XpenselyServer/docs/API.md

# 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`

_TODO_