Nextus-Web/README.md

# NexusWeb

A lightweight, high-performance HTTP server library built on top of Netty. NexusWeb provides a fluent API for routing, middleware, CORS handling, and rate limiting — all running on Java virtual threads for maximum concurrency.

## Features

- **Netty-based** — uses epoll/kqueue/NIO automatically based on the platform
- **Virtual thread dispatch** — each request is handled on a Java 21 virtual thread
- **Trie-based router** — supports static paths, path parameters (`{id}`), and wildcards (`*`)
- **Annotation-based controllers** — define routes declaratively with `@Controller`, `@GET`, `@POST`, etc.
- **Middleware chain** — attach cross-cutting logic to all routes
- **CORS support** — configurable origins, methods, headers, credentials, and preflight caching
- **Rate limiting** — four algorithm implementations with per-IP, per-token, or custom key strategies
- **JSON I/O** — built-in Jackson integration for request parsing and response serialization

---

## Quick Start

```java
Router router = new Router();

router.get("/hello", (req, res) -> {
    res.status(200).json("{\"message\":\"Hello, World!\"}");
});

HttpServer.builder(8080, router).start();
```

---

## Routing

Routes are registered with `GET`, `POST`, `PUT`, `DELETE`, or the generic `register` method.

```java
Router router = new Router();

// Static path
router.get("/users", (req, res) -> { ... });

// Path parameter
router.get("/users/{id}", (req, res) -> {
    String id = req.pathParam("id");
    res.json("{\"id\":\"" + id + "\"}");
});

// Wildcard
router.get("/files/*", (req, res) -> { ... });

// POST with JSON body
router.post("/users", (req, res) -> {
    MyDto dto = req.jsonAs(MyDto.class);
    res.status(201).json(dto);
});
```

### Router resolution

| Outcome | HTTP Status |
|---|---|
| Path and method match | Handler is called |
| Path exists, method not registered | `405 Method Not Allowed` + `Allow` header |
| Path not found | `404 Not Found` |

### Annotation-based Controllers

Instead of registering lambdas on the `Router` directly, you can declare routes as methods on a class and register the whole object at once via `AnnotationScanner`.

**Annotations**

| Annotation | Target | Description |
|---|---|---|
| `@Controller("prefix")` | class | Optional path prefix applied to all methods in the class |
| `@GET("/path")` | method | Maps a `GET` route |
| `@POST("/path")` | method | Maps a `POST` route |
| `@PUT("/path")` | method | Maps a `PUT` route |
| `@DELETE("/path")` | method | Maps a `DELETE` route |
| `@PATCH("/path")` | method | Maps a `PATCH` route |
| `@Route(method="…", path="…")` | method | Generic — any standard HTTP method by name |
| `@CUSTOM(method="…", value="…")` | method | Custom/non-standard HTTP methods |

Every annotated method **must** have the exact signature `(Request req, Response res)` and return `void`.

```java
@Controller("/users")
public class UserController {

    @GET("")
    public void list(Request req, Response res) {
        res.json("[{\"id\":1}]");
    }

    @GET("/{id}")
    public void get(Request req, Response res) {
        res.json("{\"id\":\"" + req.pathParam("id") + "\"}");
    }

    @POST("")
    public void create(Request req, Response res) {
        UserDto dto = req.jsonAs(UserDto.class);
        // ... persist ...
        res.status(201).json(dto);
    }

    @PUT("/{id}")
    public void update(Request req, Response res) {
        // ...
    }

    @DELETE("/{id}")
    public void delete(Request req, Response res) {
        res.status(204);
    }

    @PATCH("/{id}")
    public void patch(Request req, Response res) {
        // ...
    }
}
```

Register the controller with:

```java
Router router = new Router();
AnnotationScanner.register(router, new UserController());

HttpServer.builder(8080, router).start();
```

`AnnotationScanner.register` prints each registered route to stdout:

```
Registered: GET    /users                         -> UserController.list
Registered: GET    /users/{id}                    -> UserController.get
Registered: POST   /users                         -> UserController.create
```

Multiple controllers can be registered on the same router:

```java
AnnotationScanner.register(router, new UserController());
AnnotationScanner.register(router, new OrderController());
```

---

### Middleware

Middleware runs before the matched handler on every request.

```java
router.use((req, res) -> {
    res.header("X-Request-Id", UUID.randomUUID().toString());
});
```

---

## Request API

```java
req.pathParam("id")           // path parameter, e.g. from /users/{id}
req.queryParam("search")      // first value of ?search=
req.queryParams("tag")        // all values of ?tag= as List<String>
req.header("Authorization")   // raw header value
req.body()                    // raw body as UTF-8 String
req.json()                    // body parsed as Jackson JsonNode
req.jsonAs(MyDto.class)       // body deserialized into a POJO
req.method()                  // HttpMethod
req.path()                    // decoded path without query string
```

`json()` and `jsonAs()` throw `BadRequestException` (→ `400`) on malformed JSON.

---

## Response API

```java
res.status(201)
   .header("X-Custom", "value")
   .json("{\"ok\":true}");    // sets Content-Type: application/json

res.text("plain text");       // sets Content-Type: text/plain
res.json(someObject);         // serializes POJO via Jackson
```

---

## CORS

```java
CorsConfig config = CorsConfig.builder()
        .allowedOrigins("https://app.example.com")
        .allowedMethods(HttpMethod.GET, HttpMethod.POST)
        .allowedHeaders("Content-Type", "Authorization")
        .allowCredentials(true)
        .maxAgeSeconds(3600)
        .build();

CorsHandler cors = new CorsHandler(config);

HttpServer.builder(8080, router)
        .withCorsHandler(cors)
        .start();
```

Use `CorsConfig.permissive()` for a development preset that allows any origin with common methods and headers (incompatible with `allowCredentials`).

**Preflight requests** (`OPTIONS` + `Access-Control-Request-Method`) are handled automatically and short-circuit before the router.

---

## Rate Limiting

### Algorithms

| Class | Description |
|---|---|
| `TokenBucketLimiter` | Smooth bursts up to a configurable capacity, refills at a steady rate |
| `FixedWindowLimiter` | Hard limit per fixed time window |
| `SlidingWindowLimiter` | Weighted sliding window — reduces boundary spikes vs. fixed window |
| `LeakyBucketLimiter` | Constant outflow rate; excess requests are rejected immediately |

### Configuration

```java
RateLimitConfig config = RateLimitConfig.builder()
        // Global rule for all routes — keyed by client IP
        .global(new TokenBucketLimiter(100, 200), KeyResolver.clientIp())
        // Stricter rule for a specific path
        .forPath("/login", new FixedWindowLimiter(5, 60_000), KeyResolver.clientIp())
        // Rule for an entire path prefix
        .forPrefix("/api/", new SlidingWindowLimiter(1000, 1000), KeyResolver.userOrIp())
        .build();

RateLimitGate gate = new RateLimitGate(config);

HttpServer.builder(8080, router)
        .withRateLimitGate(gate)
        .start();
```

When the limit is exceeded the server responds with `429 Too Many Requests` and a `Retry-After` header.

### Response headers

Every response automatically includes:

| Header | Description |
|---|---|
| `X-RateLimit-Limit` | Configured limit for the matched rule |
| `X-RateLimit-Remaining` | Remaining requests in the current window |
| `Retry-After` | Seconds until the client may retry (only on `429`) |

### Key resolvers

| Factory | Behaviour |
|---|---|
| `KeyResolver.clientIp()` | Uses `X-Forwarded-For` if present, otherwise the remote IP |
| `KeyResolver.userOrIp()` | Uses the Bearer token if present (`u:<token>`), otherwise the client IP (`ip:<addr>`) |
| Custom lambda | `(req, remoteAddr) -> myKey(req)` |

---

## Full Example

```java
// Controller class
@Controller("/users")
public class UserController {

    @GET("")
    public void list(Request req, Response res) {
        res.json("[{\"id\":1}]");
    }

    @GET("/{id}")
    public void get(Request req, Response res) {
        res.json("{\"id\":\"" + req.pathParam("id") + "\"}");
    }

    @POST("")
    public void create(Request req, Response res) {
        UserDto dto = req.jsonAs(UserDto.class);
        res.status(201).json(dto);
    }
}

// Server setup
Router router = new Router();

// Middleware — add request ID to every response
router.use((req, res) -> res.header("X-Request-Id", UUID.randomUUID().toString()));

AnnotationScanner.register(router, new UserController());

CorsHandler cors = new CorsHandler(CorsConfig.permissive());

RateLimitConfig rlConfig = RateLimitConfig.builder()
        .global(new TokenBucketLimiter(200, 400), KeyResolver.clientIp())
        .build();
RateLimitGate gate = new RateLimitGate(rlConfig);

HttpServer.builder(8080, router)
        .withCorsHandler(cors)
        .withRateLimitGate(gate)
        .start();
```

---

## Requirements

- Java 26+
- Netty 4.x
- Jackson (tools.jackson)