ci: improve dev pipeline with Dokploy webhook and Docker layer caching

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>

.gitea/workflows/build.yml

@@ -12,43 +12,45 @@ jobs:
     steps:
       # 1. Checkout the code
       - name: Checkout code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v4
 
-      # 2. Set up Java and Maven
+      # 2. Set up Java and Maven (includes Maven, no separate install needed)
       - name: Set up JDK (Eclipse Temurin)
-        uses: actions/setup-java@v3
+        uses: actions/setup-java@v4
         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
+      # 3. Build the Spring Boot application
       - name: Build Spring Boot Application
-        run: |
-          mvn clean package -DskipTests
+        run: mvn clean package -DskipTests
 
-      # 5. Set up Docker
-      - name: Set up Docker
-        run: |
-          docker --version
+      # 4. Set up Docker Buildx (enables layer caching)
+      - name: Set up Docker Buildx
+        uses: docker/setup-buildx-action@v3
 
-      # 6. Build the Docker image
-      - name: Build and Package Docker Image
-        run: |
-          docker build -t tea.zendric.de/cedric/xpensely-server:latest .
-
-      # 7. Docker login
+      # 5. Docker login
       - name: Login to Docker Registry
+        uses: docker/login-action@v3
+        with:
+          registry: tea.zendric.de
+          username: ${{ secrets.TEAUSER }}
+          password: ${{ secrets.TEAPASSWORD }}
+
+      # 6. Build and push Docker image with layer caching
+      - name: Build and Push Docker Image
+        uses: docker/build-push-action@v5
+        with:
+          context: .
+          push: true
+          tags: tea.zendric.de/cedric/xpensely-server:latest
+          cache-from: type=registry,ref=tea.zendric.de/cedric/xpensely-server:buildcache
+          cache-to: type=registry,ref=tea.zendric.de/cedric/xpensely-server:buildcache,mode=max
+
+      # 7. Trigger Dokploy to redeploy the dev server automatically
+      - name: Trigger Dokploy Redeploy
         run: |
-          echo "${{ secrets.TEAPASSWORD }}" | docker login tea.zendric.de -u ${{ secrets.TEAUSER }} --password-stdin
-      # 8. Push Docker image
-      - name: Push the Docker Image to registry
-        run: |
-          docker push tea.zendric.de/cedric/xpensely-server:latest
+          curl -X POST "${{ secrets.DOKPLOY_WEBHOOK_URL }}" \
+            -H "Authorization: Bearer ${{ secrets.DOKPLOY_TOKEN }}" \
+            --fail

.gitignore

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

docs/API.md

@@ -0,0 +1,704 @@
+# Xpensely Server — API Reference
+
+> Last updated: 2026-05-09 · Branch: `feature/security-hardening`
+
+## 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"` |
+| 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
+```
+
+---
+
+### 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:** `200 OK` — 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 |
+|--------|-----------|
+| 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 |
+|--------|-----------|
+| 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 the deleted [AppUser](#appuser).
+
+**Error responses:**
+| Status | Condition |
+|--------|-----------|
+| 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 response:** `200 OK` — array of [ExpenseList](#expenselist).
+
+```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:** `200 OK` — 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:** `200 OK` — returns the deleted [Expense](#expense).
+
+**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 the [ExpenseList](#expenselist) the caller joined.
+
+**Error responses:**
+| Status | Condition |
+|--------|-----------|
+| 400 | Validation failure or invite code not found / expired |
+| 403 | Caller is already the owner of this list |
+
+---
+
+## 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.

docs/superpowers/specs/2026-05-04-security-hardening-design.md

@@ -1,166 +0,0 @@
-# Security Hardening Design — XpenselyServer
-**Date:** 2026-05-04  
-**Scope:** Input validation, authorization enforcement, rate limiting
-
----
-
-## Problem Statement
-
-The XpenselyServer has three security gaps:
-
-1. **No input validation** — request bodies are accepted without any field constraints, allowing null fields, negative amounts, oversized strings, and malformed data to reach the database layer.
-2. **Authorization bypass** — every endpoint trusts caller-supplied user IDs from query params or request bodies rather than the authenticated JWT. Any authenticated user can read or destroy another user's expense lists.
-3. **No rate limiting** — no protection against brute-force or abuse of sensitive endpoints (invite generation, account creation, invite acceptance).
-
----
-
-## Section 1 — Input Validation
-
-### Dependency
-Add `spring-boot-starter-validation` to `pom.xml`.
-
-### Request Model Constraints
-
-**`AppUserCreateRequest`**
-- `username`: `@NotBlank @Size(min=3, max=30) @Pattern(regexp="^[a-zA-Z0-9_.-]+$")`
-- `googleId`: `@NotBlank`
-
-**`ExpenseInput`**
-- `title`: `@NotBlank @Size(max=100)`
-- `owner`: `@NotBlank`
-- `amount`: `@NotNull @DecimalMin("0.01")`
-- `date`: `@NotNull`
-- `category`: `@NotBlank`
-
-**`ExpenseChangeRequest`** — same constraints as `ExpenseInput` for the corresponding fields.
-
-**`InviteRequest`**
-- `inviteCode`: `@NotBlank @Size(min=6, max=6)`
-- `userId` field removed (derived from JWT — see Section 2)
-
-### Controller Changes
-Add `@Valid` to every `@RequestBody` parameter in `AppUserController` and `ExpenseListController`.
-
-### Error Handling
-Add a `@ControllerAdvice` class `GlobalExceptionHandler` that:
-- Catches `MethodArgumentNotValidException` → returns `400 Bad Request` with a map of `{ field: errorMessage }` pairs
-- Catches `IllegalArgumentException` → returns `400 Bad Request` with the exception message
-
-This replaces the current pattern of returning `500 INTERNAL_SERVER_ERROR` or `417 EXPECTATION_FAILED` for validation failures.
-
-### Cleanup
-Remove the stray `@Id` and `@GeneratedValue` JPA annotations from `ExpenseInput` — it is a DTO, not an entity.
-
----
-
-## Section 2 — Authorization Model
-
-### Core Principle
-Stop trusting caller-supplied user IDs. Derive the authenticated user from the JWT on every request.
-
-### New Component: `AuthenticatedUserResolver`
-A `@Component` with a single method:
-```java
-AppUser resolveCurrentUser(Authentication auth)
-```
-- Extracts the `sub` claim (Google ID) from the JWT
-- Calls `UserService.getUserByGoogleId(sub)` to return the `AppUser`
-- Throws `ResponseStatusException(403)` if no user is found for the JWT subject
-
-### Endpoint Changes
-
-| Endpoint | Change |
-|---|---|
-| `GET /api/expenselist/all` | **Removed** — no legitimate non-admin use case |
-| `GET /api/expenselist/byUser?userId=X` | **Replaced** by `GET /api/expenselist/mine` — returns lists for the JWT user, no param |
-| `GET /api/expenselist/byUsername?username=X` | **Removed** — redundant with `/mine` |
-| `GET /api/expenselist/byId?id=X` | **Guard added** — 403 if authenticated user is neither owner nor sharedWith |
-| `DELETE /api/expenselist/{id}` | **Guard added** — 403 if authenticated user is not the owner |
-| `POST /api/expenselist/{id}/add` | **Guard added** — 403 if authenticated user is not owner or sharedWith |
-| `PUT /api/expenselist/{id}/update` | **Guard added** — 403 if authenticated user is not owner or sharedWith |
-| `DELETE /api/expenselist/{id}/delete` | **Guard added** — 403 if authenticated user is not owner or sharedWith |
-| `POST /api/expenselist/{listId}/invite` | **Guard added** — 403 if authenticated user is not the owner |
-| `POST /api/expenselist/accept-invite` | **`userId` removed from body** — derived from JWT instead |
-| `GET /api/users?id=X` | **Guard added** — 403 if id doesn't match JWT user's id |
-| `GET /api/users/byGoogleId?id=X` | **Guard added** — 403 if id doesn't match JWT sub |
-| `DELETE /api/users?id=X` | **Guard added** — 403 if id doesn't match JWT user's id |
-
-### Ownership Check Helper
-Each guard is implemented as a private method in its controller (2–3 lines):
-```java
-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);
-}
-```
-
-### Test Profile
-The `@Profile("test")` security chain in `SecurityConfig` is untouched. Existing tests continue to work without authentication.
-
----
-
-## Section 3 — Rate Limiting
-
-### Dependency
-Add `bucket4j-core` to `pom.xml`. In-memory storage — no external cache needed for single-instance deployment.
-
-### Implementation
-A `RateLimitFilter` extending `OncePerRequestFilter`, registered as a `@Component` with `@Profile("!test")`:
-
-- **Key for authenticated requests:** JWT `sub` claim (per-user bucket)
-- **Key for unauthenticated requests:** remote IP address (pre-auth fallback)
-- Buckets stored in a `ConcurrentHashMap<String, Bucket>`
-
-### Limits
-
-| Endpoint pattern | Limit |
-|---|---|
-| All endpoints (default) | 60 requests / minute |
-| `POST /api/expenselist/*/invite` | 5 requests / minute |
-| `POST /api/expenselist/accept-invite` | 10 requests / minute |
-| `POST /api/users/createUser` | 3 requests / minute |
-
-Sensitive endpoints get their own per-user bucket independent of the general bucket.
-
-### Response
-When a bucket is exhausted: `429 Too Many Requests` with a `Retry-After: <seconds>` header indicating time until refill.
-
----
-
-## Architecture Summary
-
-```
-Request
-  └── RateLimitFilter (per-user/IP buckets)
-        └── SecurityFilterChain (JWT validation)
-              └── Controller
-                    ├── @Valid on @RequestBody → GlobalExceptionHandler on failure
-                    ├── AuthenticatedUserResolver → AppUser from JWT sub
-                    └── assertOwner / assertMember → 403 on violation
-```
-
-No new service layer is introduced. The authorization checks are lightweight and local to each controller method.
-
----
-
-## Files Affected
-
-| File | Change |
-|---|---|
-| `pom.xml` | Add `spring-boot-starter-validation`, `bucket4j-core` |
-| `model/AppUserCreateRequest.java` | Add validation annotations |
-| `model/ExpenseInput.java` | Add validation annotations, remove JPA annotations |
-| `model/ExpenseChangeRequest.java` | Add validation annotations |
-| `model/InviteRequest.java` | Add validation annotations, remove `userId` field |
-| `controller/AppUserController.java` | Add `@Valid`, ownership guards, use `AuthenticatedUserResolver` |
-| `controller/ExpenseListController.java` | Add `@Valid`, ownership guards, remove/rename endpoints, use `AuthenticatedUserResolver` |
-| `security/AuthenticatedUserResolver.java` | **New** — resolves JWT sub to `AppUser` |
-| `security/RateLimitFilter.java` | **New** — per-user/IP rate limiting |
-| `controller/GlobalExceptionHandler.java` | **New** — structured 400/403 error responses |

pom.xml

@@ -1,119 +1,135 @@
 <?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">
-	<modelVersion>4.0.0</modelVersion>
-	<parent>
-		<groupId>org.springframework.boot</groupId>
-		<artifactId>spring-boot-starter-parent</artifactId>
-		<version>3.4.1</version>
-		<relativePath/> <!-- lookup parent from repository -->
-	</parent>
-	<groupId>de.zendric.app</groupId>
-	<artifactId>XpenselyServer</artifactId>
-	<version>1.0.0</version>
-	<name>XpenselyServer</name>
-	<description>XpenselyServer used to handle the Xpensely App</description>
-	<url/>
-	<licenses>
-		<license/>
-	</licenses>
-	<developers>
-		<developer/>
-	</developers>
-	<scm>
-		<connection/>
-		<developerConnection/>
-		<tag/>
-		<url/>
-	</scm>
-	<properties>
-		<java.version>17</java.version>
-	</properties>
-	<dependencies>
-		<dependency>
-			<groupId>org.springframework.boot</groupId>
-			<artifactId>spring-boot-starter-data-jpa</artifactId>
-		</dependency>
-		<dependency>
-			<groupId>org.springframework.boot</groupId>
-			<artifactId>spring-boot-starter-security</artifactId>
-		</dependency>
-		<dependency>
-			<groupId>org.springframework.boot</groupId>
-			<artifactId>spring-boot-starter-validation</artifactId>
-		</dependency>
-		<dependency>
-			<groupId>com.bucket4j</groupId>
-			<artifactId>bucket4j-core</artifactId>
-			<version>8.10.1</version>
-		</dependency>
-		<dependency>
-        	<groupId>org.springframework.boot</groupId>
-        	<artifactId>spring-boot-starter-oauth2-resource-server</artifactId>
-    	</dependency>
-			<dependency>
-			<groupId>org.springframework.boot</groupId>
-			<artifactId>spring-boot-starter-oauth2-client</artifactId>
-		</dependency>
-		<dependency>
-			<groupId>org.springframework.boot</groupId>
-			<artifactId>spring-boot-starter-web</artifactId>
-		</dependency>
-		<dependency>
-			<groupId>org.springframework.boot</groupId>
-			<artifactId>spring-boot-devtools</artifactId>
-			<scope>runtime</scope>
-			<optional>true</optional>
-		</dependency>
-		<dependency>
-			<groupId>org.postgresql</groupId>
-			<artifactId>postgresql</artifactId>
-			<scope>runtime</scope>
-		</dependency>
-		<dependency>
-			<groupId>org.projectlombok</groupId>
-			<artifactId>lombok</artifactId>
-			<optional>true</optional>
-		</dependency>
-		<dependency>
-			<groupId>org.springframework.boot</groupId>
-			<artifactId>spring-boot-starter-test</artifactId>
-			<scope>test</scope>
-		</dependency>
-		<dependency>
-			<groupId>org.springframework.security</groupId>
-			<artifactId>spring-security-test</artifactId>
-			<scope>test</scope>
-		</dependency>
-	</dependencies>
+         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>
+    <parent>
+        <groupId>org.springframework.boot</groupId>
+        <artifactId>spring-boot-starter-parent</artifactId>
+        <version>4.0.6</version>
+        <relativePath/> <!-- lookup parent from repository -->
+    </parent>
+    <groupId>de.zendric.app</groupId>
+    <artifactId>XpenselyServer</artifactId>
+    <version>1.0.0</version>
+    <name>XpenselyServer</name>
+    <description>XpenselyServer used to handle the Xpensely App</description>
+    <url/>
+    <licenses>
+        <license/>
+    </licenses>
+    <developers>
+        <developer/>
+    </developers>
+    <scm>
+        <connection/>
+        <developerConnection/>
+        <tag/>
+        <url/>
+    </scm>
+    <properties>
+        <java.version>21</java.version>
+        <lombok.version>1.18.46</lombok.version>
+    </properties>
+    <dependencies>
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-data-jpa</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-security</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-validation</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>com.bucket4j</groupId>
+            <artifactId>bucket4j-core</artifactId>
+            <version>8.10.1</version>
+        </dependency>
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-oauth2-resource-server</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-oauth2-client</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-web</artifactId>
+        </dependency>
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-devtools</artifactId>
+            <scope>runtime</scope>
+            <optional>true</optional>
+        </dependency>
+        <dependency>
+            <groupId>org.postgresql</groupId>
+            <artifactId>postgresql</artifactId>
+            <scope>runtime</scope>
+        </dependency>
+        <dependency>
+            <groupId>org.projectlombok</groupId>
+            <artifactId>lombok</artifactId>
+            <optional>true</optional>
+        </dependency>
+        <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>
-		<plugins>
-			<plugin>
-				<groupId>org.apache.maven.plugins</groupId>
-				<artifactId>maven-compiler-plugin</artifactId>
-				<configuration>
-					<annotationProcessorPaths>
-						<path>
-							<groupId>org.projectlombok</groupId>
-							<artifactId>lombok</artifactId>
-						</path>
-					</annotationProcessorPaths>
-				</configuration>
-			</plugin>
-			<plugin>
-				<groupId>org.springframework.boot</groupId>
-				<artifactId>spring-boot-maven-plugin</artifactId>
-				<configuration>
-					<excludes>
-						<exclude>
-							<groupId>org.projectlombok</groupId>
-							<artifactId>lombok</artifactId>
-						</exclude>
-					</excludes>
-				</configuration>
-			</plugin>
-		</plugins>
-	</build>
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-compiler-plugin</artifactId>
+                <configuration>
+                    <annotationProcessorPaths>
+                        <path>
+                            <groupId>org.projectlombok</groupId>
+                            <artifactId>lombok</artifactId>
+                        </path>
+                    </annotationProcessorPaths>
+                </configuration>
+            </plugin>
+            <plugin>
+                <groupId>org.springframework.boot</groupId>
+                <artifactId>spring-boot-maven-plugin</artifactId>
+                <configuration>
+                    <excludes>
+                        <exclude>
+                            <groupId>org.projectlombok</groupId>
+                            <artifactId>lombok</artifactId>
+                        </exclude>
+                    </excludes>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
 
 </project>

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

@@ -9,7 +9,6 @@ import org.springframework.web.server.ResponseStatusException;
 
 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;
 
@@ -47,15 +46,9 @@ public class AppUserController {
 
     @PostMapping("/createUser")
     public ResponseEntity<AppUser> createUser(@RequestBody @Valid AppUserCreateRequest userRequest) {
-        try {
-            AppUser convertedUser = userRequest.convertToAppUser();
-            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);
-        }
+        AppUser convertedUser = userRequest.convertToAppUser();
+        AppUser nUser = userService.createUser(convertedUser);
+        return new ResponseEntity<>(nUser, HttpStatus.CREATED);
     }
 
     @DeleteMapping

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

@@ -5,6 +5,8 @@ import java.util.List;
 import java.util.Optional;
 
 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.security.core.Authentication;
@@ -21,6 +23,8 @@ import de.zendric.app.xpensely_server.services.UserService;
 @RequestMapping("/api/expenselist")
 public class ExpenseListController {
 
+    private static final Logger log = LoggerFactory.getLogger(ExpenseListController.class);
+
     private final ExpenseListService expenseListService;
     private final UserService userService;
     private final CategoryService categoryService;
@@ -54,22 +58,18 @@ public class ExpenseListController {
     }
 
     @PostMapping("/create")
-    public ResponseEntity<ExpenseList> create(@RequestBody ExpenseList expenseList,
+    public ResponseEntity<ExpenseList> create(@RequestBody @Valid CreateExpenseListRequest request,
             Authentication authentication) {
-        try {
-            AppUser authenticatedUser = authenticatedUserResolver.resolveCurrentUser(authentication);
-            expenseList.setOwner(authenticatedUser);
-            XpenselyStandardCategories standardCategories = categoryService.getDefaultCategories();
-            expenseList.setXpenselyStandardCategories(standardCategories);
-            expenseList.setSharedWith(null);
-            ExpenseList savedItem = expenseListService.createList(expenseList);
-            return new ResponseEntity<>(savedItem, HttpStatus.CREATED);
-        } catch (ResponseStatusException e) {
-            throw e;
-        } catch (Exception e) {
-            e.printStackTrace();
-            return new ResponseEntity<>(null, HttpStatus.EXPECTATION_FAILED);
-        }
+        AppUser authenticatedUser = authenticatedUserResolver.resolveCurrentUser(authentication);
+        ExpenseList expenseList = new ExpenseList();
+        expenseList.setName(request.getName());
+        expenseList.setOwner(authenticatedUser);
+        XpenselyStandardCategories standardCategories = categoryService.getDefaultCategories();
+        expenseList.setXpenselyStandardCategories(standardCategories);
+        expenseList.setSharedWith(null);
+        ExpenseList savedItem = expenseListService.createList(expenseList);
+        log.debug("Created expense list '{}' for user {}", savedItem.getName(), authenticatedUser.getId());
+        return new ResponseEntity<>(savedItem, HttpStatus.CREATED);
     }
 
     @DeleteMapping("{id}")
@@ -79,12 +79,8 @@ public class ExpenseListController {
         if (listOpt.isEmpty())
             return new ResponseEntity<>(HttpStatus.NOT_FOUND);
         assertOwner(user, listOpt.get());
-        try {
-            expenseListService.deleteById(id);
-            return new ResponseEntity<>(HttpStatus.NO_CONTENT);
-        } catch (Exception e) {
-            return new ResponseEntity<>(HttpStatus.EXPECTATION_FAILED);
-        }
+        expenseListService.deleteById(id);
+        return new ResponseEntity<>(HttpStatus.NO_CONTENT);
     }
 
     @PostMapping("/{id}/add")
@@ -97,14 +93,10 @@ public class ExpenseListController {
         if (listOpt.isEmpty())
             return new ResponseEntity<>(HttpStatus.NOT_FOUND);
         assertMember(user, listOpt.get());
-        try {
-            AppUser expenseOwner = userService.getUserByName(expenseInput.getOwner());
-            Expense expense = expenseInput.convertToExpense(expenseOwner.getId());
-            Expense addedExpense = expenseListService.addExpenseToList(expenseListId, expense);
-            return new ResponseEntity<>(addedExpense, HttpStatus.CREATED);
-        } catch (Exception e) {
-            return new ResponseEntity<>(null, HttpStatus.INTERNAL_SERVER_ERROR);
-        }
+        AppUser expenseOwner = userService.getUserByName(expenseInput.getOwner());
+        Expense expense = expenseInput.convertToExpense(expenseOwner.getId());
+        Expense addedExpense = expenseListService.addExpenseToList(expenseListId, expense);
+        return new ResponseEntity<>(addedExpense, HttpStatus.CREATED);
     }
 
     @PutMapping("/{id}/update")
@@ -115,16 +107,12 @@ public class ExpenseListController {
         AppUser user = authenticatedUserResolver.resolveCurrentUser(authentication);
         Optional<ExpenseList> expenseListOpt = expenseListService.findById(expenseListId);
         if (expenseListOpt.isEmpty())
-            return new ResponseEntity<>(null, HttpStatus.NOT_FOUND);
+            return new ResponseEntity<>(HttpStatus.NOT_FOUND);
         assertMember(user, expenseListOpt.get());
-        try {
-            AppUser expenseOwner = userService.getUserByName(expenseChangeRequest.getOwnerName());
-            Expense expense = expenseChangeRequest.convertToExpense(expenseOwner.getId(), expenseListOpt.get());
-            Expense updatedExpense = expenseListService.updateExpense(expenseListId, expense);
-            return new ResponseEntity<>(updatedExpense, HttpStatus.OK);
-        } catch (Exception e) {
-            return new ResponseEntity<>(null, HttpStatus.BAD_REQUEST);
-        }
+        AppUser expenseOwner = userService.getUserByName(expenseChangeRequest.getOwnerName());
+        Expense expense = expenseChangeRequest.convertToExpense(expenseOwner.getId(), expenseListOpt.get());
+        Expense updatedExpense = expenseListService.updateExpense(expenseListId, expense);
+        return new ResponseEntity<>(updatedExpense, HttpStatus.OK);
     }
 
     @DeleteMapping("/{id}/delete")
@@ -137,12 +125,8 @@ public class ExpenseListController {
         if (listOpt.isEmpty())
             return new ResponseEntity<>(HttpStatus.NOT_FOUND);
         assertMember(user, listOpt.get());
-        try {
-            expenseListService.deleteExpenseFromList(expenseListId, expenseId);
-            return new ResponseEntity<>(HttpStatus.NO_CONTENT);
-        } catch (Exception e) {
-            return new ResponseEntity<>(null, HttpStatus.EXPECTATION_FAILED);
-        }
+        expenseListService.deleteExpenseFromList(expenseListId, expenseId);
+        return new ResponseEntity<>(HttpStatus.NO_CONTENT);
     }
 
     @PostMapping("/{listId}/invite")

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

@@ -1,11 +1,16 @@
 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;
@@ -13,6 +18,8 @@ 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<>();
@@ -27,4 +34,36 @@ public class GlobalExceptionHandler {
         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/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/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/SecurityConfig.java

@@ -6,7 +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.BearerTokenAuthenticationFilter;
+import org.springframework.security.oauth2.server.resource.web.authentication.BearerTokenAuthenticationFilter;
 import org.springframework.security.web.SecurityFilterChain;
 
 @Configuration
@@ -18,7 +18,7 @@ public class SecurityConfig {
                 http
                                 .authorizeHttpRequests(auth -> auth
                                                 .anyRequest().permitAll())
-                                .csrf().disable();
+                                .csrf(csrf -> csrf.disable());
 
                 return http.build();
         }
@@ -33,7 +33,7 @@ public class SecurityConfig {
                                                 .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();
-        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;
+        return repository.findByOwnerIdOrSharedWithId(id);
     }
 
     public List<ExpenseList> findByUsername(String username) {
-        List<ExpenseList> allLists = repository.findAll();
-        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;
-
+        return repository.findByOwnerUsernameOrSharedWithUsername(username);
     }
 
     public Expense addExpenseToList(Long expenseListId, Expense expense) {
-        // find expenseList
         ExpenseList expenseList = repository.findById(expenseListId)
-                .orElseThrow(() -> new RuntimeException("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
+                .orElseThrow(() -> new ResourceNotFoundException("ExpenseList not found with id: " + expenseListId));
         expenseList.addExpense(expense);
-        // save
         repository.save(expenseList);
-
-        Expense newExpense = new Expense();
-        for (Expense e : expenseList.getExpenses()) {
-            if (!existingId.contains(e.getId())) {
-                newExpense = e;
-                break;
-            }
-        }
-        return newExpense;
+        return expense;
     }
 
     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);
-        if (user.isPresent()) {
-            return user.get();
-        } else
-            return null;
+        return userRepository.findById(id)
+                .orElseThrow(() -> new ResourceNotFoundException("User not found with id: " + id));
     }
 
     public AppUser deleteUserById(Long id) {
-        Optional<AppUser> user = userRepository.findById(id);
-        if (user.isPresent()) {
-            userRepository.deleteById(id);
-            return user.get();
-        } else
-            return null;
+        AppUser user = userRepository.findById(id)
+                .orElseThrow(() -> new ResourceNotFoundException("User not found with id: " + id));
+        userRepository.deleteById(id);
+        return user;
     }
 
     public AppUser getUserByName(String username) {
-        Optional<AppUser> optUser = userRepository.findByUsername(username);
-        if (optUser.isPresent()) {
-            return optUser.get();
-        } else
-            return null;
+        return userRepository.findByUsername(username)
+                .orElseThrow(() -> new ResourceNotFoundException("User not found: " + username));
     }
 
     public AppUser getUserByGoogleId(String id) {
-        Optional<AppUser> optUser = userRepository.findByGoogleId(id);
-        if (optUser.isPresent()) {
-            return optUser.get();
-        } else
-            return null;
+        return userRepository.findByGoogleId(id)
+                .orElseThrow(() -> new ResourceNotFoundException("User not found with Google ID: " + id));
     }
-
 }

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
-        private ExpenseListRepository expenseListRepository;
+    @Autowired
+    private ExpenseListRepository expenseListRepository;
 
-        @Test
-        void testFindExpenseListById() {
-                // Assuming an ExpenseList with id = 1 exists in your test DB.
-                Optional<ExpenseList> optionalExpenseList = expenseListRepository.findById(1L);
+    @Test
+    void saveAndFindById_returnsExpenseList() {
+        ExpenseList list = new ExpenseList();
+        list.setName("Groceries");
+        ExpenseList saved = expenseListRepository.save(list);
 
-                ExpenseList expenseList = optionalExpenseList.get();
-                System.out.println("ExpenseList name: " + expenseList.getName());
-        }
+        Optional<ExpenseList> found = expenseListRepository.findById(saved.getId());
+
+        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

@@ -6,8 +6,11 @@ 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.test.autoconfigure.web.servlet.AutoConfigureMockMvc;
-import org.springframework.boot.test.autoconfigure.web.servlet.WebMvcTest;
+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;
@@ -18,7 +21,11 @@ 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(AppUserController.class)
+@WebMvcTest(value = AppUserController.class, excludeAutoConfiguration = {
+        OAuth2ClientAutoConfiguration.class,
+        OAuth2ClientWebSecurityAutoConfiguration.class,
+        OAuth2ResourceServerAutoConfiguration.class
+})
 @AutoConfigureMockMvc(addFilters = false)
 @ActiveProfiles("test")
 class AppUserControllerTest {

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

@@ -9,8 +9,11 @@ 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.test.autoconfigure.web.servlet.AutoConfigureMockMvc;
-import org.springframework.boot.test.autoconfigure.web.servlet.WebMvcTest;
+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;
@@ -24,7 +27,11 @@ 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(ExpenseListController.class)
+@WebMvcTest(value = ExpenseListController.class, excludeAutoConfiguration = {
+        OAuth2ClientAutoConfiguration.class,
+        OAuth2ClientWebSecurityAutoConfiguration.class,
+        OAuth2ResourceServerAutoConfiguration.class
+})
 @AutoConfigureMockMvc(addFilters = false)
 @ActiveProfiles("test")
 class ExpenseListControllerTest {
@@ -132,4 +139,43 @@ class ExpenseListControllerTest {
         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/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");
+    }
+}