# Technical Documentation: AsyncHttpClient/async-http-client

> ℹ️ **Provenance:** Multi-Tier Ingestion · [CodeWiki Reference](https://codewiki.google/github.com/AsyncHttpClient/async-http-client)

### Overview (`README.md`)

# Async Http Client

[](https://github.com/AsyncHttpClient/async-http-client/actions/workflows/builds.yml)
[](https://central.sonatype.com/artifact/org.asynchttpclient/async-http-client)
[](https://www.apache.org/licenses/LICENSE-2.0)

AsyncHttpClient (AHC) is a high-performance, asynchronous HTTP client for Java
built on top of [Netty](https://github.com/netty/netty).
It supports HTTP/1.1, HTTP/2, and WebSocket protocols.

## Table of Contents

- [Features](#features)
- [Requirements](#requirements)
- [Installation](#installation)
- [Quick Start](#quick-start)
- [Configuration](#configuration)
- [HTTP Requests](#http-requests)
- [Handling Responses](#handling-responses)
- [HTTP/2](#http2)
- [WebSocket](#websocket)
- [Authentication](#authentication)
- [Proxy Support](#proxy-support)
- [Community](#community)
- [License](#license)

## Features

- **HTTP/2 with multiplexing** — enabled by default over TLS via ALPN,
  with connection multiplexing and GOAWAY handling
- **HTTP/1.1 and HTTP/1.0** — connection pooling and keep-alive
- **WebSocket** — text, binary, and ping/pong frame support
- **Asynchronous API** — non-blocking I/O with `ListenableFuture`
  and `CompletableFuture`
- **Compression** — automatic gzip, deflate, Brotli, and Zstd decompression
- **Authentication** — Basic, Digest, NTLM, SPNEGO/Kerberos, and SCRAM-SHA-256
- **Proxy** — HTTP, SOCKS4, and SOCKS5 with CONNECT tunneling
- **Native transports** — optional Epoll, KQueue, and io_uring
- **Request/response filters** — intercept and transform at each stage
- **Cookie management** — RFC 6265-compliant cookie store
- **Multipart uploads** — file, byte array, input stream, and string parts
- **Resumable downloads** — built-in `ResumableIOExceptionFilter`

## Requirements

Java 11+

## Installation

**Maven:**

```xml
<dependency>
    <groupId>org.asynchttpclient</groupId>
    <artifactId>async-http-client</artifactId>
    <version>3.0.14</version>
</dependency>
```

**Gradle:**

```groovy
implementation 'org.asynchttpclient:async-http-client:3.0.14'
```

<details>
<summary><b>Optional: Native Transport</b></summary>

For lower-latency I/O on Linux, add a native transport dependency:

```xml

<dependency>
    <groupId>io.netty</groupId>
    <artifactId>netty-transport-native-epoll</artifactId>
    <classifier>linux-x86_64</classifier>
</dependency>


<dependency>
    <groupId>io.netty</groupId>
    <artifactId>netty-transport-native-io_uring</artifactId>
    <classifier>linux-x86_64</classifier>
</dependency>
```

Then enable in config:

```java
AsyncHttpClient client = asyncHttpClient(config().setUseNativeTransport(true));
```

</details>

<details>
<summary><b>Optional: Brotli / Zstd Compression</b></summary>

```xml
<dependency>
    <groupId>com.aayushatharva.brotli4j</groupId>
    <artifactId>brotli4j</artifactId>
    <version>1.20.0</version>
</dependency>

<dependency>
    <groupId>com.github.luben</groupId>
    <artifactId>zstd-jni</artifactId>
    <version>1.5.7-7</version>
</dependency>
```

</details>

## Quick Start

Import the DSL helpers:

```java
import static org.asynchttpclient.Dsl.*;
```

Create a client, execute a request, and read the response:

```java
try (AsyncHttpClient client = asyncHttpClient()) {
    // Asynchronous
    client.prepareGet("https://www.example.com/")
        .execute()
        .toCompletableFuture()
        .thenApply(Response::getResponseBody)
        .thenAccept(System.out::println)
        .join();

    // Synchronous (blocking)
    Response response = client.prepareGet("https://www.example.com/")
        .execute()
        .get();
}
```

> **Note:** `AsyncHttpClient` instances are long-lived, shared resources.
> Always close them when done. Creating a new client per request will degrade
> performance due to repeated thread pool and connection pool creation.

## Configuration

Use `config()` to build an `AsyncHttpClientConfig`:

```java
AsyncHttpClient client = asyncHttpClient(config()
    .setConnectTimeout(Duration.ofSeconds(5))
    .setRequestTimeout(Duration.ofSeconds(30))
    .setMaxConnections(500)
    .setMaxConnectionsPerHost(100)
    .setFollowRedirect(true)
    .setMaxRedirects(5)
    .setCompressionEnforced(true));
```

## HTTP Requests

### Sending Requests

**Bound** — build directly from the client:

```java
Response response = client
    .prepareGet("https://api.example.com/users")
    .addHeader("Accept", "application/json")
    .addQueryParam("page", "1")
    .execute()
    .get();
```

**Unbound** — build standalone via DSL, then execute:

```java
Request request = get("https://api.example.com/users")
    .addHeader("Accept", "application/json")
    .addQueryParam("page", "1")
    .build();

Response response = client.executeRequest(request).get();
```

Methods: `GET`, `POST`, `PUT`, `DELETE`, `PATCH`, `HEAD`, `OPTIONS`, `TRACE`.

### Request Bodies

Use `setBody` to attach a body. Supported types:

| Type | Description |
|---|---|
| `String` | Text content |
| `byte[]` | Raw bytes |
| `ByteBuffer` | NIO buffer |
| `InputStream` | Streaming input |
| `File` | File content |
| `Publisher<ByteBuf>` | Reactive stream |
| `BodyGenerator` | Custom body generation |

```java
Response response = client
    .preparePost("https://api.example.com/data")
    .setHeader("Content-Type", "application/json")
    .setBody("{\"name\": \"value\"}")
    .execute()
    .get();
```

For streaming bodies, see `FeedableBodyGenerator` which lets you push chunks
asynchronously.

### Multipart Uploads

```java
Response response = client
    .preparePost("https://api.example.com/upload")
    .addBodyPart(new FilePart("file", new File("report.pdf"), "application/pdf"))
    .addBodyPart(new StringPart("description", "Monthly report"))
    .execute()
    .get();
```

Part types: `FilePart`, `ByteArrayPart`, `InputStreamPart`, `StringPart`.

## Handling Responses

### Blocking

```java
Response response = client.prepareGet("https://www.example.com/").execute().get();
```

> Useful for debugging, but defeats the purpose of an async client in production.

### ListenableFuture

`execute()` returns a `ListenableFuture` that supports completion listeners:

```java
ListenableFuture<Response> future = client
    .prepareGet("https://www.example.com/")
    .execute();

future.addListener(() -> {
    Response response = future.get();
    System.out.println(response.getStatusCode());
}, executor);
```

> If `executor` is `null`, the callback runs on the Netty I/O thread.
> **Never block** inside I/O thread callbacks.

### CompletableFuture

```java
client.prepareGet("https://www.example.com/")
    .execute()
    .toCompletableFuture()
    .thenApply(Response::getResponseBody)
    .thenAccept(System.out::println)
    .join();
```

### AsyncCompletionHandler

For most async use cases, extend `AsyncCompletionHandler` — it buffers the
full response and gives you a single `onCompleted(Response)` callback:

```java
client.prepareGet("https://www.example.com/")
    .execute(new AsyncCompletionHandler<String>() {
        @Override
        public String onCompleted(Response response) {
            return response.getResponseBody();
        }
    });
```

### AsyncHandler

For fine-grained control, implement `AsyncHandler` directly. This lets you
inspect status, headers, and body chunks as they arrive and abort early:

```java
Future<Integer> future = client
    .prepareGet("https://www.example.com/")
    .execute(new AsyncHandler<>() {
        private int status;

        @Override
        public State onStatusReceived(HttpResponseStatus s) {
            status = s.getStatusCode();
            return State.CONTINUE;
        }

        @Override
        public State onHeadersReceived(HttpHeaders headers) {
            return State.CONTINUE;
        }

        @Override
        public State onBodyPartReceived(HttpResponseBodyPart part) {
            return State.ABORT; // stop early — we only needed the status
        }

        @Override
        public Integer onCompleted() {
            return status;
        }

        @Override
        public void onThrowable(Throwable t) {
            t.printStackTrace();
        }
    });
```

## HTTP/2

HTTP/2 is **enabled by default** for HTTPS connections via ALPN negotiation.
The client uses HTTP/2 when the server supports it and falls back to HTTP/1.1
otherwise. No additional configuration is required.

- **Connection multiplexing** — concurrent streams over a single TCP connection
- **GOAWAY handling** — graceful connection draining on server shutdown
- **PING keepalive** — configurable ping frames to keep connections alive

### HTTP/2 Configuration

```java
AsyncHttpClient client = asyncHttpClient(config()
    .setHttp2MaxConcurrentStreams(100)
    .setHttp2InitialWindowSize(65_535)
    .setHttp2MaxFrameSize(16_384)
    .setHttp2MaxHeaderListSize(8_192)
    .setHttp2PingInterval(Duration.ofSeconds(30))  // keepalive pings
    .setHttp2CleartextEnabled(true));               // h2c prior knowledge
```

When a handler suspends a response with `ResponseBodyControl`, the HTTP/2
per-stream window remains the buffering bound for that response. While at least
one response on a connection is suspended, AHC continues returning
connection-level credit so it cannot stall sibling streams. Connections with no
active suspension retain the normal 65,535-byte shared connection-window bound.
Once the last suspension ends, normal connection accounting resumes, although
credit already returned and data already queued cannot be revoked. Aggregate
queued response data during suspension can scale with the number of concurrent
streams. Use `http2InitialWindowSize` and `http2MaxConcurrentStreams` together
when an application needs a tighter aggregate bound.

To force HTTP/1.1, disable HTTP/2:

```java
AsyncHttpClient client = asyncHttpClient(config().setHttp2Enabled(false));
```

## WebSocket

```java
WebSocket ws = client
    .prepareGet("wss://echo.example.com/")
    .execute(new WebSocketUpgradeHandler.Builder()
        .addWebSocketListener(new WebSocketListener() {
            @Override
            public void onOpen(WebSocket ws) {
                ws.sendTextFrame("Hello!");
            }

            @Override
            public void onTextFrame(String payload, boolean finalFragment, int rsv) {
                System.out.println(payload);
            }

            @Override
            public void onClose(WebSocket ws, int code, String reason) {}

            @Override
            public void onError(Throwable t) { t.printStackTrace(); }
        })
        .build())
    .get();
```

## Authentication

```java
// Client-wide Basic auth
AsyncHttpClient client = asyncHttpClient(config()
    .setRealm(basicAuthRealm("user", "password")));

// Per-request Digest auth
Response response = client
    .prepareGet("https://api.example.com/protected")
    .setRealm(digestAuthRealm("user", "password").build())
    .execute()
    .get();

// SCRAM-SHA-256 (RFC 7804)
Response response = client
    .prepareGet("https://api.example.com/protected")
    .setRealm(scramSha256AuthRealm("user", "password").build())
    .execute()
    .get();
```

Supported schemes: **Basic**, **Digest**, **NTLM**, **SPNEGO/Kerberos**, **SCRAM-SHA-256**.

## Proxy Support

```java
// HTTP proxy
AsyncHttpClient client = asyncHttpClient(config()
    .setProxyServer(proxyServer("proxy.example.com", 8080)));

// Authenticated proxy
AsyncHttpClient client = asyncHttpClient(config()
    .setProxyServer(proxyServer("proxy.example.com", 8080)
        .setRealm(basicAuthRealm("proxyUser", "proxyPassword"))));
```

SOCKS4 and SOCKS5 proxies are also supported.

## Community

- [GitHub Discussions](https://github.com/AsyncHttpClient/async-http-client/discussions) — questions, ideas, and general discussion
- [Issue Tracker](https://github.com/AsyncHttpClient/async-http-client/issues) — bug reports and feature requests

## License

[Apache License 2.0](LICENSE.txt)

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