Async Http Client

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

Table of Contents

• Features
• Requirements
• Installation
• Quick Start
• Configuration
• HTTP Requests
• Handling Responses
• HTTP/2
• WebSocket
• Authentication
• Proxy Support
• Community
• 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:

<dependency>
    <groupId>org.asynchttpclient</groupId>
    <artifactId>async-http-client</artifactId>
    <version>3.0.8</version>
</dependency>

Gradle:

implementation 'org.asynchttpclient:async-http-client:3.0.8'

Optional: Native Transport

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

<!-- Epoll (Linux) -->
<dependency>
    <groupId>io.netty</groupId>
    <artifactId>netty-transport-native-epoll</artifactId>
    <classifier>linux-x86_64</classifier>
</dependency>

<!-- io_uring (Linux) -->
<dependency>
    <groupId>io.netty</groupId>
    <artifactId>netty-transport-native-io_uring</artifactId>
    <classifier>linux-x86_64</classifier>
</dependency>

Then enable in config:

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

Optional: Brotli / Zstd Compression

<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>

Quick Start

Import the DSL helpers:

import static org.asynchttpclient.Dsl.*;

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

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:

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:

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:

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

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

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

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:

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

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:

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:

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

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

To force HTTP/1.1, disable HTTP/2:

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

WebSocket

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

// 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

// 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 — questions, ideas, and general discussion
• Issue Tracker — bug reports and feature requests

License

Apache License 2.0