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>3.0.12</version>
</dependency>

Gradle:

implementation 'org.asynchttpclient:async-http-client:3.0.12'

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

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:

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