Optional<ExpenseList> listOpt = expenseListService.findById(id);
-            return new ResponseEntity<>(HttpStatus.NO_CONTENT);
+        if (listOpt.isEmpty())
-        } catch (Exception e) {
+            return new ResponseEntity<>(HttpStatus.NOT_FOUND);
-            return new ResponseEntity<>(HttpStatus.EXPECTATION_FAILED);
+        assertOwner(user, listOpt.get());
-        }
+        expenseListService.deleteById(id);
+        return new ResponseEntity<>(HttpStatus.NO_CONTENT);
     }
 
     @PostMapping("/{id}/add")
     public ResponseEntity<Expense> addExpenseToList(
             @PathVariable("id") Long expenseListId,
-            @RequestBody ExpenseInput expenseInput) {
+            @RequestBody @Valid ExpenseInput expenseInput,
-        try {
+            Authentication authentication) {
-            AppUser expenseOwner = userService.getUserByName(expenseInput.getOwner());
+        AppUser user = authenticatedUserResolver.resolveCurrentUser(authentication);
-            Expense expense = expenseInput.convertToExpense(expenseOwner.getId());
+        Optional<ExpenseList> listOpt = expenseListService.findById(expenseListId);
-
+        if (listOpt.isEmpty())
-            Expense addedExpense = expenseListService.addExpenseToList(expenseListId, expense);
+            return new ResponseEntity<>(HttpStatus.NOT_FOUND);
-            return new ResponseEntity<>(addedExpense, HttpStatus.CREATED);
+        assertMember(user, listOpt.get());
-        } catch (Exception e) {
+        AppUser expenseOwner = userService.getUserByName(expenseInput.getOwner());
-            return new ResponseEntity<>(null, HttpStatus.INTERNAL_SERVER_ERROR);
+        Expense expense = expenseInput.convertToExpense(expenseOwner.getId());
-        }
+        Expense addedExpense = expenseListService.addExpenseToList(expenseListId, expense);
+        return new ResponseEntity<>(addedExpense, HttpStatus.CREATED);
     }
 
     @PutMapping("/{id}/update")
     public ResponseEntity<Expense> updateExpenseInList(
             @PathVariable("id") Long expenseListId,
-            @RequestBody ExpenseChangeRequest expenseChangeRequest) {
+            @RequestBody @Valid ExpenseChangeRequest expenseChangeRequest,
-        try {
+            Authentication authentication) {
-            AppUser expenseOwner = userService.getUserByName(expenseChangeRequest.getOwnerName());
+        AppUser user = authenticatedUserResolver.resolveCurrentUser(authentication);
-            Optional<ExpenseList> expenseList = expenseListService.findById(expenseListId);
+        Optional<ExpenseList> expenseListOpt = expenseListService.findById(expenseListId);
-            if (expenseList.isPresent()) {
+        if (expenseListOpt.isEmpty())
-                Expense expense = expenseChangeRequest.convertToExpense(expenseOwner.getId(), expenseList.get());
+            return new ResponseEntity<>(HttpStatus.NOT_FOUND);
-
+        assertMember(user, expenseListOpt.get());
-                Expense addedExpense = expenseListService.updateExpense(expenseListId, expense);
+        AppUser expenseOwner = userService.getUserByName(expenseChangeRequest.getOwnerName());
-                return new ResponseEntity<>(addedExpense, HttpStatus.CREATED);
+        Expense expense = expenseChangeRequest.convertToExpense(expenseOwner.getId(), expenseListOpt.get());
-            }
+        Expense updatedExpense = expenseListService.updateExpense(expenseListId, expense);
-            return new ResponseEntity<>(null, HttpStatus.BAD_REQUEST);
+        return new ResponseEntity<>(updatedExpense, HttpStatus.OK);
-
-        } catch (Exception e) {
-            return new ResponseEntity<>(null, HttpStatus.BAD_REQUEST);
-        }
     }
 
     @DeleteMapping("/{id}/delete")
     public ResponseEntity<Expense> deleteExpenseFromList(
             @PathVariable("id") Long expenseListId,
-            @RequestParam("expenseId") Long expenseId) {
+            @RequestParam("expenseId") Long expenseId,
-        try {
+            Authentication authentication) {
-            expenseListService.deleteExpenseFromList(expenseListId, expenseId);
+        AppUser user = authenticatedUserResolver.resolveCurrentUser(authentication);
-            return new ResponseEntity<>(HttpStatus.NO_CONTENT);
+        Optional<ExpenseList> listOpt = expenseListService.findById(expenseListId);
-        } catch (Exception e) {
+        if (listOpt.isEmpty())
-            return new ResponseEntity<>(null, HttpStatus.EXPECTATION_FAILED);
+            return new ResponseEntity<>(HttpStatus.NOT_FOUND);
-        }
+        assertMember(user, listOpt.get());
+        expenseListService.deleteExpenseFromList(expenseListId, expenseId);
+        return new ResponseEntity<>(HttpStatus.NO_CONTENT);
     }
 
     @PostMapping("/{listId}/invite")
-    public ResponseEntity<String> generateInvite(@PathVariable Long listId) {
+    public ResponseEntity<String> generateInvite(@PathVariable Long listId, Authentication authentication) {
+        AppUser user = authenticatedUserResolver.resolveCurrentUser(authentication);
+        Optional<ExpenseList> listOpt = expenseListService.findById(listId);
+        if (listOpt.isEmpty())
+            return new ResponseEntity<>(HttpStatus.NOT_FOUND);
+        assertOwner(user, listOpt.get());
         String inviteCode = expenseListService.generateInviteCode(listId);
         return ResponseEntity.ok(inviteCode);
     }
 
     @PostMapping("/accept-invite")
-    public ResponseEntity<?> acceptInvite(@RequestBody InviteRequest inviteRequest) {
+    public ResponseEntity<?> acceptInvite(@RequestBody @Valid InviteRequest inviteRequest,
+            Authentication authentication) {
+        AppUser authenticatedUser = authenticatedUserResolver.resolveCurrentUser(authentication);
         ExpenseList list = expenseListService.findByInviteCode(inviteRequest.getInviteCode());
 
         if (list == null || list.getInviteCodeExpiration() == null ||
@@ -200,21 +153,24 @@ class ExpenseListController {
         if (list.getSharedWith() != null) {
             return ResponseEntity.status(HttpStatus.IM_USED).body("List has already been shared");
         }
-        if (list.getOwner().getId() == inviteRequest.getUserId()) {
+        if (list.getOwner().getId().equals(authenticatedUser.getId())) {
-            return ResponseEntity.status(HttpStatus.BAD_REQUEST).body("You cant join your own List");
+            return ResponseEntity.status(HttpStatus.BAD_REQUEST).body("You cannot join your own list");
-        }
-        AppUser user = null;
-        try {
-            user = userService.getUser(inviteRequest.getUserId());
-        } catch (Exception e) {
-            throw new RuntimeException("User not found");
-        }
-        if (user != null) {
-            list.setSharedWith(user);
-            expenseListService.save(list);
-        } else {
-            throw new RuntimeException("User not found");
         }
+        list.setSharedWith(authenticatedUser);
+        expenseListService.save(list);
         return ResponseEntity.ok("User added to the list");
     }
+
+    private void assertOwner(AppUser authenticated, ExpenseList list) {
+        if (!list.getOwner().getId().equals(authenticated.getId()))
+            throw new ResponseStatusException(HttpStatus.FORBIDDEN);
+    }
+
+    private void assertMember(AppUser authenticated, ExpenseList list) {
+        boolean isOwner = list.getOwner().getId().equals(authenticated.getId());
+        boolean isShared = list.getSharedWith() != null
+                && list.getSharedWith().getId().equals(authenticated.getId());
+        if (!isOwner && !isShared)
+            throw new ResponseStatusException(HttpStatus.FORBIDDEN);
+    }
 }

src/main/java/de/zendric/app/xpensely_server/controller/GlobalExceptionHandler.java

@@ -0,0 +1,69 @@
+package de.zendric.app.xpensely_server.controller;
+
+import de.zendric.app.xpensely_server.model.Exception.ResourceNotFoundException;
+import de.zendric.app.xpensely_server.model.Exception.UsernameAlreadyExistsException;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.springframework.http.HttpStatus;
+import org.springframework.http.ResponseEntity;
+import org.springframework.validation.FieldError;
+import org.springframework.web.bind.MethodArgumentNotValidException;
+import org.springframework.web.bind.annotation.ExceptionHandler;
+import org.springframework.web.bind.annotation.RestControllerAdvice;
+import org.springframework.web.server.ResponseStatusException;
+
+import java.util.HashMap;
+import java.util.Map;
+
+@RestControllerAdvice
+public class GlobalExceptionHandler {
+
+    private static final Logger log = LoggerFactory.getLogger(GlobalExceptionHandler.class);
+
+    @ExceptionHandler(MethodArgumentNotValidException.class)
+    public ResponseEntity<Map<String, String>> handleValidationErrors(MethodArgumentNotValidException ex) {
+        Map<String, String> errors = new HashMap<>();
+        for (FieldError fieldError : ex.getBindingResult().getFieldErrors()) {
+            errors.put(fieldError.getField(), fieldError.getDefaultMessage());
+        }
+        return ResponseEntity.status(HttpStatus.BAD_REQUEST).body(errors);
+    }
+
+    @ExceptionHandler(IllegalArgumentException.class)
+    public ResponseEntity<Map<String, String>> handleIllegalArgument(IllegalArgumentException ex) {
+        return ResponseEntity.status(HttpStatus.BAD_REQUEST)
+                .body(Map.of("error", ex.getMessage()));
+    }
+
+    @ExceptionHandler(ResourceNotFoundException.class)
+    public ResponseEntity<Map<String, String>> handleNotFound(ResourceNotFoundException ex) {
+        return ResponseEntity.status(HttpStatus.NOT_FOUND)
+                .body(Map.of("error", ex.getMessage()));
+    }
+
+    @ExceptionHandler(UsernameAlreadyExistsException.class)
+    public ResponseEntity<Map<String, String>> handleUsernameConflict(UsernameAlreadyExistsException ex) {
+        return ResponseEntity.status(HttpStatus.CONFLICT)
+                .body(Map.of("error", ex.getMessage()));
+    }
+
+    @ExceptionHandler(ResponseStatusException.class)
+    public ResponseEntity<Map<String, String>> handleResponseStatus(ResponseStatusException ex) {
+        return ResponseEntity.status(ex.getStatusCode())
+                .body(Map.of("error", ex.getReason() != null ? ex.getReason() : ex.getMessage()));
+    }
+
+    @ExceptionHandler(RuntimeException.class)
+    public ResponseEntity<Map<String, String>> handleRuntime(RuntimeException ex) {
+        log.error("Unhandled runtime exception", ex);
+        return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR)
+                .body(Map.of("error", "An unexpected error occurred"));
+    }
+
+    @ExceptionHandler(Exception.class)
+    public ResponseEntity<Map<String, String>> handleGeneric(Exception ex) {
+        log.error("Unhandled exception", ex);
+        return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR)
+                .body(Map.of("error", "An unexpected error occurred"));
+    }
+}

src/main/java/de/zendric/app/xpensely_server/controller/HomeController.java

@@ -1,13 +1,30 @@
 package de.zendric.app.xpensely_server.controller;
 
+import org.springframework.boot.info.BuildProperties;
 import org.springframework.web.bind.annotation.GetMapping;
 import org.springframework.web.bind.annotation.RestController;
 
+import java.util.Map;
+
 @RestController
 class HomeController {
 
+    private final BuildProperties buildProperties;
+
+    HomeController(BuildProperties buildProperties) {
+        this.buildProperties = buildProperties;
+    }
+
     @GetMapping("/")
     public String getAll() {
         return "Welcome";
     }
+
+    @GetMapping("/api/version")
+    public Map<String, String> version() {
+        return Map.of(
+                "version", buildProperties.getVersion(),
+                "builtAt", buildProperties.getTime().toString()
+        );
+    }
 }

src/main/java/de/zendric/app/xpensely_server/model/AppUserCreateRequest.java

@@ -1,21 +1,25 @@
 package de.zendric.app.xpensely_server.model;
 
-import jakarta.persistence.Column;
+import jakarta.validation.constraints.NotBlank;
+import jakarta.validation.constraints.Pattern;
+import jakarta.validation.constraints.Size;
 import lombok.Data;
 
 @Data
 public class AppUserCreateRequest {
 
-    @Column(name = "username", nullable = false, unique = true)
+    @NotBlank(message = "Username is required")
+    @Size(min = 3, max = 30, message = "Username must be between 3 and 30 characters")
+    @Pattern(regexp = "^[a-zA-Z0-9_.\\-]+$", message = "Username may only contain letters, digits, underscores, dots, and hyphens")
     private String username;
 
+    @NotBlank(message = "Google ID is required")
     private String googleId;
 
     public AppUser convertToAppUser() {
         AppUser appUser = new AppUser();
         appUser.setGoogleId(googleId);
         appUser.setUsername(username);
-
         return appUser;
     }
 }

src/main/java/de/zendric/app/xpensely_server/model/CreateExpenseListRequest.java

@@ -0,0 +1,17 @@
+package de.zendric.app.xpensely_server.model;
+
+import jakarta.validation.constraints.NotBlank;
+import jakarta.validation.constraints.Size;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import lombok.Setter;
+
+@Getter
+@Setter
+@NoArgsConstructor
+public class CreateExpenseListRequest {
+
+    @NotBlank(message = "List name is required")
+    @Size(max = 100, message = "List name must not exceed 100 characters")
+    private String name;
+}

src/main/java/de/zendric/app/xpensely_server/model/Exception/ResourceNotFoundException.java

@@ -0,0 +1,11 @@
+package de.zendric.app.xpensely_server.model.Exception;
+
+import org.springframework.http.HttpStatus;
+import org.springframework.web.bind.annotation.ResponseStatus;
+
+@ResponseStatus(HttpStatus.NOT_FOUND)
+public class ResourceNotFoundException extends RuntimeException {
+    public ResourceNotFoundException(String message) {
+        super(message);
+    }
+}

src/main/java/de/zendric/app/xpensely_server/model/ExpenseChangeRequest.java

@@ -2,6 +2,10 @@ package de.zendric.app.xpensely_server.model;
 
 import java.time.LocalDate;
 
+import jakarta.validation.constraints.DecimalMin;
+import jakarta.validation.constraints.NotBlank;
+import jakarta.validation.constraints.NotNull;
+import jakarta.validation.constraints.Size;
 import lombok.AllArgsConstructor;
 import lombok.Data;
 import lombok.NoArgsConstructor;
@@ -12,12 +16,25 @@ import lombok.NoArgsConstructor;
 public class ExpenseChangeRequest {
 
     private Long id;
+
+    @NotBlank(message = "Title is required")
+    @Size(max = 100, message = "Title must not exceed 100 characters")
     private String title;
+
+    @NotBlank(message = "Owner name is required")
     private String ownerName;
+
+    @NotNull(message = "Amount is required")
+    @DecimalMin(value = "0.01", message = "Amount must be greater than zero")
     private Double amount;
+
     private Double personalUseAmount;
     private Double otherPersonAmount;
+
+    @NotNull(message = "Date is required")
     private LocalDate date;
+
+    @NotBlank(message = "Category is required")
     private String category;
 
     public Expense convertToExpense(Long userId, ExpenseList expenseList) {
@@ -38,4 +55,4 @@ public class ExpenseChangeRequest {
 
         return expense;
     }
-}
+}

src/main/java/de/zendric/app/xpensely_server/model/ExpenseInput.java

@@ -2,9 +2,10 @@ package de.zendric.app.xpensely_server.model;
 
 import java.time.LocalDate;
 
-import jakarta.persistence.GeneratedValue;
+import jakarta.validation.constraints.DecimalMin;
-import jakarta.persistence.GenerationType;
+import jakarta.validation.constraints.NotBlank;
-import jakarta.persistence.Id;
+import jakarta.validation.constraints.NotNull;
+import jakarta.validation.constraints.Size;
 import lombok.AllArgsConstructor;
 import lombok.Getter;
 import lombok.NoArgsConstructor;
@@ -16,19 +17,26 @@ import lombok.Setter;
 @NoArgsConstructor
 public class ExpenseInput {
 
-    @Id
-    @GeneratedValue(strategy = GenerationType.IDENTITY)
     private Long id;
 
+    @NotBlank(message = "Title is required")
+    @Size(max = 100, message = "Title must not exceed 100 characters")
     private String title;
 
+    @NotBlank(message = "Owner is required")
     private String owner;
 
+    @NotNull(message = "Amount is required")
+    @DecimalMin(value = "0.01", message = "Amount must be greater than zero")
     private Double amount;
+
     private Double personalUseAmount;
     private Double otherPersonAmount;
 
+    @NotNull(message = "Date is required")
     private LocalDate date;
+
+    @NotBlank(message = "Category is required")
     private String category;
 
     private ExpenseList expenseList;

src/main/java/de/zendric/app/xpensely_server/model/InviteRequest.java

@@ -1,5 +1,7 @@
 package de.zendric.app.xpensely_server.model;
 
+import jakarta.validation.constraints.NotBlank;
+import jakarta.validation.constraints.Size;
 import lombok.AllArgsConstructor;
 import lombok.Data;
 import lombok.NoArgsConstructor;
@@ -8,6 +10,8 @@ import lombok.NoArgsConstructor;
 @AllArgsConstructor
 @NoArgsConstructor
 public class InviteRequest {
+
+    @NotBlank(message = "Invite code is required")
+    @Size(min = 6, max = 6, message = "Invite code must be exactly 6 characters")
     private String inviteCode;
-    private Long userId;
 }

src/main/java/de/zendric/app/xpensely_server/repo/ExpenseListRepository.java

@@ -3,13 +3,22 @@ package de.zendric.app.xpensely_server.repo;
 import java.util.List;
 
 import org.springframework.data.jpa.repository.JpaRepository;
+import org.springframework.data.jpa.repository.Query;
+import org.springframework.data.repository.query.Param;
 import org.springframework.stereotype.Repository;
 
 import de.zendric.app.xpensely_server.model.ExpenseList;
 
 @Repository
 public interface ExpenseListRepository extends JpaRepository<ExpenseList, Long> {
+
     List<ExpenseList> findByOwnerId(Long ownerId);
 
     ExpenseList findByInviteCode(String inviteCode);
-}
+
+    @Query("SELECT el FROM ExpenseList el WHERE el.owner.id = :userId OR el.sharedWith.id = :userId")
+    List<ExpenseList> findByOwnerIdOrSharedWithId(@Param("userId") Long userId);
+
+    @Query("SELECT el FROM ExpenseList el WHERE el.owner.username = :username OR el.sharedWith.username = :username")
+    List<ExpenseList> findByOwnerUsernameOrSharedWithUsername(@Param("username") String username);
+}

src/main/java/de/zendric/app/xpensely_server/security/AuthenticatedUserResolver.java

@@ -0,0 +1,36 @@
+package de.zendric.app.xpensely_server.security;
+
+import de.zendric.app.xpensely_server.model.AppUser;
+import de.zendric.app.xpensely_server.services.UserService;
+import org.springframework.http.HttpStatus;
+import org.springframework.security.core.Authentication;
+import org.springframework.security.oauth2.jwt.Jwt;
+import org.springframework.stereotype.Component;
+import org.springframework.web.server.ResponseStatusException;
+
+@Component
+public class AuthenticatedUserResolver {
+
+    private final UserService userService;
+
+    public AuthenticatedUserResolver(UserService userService) {
+        this.userService = userService;
+    }
+
+    public AppUser resolveCurrentUser(Authentication authentication) {
+        if (authentication == null) {
+            throw new ResponseStatusException(HttpStatus.FORBIDDEN, "Not authenticated");
+        }
+        Jwt jwt = (Jwt) authentication.getPrincipal();
+        String googleId = jwt.getSubject();
+        try {
+            AppUser user = userService.getUserByGoogleId(googleId);
+            if (user == null) {
+                throw new ResponseStatusException(HttpStatus.FORBIDDEN, "User not registered");
+            }
+            return user;
+        } catch (IllegalArgumentException e) {
+            throw new ResponseStatusException(HttpStatus.FORBIDDEN, "User not registered");
+        }
+    }
+}

src/main/java/de/zendric/app/xpensely_server/security/RateLimitFilter.java

@@ -0,0 +1,61 @@
+package de.zendric.app.xpensely_server.security;
+
+import io.github.bucket4j.Bandwidth;
+import io.github.bucket4j.Bucket;
+import jakarta.servlet.FilterChain;
+import jakarta.servlet.ServletException;
+import jakarta.servlet.http.HttpServletRequest;
+import jakarta.servlet.http.HttpServletResponse;
+import org.springframework.http.HttpStatus;
+import org.springframework.security.core.Authentication;
+import org.springframework.security.core.context.SecurityContextHolder;
+import org.springframework.security.oauth2.jwt.Jwt;
+import org.springframework.web.filter.OncePerRequestFilter;
+
+import java.io.IOException;
+import java.time.Duration;
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+
+public class RateLimitFilter extends OncePerRequestFilter {
+
+    private static final int REQUESTS_PER_MINUTE = 60;
+
+    private final Map<String, Bucket> buckets = new ConcurrentHashMap<>();
+
+    @Override
+    protected void doFilterInternal(HttpServletRequest request,
+                                    HttpServletResponse response,
+                                    FilterChain filterChain) throws ServletException, IOException {
+        String key = resolveKey(request);
+        Bucket bucket = buckets.computeIfAbsent(key, k -> newBucket());
+
+        if (bucket.tryConsume(1)) {
+            filterChain.doFilter(request, response);
+        } else {
+            response.setStatus(HttpStatus.TOO_MANY_REQUESTS.value());
+            response.getWriter().write("Rate limit exceeded");
+        }
+    }
+
+    private String resolveKey(HttpServletRequest request) {
+        Authentication auth = SecurityContextHolder.getContext().getAuthentication();
+        if (auth != null && auth.getPrincipal() instanceof Jwt jwt) {
+            return "user:" + jwt.getSubject();
+        }
+        String ip = request.getHeader("X-Forwarded-For");
+        if (ip != null && !ip.isBlank()) {
+            return "ip:" + ip.split(",")[0].trim();
+        }
+        return "ip:" + request.getRemoteAddr();
+    }
+
+    private Bucket newBucket() {
+        return Bucket.builder()
+                .addLimit(Bandwidth.builder()
+                        .capacity(REQUESTS_PER_MINUTE)
+                        .refillGreedy(REQUESTS_PER_MINUTE, Duration.ofMinutes(1))
+                        .build())
+                .build();
+    }
+}

src/main/java/de/zendric/app/xpensely_server/security/SecurityConfig.java

@@ -6,6 +6,7 @@ import org.springframework.context.annotation.Profile;
 import org.springframework.security.config.Customizer;
 import org.springframework.security.config.annotation.web.builders.HttpSecurity;
 import org.springframework.security.config.annotation.web.configuration.EnableWebSecurity;
+import org.springframework.security.oauth2.server.resource.web.authentication.BearerTokenAuthenticationFilter;
 import org.springframework.security.web.SecurityFilterChain;
 
 @Configuration
@@ -17,7 +18,7 @@ public class SecurityConfig {
                 http
                                 .authorizeHttpRequests(auth -> auth
                                                 .anyRequest().permitAll())
-                                .csrf().disable();
+                                .csrf(csrf -> csrf.disable());
 
                 return http.build();
         }
@@ -27,11 +28,12 @@ public class SecurityConfig {
         public SecurityFilterChain securityFilterChain(HttpSecurity http) throws Exception {
                 http
                                 .authorizeHttpRequests(auth -> auth
+                                                .requestMatchers("/api/version").permitAll()
                                                 .anyRequest().authenticated())
                                 .oauth2ResourceServer(oauth2 -> oauth2
                                                 .jwt(Customizer.withDefaults()))
-                                .oauth2Login(Customizer.withDefaults())
+                                .addFilterAfter(new RateLimitFilter(), BearerTokenAuthenticationFilter.class)
-                                .csrf().disable();
+                                .csrf(csrf -> csrf.disable());
 
                 return http.build();
         }

src/main/java/de/zendric/app/xpensely_server/services/ExpenseListService.java

@@ -1,37 +1,29 @@
 package de.zendric.app.xpensely_server.services;
 
 import java.time.LocalDateTime;
-import java.util.ArrayList;
-import java.util.HashSet;
 import java.util.List;
 import java.util.Optional;
 import java.util.UUID;
 
-import org.springframework.beans.factory.annotation.Autowired;
 import org.springframework.stereotype.Service;
 import org.springframework.transaction.annotation.Transactional;
 
-import de.zendric.app.xpensely_server.model.AppUser;
 import de.zendric.app.xpensely_server.model.Expense;
 import de.zendric.app.xpensely_server.model.ExpenseList;
+import de.zendric.app.xpensely_server.model.Exception.ResourceNotFoundException;
 import de.zendric.app.xpensely_server.model.XpenselyCustomCategory;
 import de.zendric.app.xpensely_server.repo.ExpenseListRepository;
 import de.zendric.app.xpensely_server.repo.ExpenseRepository;
 import de.zendric.app.xpensely_server.repo.XpenselyCustomCategoryRepository;
-import jakarta.persistence.EntityManager;
 
 @Service
 @Transactional
 public class ExpenseListService {
 
-    private ExpenseListRepository repository;
+    private final ExpenseListRepository repository;
     private final ExpenseRepository expenseRepository;
-    private XpenselyCustomCategoryRepository customCategoryRepository;
+    private final XpenselyCustomCategoryRepository customCategoryRepository;
 
-    @Autowired
-    private EntityManager entityManager;
-
-    @Autowired
     public ExpenseListService(ExpenseListRepository repository, ExpenseRepository expenseRepository,
             XpenselyCustomCategoryRepository customCategoryRepository) {
         this.repository = repository;
@@ -39,18 +31,10 @@ public class ExpenseListService {
         this.customCategoryRepository = customCategoryRepository;
     }
 
-    public List<ExpenseList> getAllLists() {
-        return repository.findAll();
-    }
-
     public ExpenseList createList(ExpenseList list) {
         return repository.save(list);
     }
 
-    public void deleteList(Long id) {
-        repository.deleteById(id);
-    }
-
     public void deleteById(Long id) {
         repository.deleteById(id);
     }
@@ -59,76 +43,29 @@ public class ExpenseListService {
         return repository.findById(id);
     }
 
-    public Iterable<ExpenseList> findAll() {
-        return repository.findAll();
-    }
-
     public ExpenseList save(ExpenseList expenseList) {
         return repository.save(expenseList);
     }
 
     public List<ExpenseList> findByUserId(Long id) {
-        List<ExpenseList> allLists = repository.findAll();
+        return repository.findByOwnerIdOrSharedWithId(id);
-        List<ExpenseList> userSpecificList = new ArrayList<>();
-        for (ExpenseList expenseList : allLists) {
-            AppUser sharedWith = expenseList.getSharedWith();
-
-            if (expenseList.getOwner().getId().equals(id)) {
-                userSpecificList.add(expenseList);
-            } else {
-                if (sharedWith != null && sharedWith.getId().equals(id)) {
-                    userSpecificList.add(expenseList);
-                }
-            }
-        }
-        return userSpecificList;
     }
 
     public List<ExpenseList> findByUsername(String username) {
-        List<ExpenseList> allLists = repository.findAll();
+        return repository.findByOwnerUsernameOrSharedWithUsername(username);
-        List<ExpenseList> userSpecificList = new ArrayList<>();
-        for (ExpenseList expenseList : allLists) {
-            AppUser sharedWith = expenseList.getSharedWith();
-
-            if (expenseList.getOwner().getUsername().equals(username)) {
-                userSpecificList.add(expenseList);
-            } else {
-                if (sharedWith != null && sharedWith.getUsername().equals(username)) {
-                    userSpecificList.add(expenseList);
-                }
-            }
-        }
-        return userSpecificList;
-
     }
 
     public Expense addExpenseToList(Long expenseListId, Expense expense) {
-        // find expenseList
         ExpenseList expenseList = repository.findById(expenseListId)
-                .orElseThrow(() -> new RuntimeException("ExpenseList not found with id: " + expenseListId));
+                .orElseThrow(() -> new ResourceNotFoundException("ExpenseList not found with id: " + expenseListId));
-        // get all added expenses
-        HashSet<Long> existingId = new HashSet<>();
-        for (Expense e : expenseList.getExpenses()) {
-            existingId.add(e.getId());
-        }
-        // add the new expense
         expenseList.addExpense(expense);
-        // save
         repository.save(expenseList);
-
+        return expense;
-        Expense newExpense = new Expense();
-        for (Expense e : expenseList.getExpenses()) {
-            if (!existingId.contains(e.getId())) {
-                newExpense = e;
-                break;
-            }
-        }
-        return newExpense;
     }
 
     public void deleteExpenseFromList(Long expenseListId, Long expenseId) {
         ExpenseList expenseList = repository.findById(expenseListId)
-                .orElseThrow(() -> new RuntimeException("ExpenseList not found with id: " + expenseListId));
+                .orElseThrow(() -> new ResourceNotFoundException("ExpenseList not found with id: " + expenseListId));
         Expense expenseToRemove = null;
         for (Expense expense : expenseList.getExpenses()) {
             if (expense.getId().equals(expenseId)) {
@@ -139,14 +76,14 @@ public class ExpenseListService {
         if (expenseToRemove != null) {
             expenseList.removeExpense(expenseToRemove);
         } else {
-            throw new RuntimeException("Expense not found with id: " + expenseId);
+            throw new ResourceNotFoundException("Expense not found with id: " + expenseId);
         }
         repository.save(expenseList);
     }
 
     public String generateInviteCode(Long listId) {
         ExpenseList list = repository.findById(listId)
-                .orElseThrow(() -> new RuntimeException("List not found"));
+                .orElseThrow(() -> new ResourceNotFoundException("List not found"));
         String inviteCode;
         if (list.getInviteCode() == null || list.getInviteCodeExpiration().isBefore(LocalDateTime.now())) {
 
@@ -168,7 +105,7 @@ public class ExpenseListService {
 
     public Expense updateExpense(Long expenseListId, Expense updatedExpense) {
         ExpenseList expenseList = repository.findById(expenseListId)
-                .orElseThrow(() -> new IllegalArgumentException("ExpenseList not found"));
+                .orElseThrow(() -> new ResourceNotFoundException("ExpenseList not found with id: " + expenseListId));
 
         if (!expenseList.getExpenses().stream()
                 .anyMatch(expense -> expense.getId().equals(updatedExpense.getId()))) {
@@ -176,7 +113,7 @@ public class ExpenseListService {
         }
 
         Expense existingExpense = expenseRepository.findById(updatedExpense.getId())
-                .orElseThrow(() -> new IllegalArgumentException("Expense not found"));
+                .orElseThrow(() -> new ResourceNotFoundException("Expense not found with id: " + updatedExpense.getId()));
         existingExpense.setTitle(updatedExpense.getTitle());
         existingExpense.setAmount(updatedExpense.getAmount());
         existingExpense.setPersonalUseAmount(updatedExpense.getPersonalUseAmount());
@@ -191,7 +128,7 @@ public class ExpenseListService {
     // TODO implement API for this
     public XpenselyCustomCategory addCustomCategory(Long expenseListId, XpenselyCustomCategory customCategory) {
         ExpenseList expenseList = repository.findById(expenseListId)
-                .orElseThrow(() -> new RuntimeException("Expense List not found"));
+                .orElseThrow(() -> new ResourceNotFoundException("Expense List not found"));
         customCategory.setExpenseList(expenseList);
 
         return customCategoryRepository.save(customCategory);
@@ -200,9 +137,9 @@ public class ExpenseListService {
     // TODO implement API for this
     public void deleteCustomCategory(Long expenseListId, Long categoryId) {
         XpenselyCustomCategory category = customCategoryRepository.findById(categoryId)
-                .orElseThrow(() -> new RuntimeException("Custom Category not found"));
+                .orElseThrow(() -> new ResourceNotFoundException("Custom Category not found"));
         if (!category.getExpenseList().getId().equals(expenseListId)) {
-            throw new RuntimeException("Category does not belong to the specified Expense List");
+            throw new IllegalArgumentException("Category does not belong to the specified Expense List");
         }
         customCategoryRepository.delete(category);
     }

src/main/java/de/zendric/app/xpensely_server/services/UserService.java

@@ -1,11 +1,11 @@
 package de.zendric.app.xpensely_server.services;
 
 import java.util.List;
-import java.util.Optional;
 
 import org.springframework.stereotype.Service;
 
 import de.zendric.app.xpensely_server.model.AppUser;
+import de.zendric.app.xpensely_server.model.Exception.ResourceNotFoundException;
 import de.zendric.app.xpensely_server.model.Exception.UsernameAlreadyExistsException;
 import de.zendric.app.xpensely_server.repo.UserRepository;
 
@@ -29,36 +29,24 @@ public class UserService {
     }
 
     public AppUser getUser(Long id) {
-        Optional<AppUser> user = userRepository.findById(id);
+        return userRepository.findById(id)
-        if (user.isPresent()) {
+                .orElseThrow(() -> new ResourceNotFoundException("User not found with id: " + id));
-            return user.get();
-        } else
-            return null;
     }
 
     public AppUser deleteUserById(Long id) {
-        Optional<AppUser> user = userRepository.findById(id);
+        AppUser user = userRepository.findById(id)
-        if (user.isPresent()) {
+                .orElseThrow(() -> new ResourceNotFoundException("User not found with id: " + id));
-            userRepository.deleteById(id);
+        userRepository.deleteById(id);
-            return user.get();
+        return user;
-        } else
-            return null;
     }
 
     public AppUser getUserByName(String username) {
-        Optional<AppUser> optUser = userRepository.findByUsername(username);
+        return userRepository.findByUsername(username)
-        if (optUser.isPresent()) {
+                .orElseThrow(() -> new ResourceNotFoundException("User not found: " + username));
-            return optUser.get();
-        } else
-            return null;
     }
 
     public AppUser getUserByGoogleId(String id) {
-        Optional<AppUser> optUser = userRepository.findByGoogleId(id);
+        return userRepository.findByGoogleId(id)
-        if (optUser.isPresent()) {
+                .orElseThrow(() -> new ResourceNotFoundException("User not found with Google ID: " + id));
-            return optUser.get();
-        } else
-            return null;
     }
-
+}
-}

src/main/resources/application.properties

@@ -3,8 +3,6 @@ spring.application.name=XpenselyServer
 
 #Security
 spring.security.enabled=false
-spring.security.oauth2.client.registration.google.client-id=${GOOGLE_CLIENT_ID}
-spring.security.oauth2.client.registration.google.client-secret=${GOOGLE_CLIENT_SECRET}
 spring.security.oauth2.resourceserver.jwt.issuer-uri=https://accounts.google.com
 
 # PostgreSQL Configuration

src/test/java/de/zendric/app/xpensely_Server/ExpenseListRepositoryTest.java

@@ -1,26 +1,47 @@
 package de.zendric.app.xpensely_Server;
 
-import java.util.Optional;
-
-import org.junit.jupiter.api.Test;
-import org.springframework.beans.factory.annotation.Autowired;
-import org.springframework.boot.test.autoconfigure.orm.jpa.DataJpaTest;
-
 import de.zendric.app.xpensely_server.model.ExpenseList;
 import de.zendric.app.xpensely_server.repo.ExpenseListRepository;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.data.jpa.test.autoconfigure.DataJpaTest;
+
+import java.util.Optional;
+
+import static org.junit.jupiter.api.Assertions.*;
 
 @DataJpaTest
 class ExpenseListRepositoryTest {
 
-        @Autowired
+    @Autowired
-        private ExpenseListRepository expenseListRepository;
+    private ExpenseListRepository expenseListRepository;
 
-        @Test
+    @Test
-        void testFindExpenseListById() {
+    void saveAndFindById_returnsExpenseList() {
-                // Assuming an ExpenseList with id = 1 exists in your test DB.
+        ExpenseList list = new ExpenseList();
-                Optional<ExpenseList> optionalExpenseList = expenseListRepository.findById(1L);
+        list.setName("Groceries");
+        ExpenseList saved = expenseListRepository.save(list);
 
-                ExpenseList expenseList = optionalExpenseList.get();
+        Optional<ExpenseList> found = expenseListRepository.findById(saved.getId());
-                System.out.println("ExpenseList name: " + expenseList.getName());
+
-        }
+        assertTrue(found.isPresent());
+        assertEquals("Groceries", found.get().getName());
+    }
+
+    @Test
+    void findById_nonExistentId_returnsEmpty() {
+        Optional<ExpenseList> found = expenseListRepository.findById(999L);
+        assertTrue(found.isEmpty());
+    }
+
+    @Test
+    void delete_removesFromRepository() {
+        ExpenseList list = new ExpenseList();
+        list.setName("To Delete");
+        ExpenseList saved = expenseListRepository.save(list);
+
+        expenseListRepository.deleteById(saved.getId());
+
+        assertTrue(expenseListRepository.findById(saved.getId()).isEmpty());
+    }
 }

src/test/java/de/zendric/app/xpensely_Server/controller/AppUserControllerTest.java

@@ -0,0 +1,102 @@
+package de.zendric.app.xpensely_Server.controller;
+
+import de.zendric.app.xpensely_server.controller.AppUserController;
+import de.zendric.app.xpensely_server.model.AppUser;
+import de.zendric.app.xpensely_server.security.AuthenticatedUserResolver;
+import de.zendric.app.xpensely_server.services.UserService;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.security.oauth2.client.autoconfigure.OAuth2ClientAutoConfiguration;
+import org.springframework.boot.security.oauth2.client.autoconfigure.servlet.OAuth2ClientWebSecurityAutoConfiguration;
+import org.springframework.boot.security.oauth2.server.resource.autoconfigure.servlet.OAuth2ResourceServerAutoConfiguration;
+import org.springframework.boot.webmvc.test.autoconfigure.AutoConfigureMockMvc;
+import org.springframework.boot.webmvc.test.autoconfigure.WebMvcTest;
+import org.springframework.http.MediaType;
+import org.springframework.test.context.ActiveProfiles;
+import org.springframework.test.context.bean.override.mockito.MockitoBean;
+import org.springframework.test.web.servlet.MockMvc;
+
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.when;
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.*;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.*;
+
+@WebMvcTest(value = AppUserController.class, excludeAutoConfiguration = {
+        OAuth2ClientAutoConfiguration.class,
+        OAuth2ClientWebSecurityAutoConfiguration.class,
+        OAuth2ResourceServerAutoConfiguration.class
+})
+@AutoConfigureMockMvc(addFilters = false)
+@ActiveProfiles("test")
+class AppUserControllerTest {
+
+    @Autowired MockMvc mockMvc;
+    @MockitoBean UserService userService;
+    @MockitoBean AuthenticatedUserResolver authenticatedUserResolver;
+
+    @Test
+    void createUser_blankUsername_returns400WithFieldError() throws Exception {
+        mockMvc.perform(post("/api/users/createUser")
+                .contentType(MediaType.APPLICATION_JSON)
+                .content("{\"username\":\"\",\"googleId\":\"gid123\"}"))
+                .andExpect(status().isBadRequest())
+                .andExpect(jsonPath("$.username").exists());
+    }
+
+    @Test
+    void createUser_invalidUsernamePattern_returns400() throws Exception {
+        mockMvc.perform(post("/api/users/createUser")
+                .contentType(MediaType.APPLICATION_JSON)
+                .content("{\"username\":\"hello world!\",\"googleId\":\"gid123\"}"))
+                .andExpect(status().isBadRequest())
+                .andExpect(jsonPath("$.username").exists());
+    }
+
+    @Test
+    void createUser_usernameTooShort_returns400() throws Exception {
+        mockMvc.perform(post("/api/users/createUser")
+                .contentType(MediaType.APPLICATION_JSON)
+                .content("{\"username\":\"ab\",\"googleId\":\"gid123\"}"))
+                .andExpect(status().isBadRequest())
+                .andExpect(jsonPath("$.username").exists());
+    }
+
+    @Test
+    void createUser_blankGoogleId_returns400() throws Exception {
+        mockMvc.perform(post("/api/users/createUser")
+                .contentType(MediaType.APPLICATION_JSON)
+                .content("{\"username\":\"validuser\",\"googleId\":\"\"}"))
+                .andExpect(status().isBadRequest())
+                .andExpect(jsonPath("$.googleId").exists());
+    }
+
+    // --- Authorization tests ---
+
+    @Test
+    void getUser_differentUser_returns403() throws Exception {
+        AppUser self = new AppUser(); self.setId(1L);
+        when(authenticatedUserResolver.resolveCurrentUser(any())).thenReturn(self);
+
+        mockMvc.perform(get("/api/users").param("id", "99"))
+                .andExpect(status().isForbidden());
+    }
+
+    @Test
+    void getUser_sameUser_returns200() throws Exception {
+        AppUser self = new AppUser(); self.setId(1L);
+        when(authenticatedUserResolver.resolveCurrentUser(any())).thenReturn(self);
+        when(userService.getUser(1L)).thenReturn(self);
+
+        mockMvc.perform(get("/api/users").param("id", "1"))
+                .andExpect(status().isOk());
+    }
+
+    @Test
+    void deleteUser_differentUser_returns403() throws Exception {
+        AppUser self = new AppUser(); self.setId(1L);
+        when(authenticatedUserResolver.resolveCurrentUser(any())).thenReturn(self);
+
+        mockMvc.perform(delete("/api/users").param("id", "99"))
+                .andExpect(status().isForbidden());
+    }
+}

src/test/java/de/zendric/app/xpensely_Server/controller/ExpenseListControllerTest.java

@@ -0,0 +1,181 @@
+package de.zendric.app.xpensely_Server.controller;
+
+import de.zendric.app.xpensely_server.controller.ExpenseListController;
+import de.zendric.app.xpensely_server.model.AppUser;
+import de.zendric.app.xpensely_server.model.ExpenseList;
+import de.zendric.app.xpensely_server.security.AuthenticatedUserResolver;
+import de.zendric.app.xpensely_server.services.CategoryService;
+import de.zendric.app.xpensely_server.services.ExpenseListService;
+import de.zendric.app.xpensely_server.services.UserService;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.security.oauth2.client.autoconfigure.OAuth2ClientAutoConfiguration;
+import org.springframework.boot.security.oauth2.client.autoconfigure.servlet.OAuth2ClientWebSecurityAutoConfiguration;
+import org.springframework.boot.security.oauth2.server.resource.autoconfigure.servlet.OAuth2ResourceServerAutoConfiguration;
+import org.springframework.boot.webmvc.test.autoconfigure.AutoConfigureMockMvc;
+import org.springframework.boot.webmvc.test.autoconfigure.WebMvcTest;
+import org.springframework.http.MediaType;
+import org.springframework.test.context.ActiveProfiles;
+import org.springframework.test.context.bean.override.mockito.MockitoBean;
+import org.springframework.test.web.servlet.MockMvc;
+
+import java.util.List;
+import java.util.Optional;
+
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.Mockito.when;
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.*;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.*;
+
+@WebMvcTest(value = ExpenseListController.class, excludeAutoConfiguration = {
+        OAuth2ClientAutoConfiguration.class,
+        OAuth2ClientWebSecurityAutoConfiguration.class,
+        OAuth2ResourceServerAutoConfiguration.class
+})
+@AutoConfigureMockMvc(addFilters = false)
+@ActiveProfiles("test")
+class ExpenseListControllerTest {
+
+    @Autowired MockMvc mockMvc;
+    @MockitoBean ExpenseListService expenseListService;
+    @MockitoBean UserService userService;
+    @MockitoBean CategoryService categoryService;
+    @MockitoBean AuthenticatedUserResolver authenticatedUserResolver;
+
+    // --- Validation tests ---
+
+    @Test
+    void addExpense_blankTitle_returns400() throws Exception {
+        mockMvc.perform(post("/api/expenselist/1/add")
+                .contentType(MediaType.APPLICATION_JSON)
+                .content("{\"title\":\"\",\"owner\":\"alice\",\"amount\":10.0,\"date\":\"2026-05-04\",\"category\":\"Food\"}"))
+                .andExpect(status().isBadRequest())
+                .andExpect(jsonPath("$.title").exists());
+    }
+
+    @Test
+    void addExpense_negativeAmount_returns400() throws Exception {
+        mockMvc.perform(post("/api/expenselist/1/add")
+                .contentType(MediaType.APPLICATION_JSON)
+                .content("{\"title\":\"Lunch\",\"owner\":\"alice\",\"amount\":-5.0,\"date\":\"2026-05-04\",\"category\":\"Food\"}"))
+                .andExpect(status().isBadRequest())
+                .andExpect(jsonPath("$.amount").exists());
+    }
+
+    @Test
+    void addExpense_nullDate_returns400() throws Exception {
+        mockMvc.perform(post("/api/expenselist/1/add")
+                .contentType(MediaType.APPLICATION_JSON)
+                .content("{\"title\":\"Lunch\",\"owner\":\"alice\",\"amount\":10.0,\"category\":\"Food\"}"))
+                .andExpect(status().isBadRequest())
+                .andExpect(jsonPath("$.date").exists());
+    }
+
+    @Test
+    void acceptInvite_blankCode_returns400() throws Exception {
+        mockMvc.perform(post("/api/expenselist/accept-invite")
+                .contentType(MediaType.APPLICATION_JSON)
+                .content("{\"inviteCode\":\"\"}"))
+                .andExpect(status().isBadRequest())
+                .andExpect(jsonPath("$.inviteCode").exists());
+    }
+
+    @Test
+    void acceptInvite_wrongCodeLength_returns400() throws Exception {
+        mockMvc.perform(post("/api/expenselist/accept-invite")
+                .contentType(MediaType.APPLICATION_JSON)
+                .content("{\"inviteCode\":\"ABC\"}"))
+                .andExpect(status().isBadRequest())
+                .andExpect(jsonPath("$.inviteCode").exists());
+    }
+
+    // --- Authorization tests ---
+
+    @Test
+    void getById_authenticatedUserNotMember_returns403() throws Exception {
+        AppUser owner = new AppUser(); owner.setId(1L);
+        AppUser requester = new AppUser(); requester.setId(2L);
+        ExpenseList list = new ExpenseList(); list.setId(1L); list.setOwner(owner);
+
+        when(expenseListService.findById(1L)).thenReturn(Optional.of(list));
+        when(authenticatedUserResolver.resolveCurrentUser(any())).thenReturn(requester);
+
+        mockMvc.perform(get("/api/expenselist/byId").param("id", "1"))
+                .andExpect(status().isForbidden());
+    }
+
+    @Test
+    void getById_authenticatedUserIsOwner_returns200() throws Exception {
+        AppUser owner = new AppUser(); owner.setId(1L);
+        ExpenseList list = new ExpenseList(); list.setId(1L); list.setOwner(owner);
+
+        when(expenseListService.findById(1L)).thenReturn(Optional.of(list));
+        when(authenticatedUserResolver.resolveCurrentUser(any())).thenReturn(owner);
+
+        mockMvc.perform(get("/api/expenselist/byId").param("id", "1"))
+                .andExpect(status().isOk());
+    }
+
+    @Test
+    void deleteList_nonOwner_returns403() throws Exception {
+        AppUser owner = new AppUser(); owner.setId(1L);
+        AppUser nonOwner = new AppUser(); nonOwner.setId(2L);
+        ExpenseList list = new ExpenseList(); list.setId(5L); list.setOwner(owner);
+
+        when(expenseListService.findById(5L)).thenReturn(Optional.of(list));
+        when(authenticatedUserResolver.resolveCurrentUser(any())).thenReturn(nonOwner);
+
+        mockMvc.perform(delete("/api/expenselist/5"))
+                .andExpect(status().isForbidden());
+    }
+
+    @Test
+    void getMine_returnsCurrentUserLists() throws Exception {
+        AppUser user = new AppUser(); user.setId(3L);
+
+        when(authenticatedUserResolver.resolveCurrentUser(any())).thenReturn(user);
+        when(expenseListService.findByUserId(3L)).thenReturn(List.of(new ExpenseList()));
+
+        mockMvc.perform(get("/api/expenselist/mine"))
+                .andExpect(status().isOk());
+    }
+
+    @Test
+    void create_returns500_onUnexpectedServiceError() throws Exception {
+        AppUser user = new AppUser();
+        user.setId(1L);
+        when(authenticatedUserResolver.resolveCurrentUser(any())).thenReturn(user);
+        when(categoryService.getDefaultCategories()).thenThrow(new RuntimeException("db down"));
+
+        mockMvc.perform(post("/api/expenselist/create")
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content("{\"name\":\"Groceries\"}"))
+                .andExpect(status().isInternalServerError());
+    }
+
+    @Test
+    void create_returns400_whenNameIsBlank() throws Exception {
+        AppUser user = new AppUser();
+        user.setId(1L);
+        when(authenticatedUserResolver.resolveCurrentUser(any())).thenReturn(user);
+
+        mockMvc.perform(post("/api/expenselist/create")
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content("{\"name\":\"\"}"))
+                .andExpect(status().isBadRequest())
+                .andExpect(jsonPath("$.name").exists());
+    }
+
+    @Test
+    void create_returns400_whenBodyIsEmpty() throws Exception {
+        AppUser user = new AppUser();
+        user.setId(1L);
+        when(authenticatedUserResolver.resolveCurrentUser(any())).thenReturn(user);
+
+        mockMvc.perform(post("/api/expenselist/create")
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content("{}"))
+                .andExpect(status().isBadRequest())
+                .andExpect(jsonPath("$.name").exists());
+    }
+}

src/test/java/de/zendric/app/xpensely_Server/security/AuthenticatedUserResolverTest.java

@@ -0,0 +1,78 @@
+package de.zendric.app.xpensely_Server.security;
+
+import de.zendric.app.xpensely_server.model.AppUser;
+import de.zendric.app.xpensely_server.security.AuthenticatedUserResolver;
+import de.zendric.app.xpensely_server.services.UserService;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.springframework.http.HttpStatus;
+import org.springframework.security.oauth2.jwt.Jwt;
+import org.springframework.security.oauth2.server.resource.authentication.JwtAuthenticationToken;
+import org.springframework.web.server.ResponseStatusException;
+
+import static org.junit.jupiter.api.Assertions.*;
+import static org.mockito.Mockito.*;
+
+class AuthenticatedUserResolverTest {
+
+    UserService userService;
+    AuthenticatedUserResolver resolver;
+
+    @BeforeEach
+    void setUp() {
+        userService = mock(UserService.class);
+        resolver = new AuthenticatedUserResolver(userService);
+    }
+
+    @Test
+    void resolveCurrentUser_validJwt_returnsAppUser() {
+        Jwt jwt = Jwt.withTokenValue("token")
+                .header("alg", "RS256")
+                .subject("google-id-123")
+                .build();
+        JwtAuthenticationToken auth = new JwtAuthenticationToken(jwt);
+
+        AppUser user = new AppUser();
+        user.setId(1L);
+        user.setGoogleId("google-id-123");
+        when(userService.getUserByGoogleId("google-id-123")).thenReturn(user);
+
+        AppUser result = resolver.resolveCurrentUser(auth);
+        assertEquals(user, result);
+    }
+
+    @Test
+    void resolveCurrentUser_userNotFound_throws403() {
+        Jwt jwt = Jwt.withTokenValue("token")
+                .header("alg", "RS256")
+                .subject("unknown-id")
+                .build();
+        JwtAuthenticationToken auth = new JwtAuthenticationToken(jwt);
+        when(userService.getUserByGoogleId("unknown-id")).thenReturn(null);
+
+        ResponseStatusException ex = assertThrows(ResponseStatusException.class,
+                () -> resolver.resolveCurrentUser(auth));
+        assertEquals(HttpStatus.FORBIDDEN, ex.getStatusCode());
+    }
+
+    @Test
+    void resolveCurrentUser_userServiceThrows_throws403() {
+        Jwt jwt = Jwt.withTokenValue("token")
+                .header("alg", "RS256")
+                .subject("gone-id")
+                .build();
+        JwtAuthenticationToken auth = new JwtAuthenticationToken(jwt);
+        when(userService.getUserByGoogleId("gone-id")).thenThrow(new IllegalArgumentException("not found"));
+
+        ResponseStatusException ex = assertThrows(ResponseStatusException.class,
+                () -> resolver.resolveCurrentUser(auth));
+        assertEquals(HttpStatus.FORBIDDEN, ex.getStatusCode());
+    }
+
+    @Test
+    void resolveCurrentUser_nullAuthentication_throws403() {
+        ResponseStatusException ex = assertThrows(ResponseStatusException.class,
+                () -> resolver.resolveCurrentUser(null));
+        assertEquals(HttpStatus.FORBIDDEN, ex.getStatusCode());
+    }
+}

src/test/java/de/zendric/app/xpensely_Server/security/RateLimitFilterTest.java

@@ -0,0 +1,89 @@
+package de.zendric.app.xpensely_Server.security;
+
+import de.zendric.app.xpensely_server.security.RateLimitFilter;
+import jakarta.servlet.FilterChain;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+import org.springframework.mock.web.MockHttpServletRequest;
+import org.springframework.mock.web.MockHttpServletResponse;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.mockito.Mockito.*;
+
+class RateLimitFilterTest {
+
+    RateLimitFilter filter;
+    FilterChain chain;
+
+    @BeforeEach
+    void setUp() {
+        filter = new RateLimitFilter();
+        chain = mock(FilterChain.class);
+    }
+
+    @Test
+    void allowsRequestUnderLimit() throws Exception {
+        MockHttpServletRequest request = new MockHttpServletRequest();
+        request.setRemoteAddr("1.2.3.4");
+        MockHttpServletResponse response = new MockHttpServletResponse();
+
+        filter.doFilter(request, response, chain);
+
+        verify(chain, times(1)).doFilter(request, response);
+        assertEquals(200, response.getStatus());
+    }
+
+    @Test
+    void blocksRequestOverLimit() throws Exception {
+        MockHttpServletRequest request = new MockHttpServletRequest();
+        request.setRemoteAddr("5.6.7.8");
+
+        for (int i = 0; i < 60; i++) {
+            filter.doFilter(request, new MockHttpServletResponse(), chain);
+        }
+
+        MockHttpServletResponse blockedResponse = new MockHttpServletResponse();
+        filter.doFilter(request, blockedResponse, chain);
+
+        assertEquals(429, blockedResponse.getStatus());
+        verify(chain, times(60)).doFilter(eq(request), any());
+    }
+
+    @Test
+    void differentIpsBucketedSeparately() throws Exception {
+        MockHttpServletRequest req1 = new MockHttpServletRequest();
+        req1.setRemoteAddr("10.0.0.1");
+        MockHttpServletRequest req2 = new MockHttpServletRequest();
+        req2.setRemoteAddr("10.0.0.2");
+
+        for (int i = 0; i < 60; i++) {
+            filter.doFilter(req1, new MockHttpServletResponse(), chain);
+        }
+
+        MockHttpServletResponse response2 = new MockHttpServletResponse();
+        filter.doFilter(req2, response2, chain);
+
+        assertEquals(200, response2.getStatus());
+    }
+
+    @Test
+    void prefersXForwardedForHeader() throws Exception {
+        MockHttpServletRequest request = new MockHttpServletRequest();
+        request.setRemoteAddr("192.168.1.1");
+        request.addHeader("X-Forwarded-For", "203.0.113.5, 10.0.0.1");
+
+        for (int i = 0; i < 60; i++) {
+            filter.doFilter(request, new MockHttpServletResponse(), chain);
+        }
+
+        MockHttpServletResponse blocked = new MockHttpServletResponse();
+        filter.doFilter(request, blocked, chain);
+        assertEquals(429, blocked.getStatus());
+
+        MockHttpServletRequest directRequest = new MockHttpServletRequest();
+        directRequest.setRemoteAddr("192.168.1.1");
+        MockHttpServletResponse directResponse = new MockHttpServletResponse();
+        filter.doFilter(directRequest, directResponse, chain);
+        assertEquals(200, directResponse.getStatus());
+    }
+}

src/test/java/de/zendric/app/xpensely_Server/services/ExpenseListServiceTest.java

@@ -0,0 +1,57 @@
+package de.zendric.app.xpensely_Server.services;
+
+import de.zendric.app.xpensely_server.model.AppUser;
+import de.zendric.app.xpensely_server.model.ExpenseList;
+import de.zendric.app.xpensely_server.repo.ExpenseListRepository;
+import de.zendric.app.xpensely_server.repo.ExpenseRepository;
+import de.zendric.app.xpensely_server.repo.XpenselyCustomCategoryRepository;
+import de.zendric.app.xpensely_server.services.ExpenseListService;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.InjectMocks;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+import java.util.List;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+import static org.mockito.Mockito.never;
+
+@ExtendWith(MockitoExtension.class)
+class ExpenseListServiceTest {
+
+    @Mock ExpenseListRepository repository;
+    @Mock ExpenseRepository expenseRepository;
+    @Mock XpenselyCustomCategoryRepository customCategoryRepository;
+
+    @InjectMocks
+    ExpenseListService service;
+
+    @Test
+    void findByUserId_usesRepositoryQuery_notFindAll() {
+        AppUser owner = new AppUser(); owner.setId(1L);
+        ExpenseList list = new ExpenseList(); list.setId(10L); list.setOwner(owner);
+        when(repository.findByOwnerIdOrSharedWithId(1L)).thenReturn(List.of(list));
+
+        List<ExpenseList> result = service.findByUserId(1L);
+
+        assertThat(result).hasSize(1);
+        verify(repository).findByOwnerIdOrSharedWithId(1L);
+        verify(repository, never()).findAll();
+    }
+
+    @Test
+    void findByUsername_usesRepositoryQuery_notFindAll() {
+        AppUser owner = new AppUser(); owner.setId(1L); owner.setUsername("alice");
+        ExpenseList list = new ExpenseList(); list.setId(10L); list.setOwner(owner);
+        when(repository.findByOwnerUsernameOrSharedWithUsername("alice")).thenReturn(List.of(list));
+
+        List<ExpenseList> result = service.findByUsername("alice");
+
+        assertThat(result).hasSize(1);
+        verify(repository).findByOwnerUsernameOrSharedWithUsername("alice");
+        verify(repository, never()).findAll();
+    }
+}

src/test/java/de/zendric/app/xpensely_Server/services/UserServiceTest.java

@@ -0,0 +1,75 @@
+package de.zendric.app.xpensely_Server.services;
+
+import de.zendric.app.xpensely_server.model.AppUser;
+import de.zendric.app.xpensely_server.model.Exception.ResourceNotFoundException;
+import de.zendric.app.xpensely_server.repo.UserRepository;
+import de.zendric.app.xpensely_server.services.UserService;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.InjectMocks;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+import java.util.Optional;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+import static org.mockito.Mockito.when;
+
+@ExtendWith(MockitoExtension.class)
+class UserServiceTest {
+
+    @Mock
+    UserRepository userRepository;
+
+    @InjectMocks
+    UserService userService;
+
+    @Test
+    void getUserByName_throwsResourceNotFound_whenUserMissing() {
+        when(userRepository.findByUsername("ghost")).thenReturn(Optional.empty());
+
+        assertThatThrownBy(() -> userService.getUserByName("ghost"))
+                .isInstanceOf(ResourceNotFoundException.class)
+                .hasMessageContaining("ghost");
+    }
+
+    @Test
+    void getUserByName_returnsUser_whenFound() {
+        AppUser user = new AppUser();
+        user.setId(1L);
+        user.setUsername("alice");
+        when(userRepository.findByUsername("alice")).thenReturn(Optional.of(user));
+
+        AppUser result = userService.getUserByName("alice");
+
+        assertThat(result.getUsername()).isEqualTo("alice");
+    }
+
+    @Test
+    void getUser_throwsResourceNotFound_whenIdMissing() {
+        when(userRepository.findById(99L)).thenReturn(Optional.empty());
+
+        assertThatThrownBy(() -> userService.getUser(99L))
+                .isInstanceOf(ResourceNotFoundException.class)
+                .hasMessageContaining("99");
+    }
+
+    @Test
+    void deleteUserById_throwsResourceNotFound_whenIdMissing() {
+        when(userRepository.findById(99L)).thenReturn(Optional.empty());
+
+        assertThatThrownBy(() -> userService.deleteUserById(99L))
+                .isInstanceOf(ResourceNotFoundException.class)
+                .hasMessageContaining("99");
+    }
+
+    @Test
+    void getUserByGoogleId_throwsResourceNotFound_whenMissing() {
+        when(userRepository.findByGoogleId("gid-404")).thenReturn(Optional.empty());
+
+        assertThatThrownBy(() -> userService.getUserByGoogleId("gid-404"))
+                .isInstanceOf(ResourceNotFoundException.class)
+                .hasMessageContaining("gid-404");
+    }
+}