Async Http Client

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The AsyncHttpClient (AHC) library allows Java applications to easily execute HTTP requests and asynchronously process HTTP responses.
The library also supports the WebSocket Protocol.

It’s built on top of Netty. It’s compiled with Java 11.

Installation

Binaries are deployed on Maven Central.
Add a dependency on the main AsyncHttpClient artifact:

Maven:

<dependencies>
    <dependency>
        <groupId>org.asynchttpclient</groupId>
        <artifactId>async-http-client</artifactId>
        <version>3.0.0</version>
    </dependency>
</dependencies>

Gradle:

dependencies {
    implementation 'org.asynchttpclient:async-http-client:3.0.0'
}

Dsl

Import the Dsl helpers to use convenient methods to bootstrap components:

import static org.asynchttpclient.Dsl.*;

Client

import static org.asynchttpclient.Dsl.*;

AsyncHttpClient asyncHttpClient=asyncHttpClient();

AsyncHttpClient instances must be closed (call the close method) once you’re done with them, typically when shutting down your application.
If you don’t, you’ll experience threads hanging and resource leaks.

AsyncHttpClient instances are intended to be global resources that share the same lifecycle as the application.
Typically, AHC will usually underperform if you create a new client for each request, as it will create new threads and connection pools for each.
It’s possible to create shared resources (EventLoop and Timer) beforehand and pass them to multiple client instances in the config. You’ll then be responsible for closing
those shared resources.

Configuration

Finally, you can also configure the AsyncHttpClient instance via its AsyncHttpClientConfig object:

import static org.asynchttpclient.Dsl.*;

AsyncHttpClient c=asyncHttpClient(config().setProxyServer(proxyServer("127.0.0.1",38080)));

HTTP

Sending Requests

Basics

AHC provides 2 APIs for defining requests: bound and unbound.
AsyncHttpClient and Dsl` provide methods for standard HTTP methods (POST, PUT, etc) but you can also pass a custom one.

import org.asynchttpclient.*;

// bound
Future<Response> whenResponse=asyncHttpClient.prepareGet("http://www.example.com/").execute();

// unbound
        Request request=get("http://www.example.com/").build();
        Future<Response> whenResponse=asyncHttpClient.executeRequest(request);

Setting Request Body

Use the setBody method to add a body to the request.

This body can be of type:

• java.io.File
• byte[]
• List<byte[]>
• String
• java.nio.ByteBuffer
• java.io.InputStream
• Publisher<io.netty.buffer.ByteBuf>
• org.asynchttpclient.request.body.generator.BodyGenerator

BodyGenerator is a generic abstraction that let you create request bodies on the fly.
Have a look at FeedableBodyGenerator if you’re looking for a way to pass requests chunks on the fly.

Multipart

Use the addBodyPart method to add a multipart part to the request.

This part can be of type:

• ByteArrayPart
• FilePart
• InputStreamPart
• StringPart

Dealing with Responses

Blocking on the Future

execute methods return a java.util.concurrent.Future. You can simply block the calling thread to get the response.

Future<Response> whenResponse=asyncHttpClient.prepareGet("http://www.example.com/").execute();
        Response response=whenResponse.get();

This is useful for debugging but you’ll most likely hurt performance or create bugs when running such code on production.
The point of using a non blocking client is to NOT BLOCK the calling thread!

Setting callbacks on the ListenableFuture

execute methods actually return a org.asynchttpclient.ListenableFuture similar to Guava’s.
You can configure listeners to be notified of the Future’s completion.

        ListenableFuture<Response> whenResponse = ???;
        Runnable callback = () - > {
            try {
               Response response = whenResponse.get();
               System.out.println(response);
            } catch (InterruptedException | ExecutionException e) {
               e.printStackTrace();
            }
        };

        java.util.concurrent.Executor executor = ???;
        whenResponse.addListener(() - > ??? , executor);

If the executor parameter is null, callback will be executed in the IO thread.
You MUST NEVER PERFORM BLOCKING operations in there, typically sending another request and block on a future.

Using custom AsyncHandlers

execute methods can take an org.asynchttpclient.AsyncHandler to be notified on the different events, such as receiving the status, the headers and body chunks.
When you don’t specify one, AHC will use a org.asynchttpclient.AsyncCompletionHandler;

AsyncHandler methods can let you abort processing early (return AsyncHandler.State.ABORT) and can let you return a computation result from onCompleted that will be used
as the Future’s result.
See AsyncCompletionHandler implementation as an example.

The below sample just capture the response status and skips processing the response body chunks.

Note that returning ABORT closes the underlying connection.

import static org.asynchttpclient.Dsl.*;

import org.asynchttpclient.*;
import io.netty.handler.codec.http.HttpHeaders;

Future<Integer> whenStatusCode = asyncHttpClient.prepareGet("http://www.example.com/")
        .execute(new AsyncHandler<Integer> () {
            private Integer status;
            
            @Override
            public State onStatusReceived(HttpResponseStatus responseStatus) throws Exception {
                status = responseStatus.getStatusCode();
                return State.ABORT;
            }
            
            @Override
            public State onHeadersReceived(HttpHeaders headers) throws Exception {
              return State.ABORT;
            }
            
            @Override
            public State onBodyPartReceived(HttpResponseBodyPart bodyPart) throws Exception {
                 return State.ABORT;
            }
        
            @Override
            public Integer onCompleted() throws Exception{
                return status;
            }
        
            @Override
            public void onThrowable(Throwable t) {
                t.printStackTrace();
            }
        });

        Integer statusCode = whenStatusCode.get();

Using Continuations

ListenableFuture has a toCompletableFuture method that returns a CompletableFuture.
Beware that canceling this CompletableFuture won’t properly cancel the ongoing request.
There’s a very good chance we’ll return a CompletionStage instead in the next release.

CompletableFuture<Response> whenResponse=asyncHttpClient
        .prepareGet("http://www.example.com/")
        .execute()
        .toCompletableFuture()
        .exceptionally(t->{ /* Something wrong happened... */  })
        .thenApply(response->{ /*  Do something with the Response */ return resp;});
        whenResponse.join(); // wait for completion

You may get the complete maven project for this simple demo
from org.asynchttpclient.example

WebSocket

Async Http Client also supports WebSocket.
You need to pass a WebSocketUpgradeHandler where you would register a WebSocketListener.

WebSocket websocket = c.prepareGet("ws://demos.kaazing.com/echo")
        .execute(new WebSocketUpgradeHandler.Builder().addWebSocketListener(
                new WebSocketListener() {

                  @Override
                  public void onOpen(WebSocket websocket) {
                    websocket.sendTextFrame("...").sendTextFrame("...");
                  }

                  @Override
                  public void onClose(WebSocket websocket) {
                    // ...
                  }

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

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

User Group

Keep up to date on the library development by joining the Asynchronous HTTP Client discussion group

GitHub Discussions