AsyncHttpClient

Official repository and docs: https://github.com/AsyncHttpClient/async-http-client

Maven Dependency

Check the latest version of async-http-client at https://mvnrepository.com/artifact/org.asynchttpclient/async-http-client

<dependency>
    <groupId>org.asynchttpclient</groupId>
    <artifactId>async-http-client</artifactId>
    <version>2.12.3</version>
</dependency>

Usage

Basic Usage

AsyncHttpClient provides 2 APIs for defining requests: bound and unbound. AsyncHttpClient and Dsl` provide methods for standard HTTP methods (POST, PUT, etc)

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

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

Client Configuration

Instead of default configuration, create a customized one with Dsl.config()

AsyncHttpClientConfig config = Dsl.config()
        .setConnectTimeout(CONN_TIMEOUT)
        .setRequestTimeout(REQ_TIMEOUT)
        .setMaxRequestRetry(100)
        .build();
AsyncHttpClient client = Dsl.asyncHttpClient(config);

Download To File

The default implementation accumulates the HTTP chunks received into an ArrayList. This could lead to high memory consumption, or an OutOfMemory exception when trying to download a large file. Use a FileChannel to write the bytes to our local file directly. We'll use the getBodyByteBuffer() method to access the body part content through a ByteBuffer.

// Open the file, enable append(not necessary in this example)
FileOutputStream os = new FileOutputStream(LOCAL_FILE, true);
// Create a default client
AsyncHttpClient client = Dsl.asyncHttpClient();
// Use Future to block till download complete, just for demostration
ListenableFuture<Response> whenResponse = client.prepareGet(FILE_URL).execute(new AsyncCompletionHandler<Response>() {

    @Override
    public State onBodyPartReceived(HttpResponseBodyPart bodyPart) throws Exception {
        os.getChannel().write(bodyPart.getBodyByteBuffer());
        return State.CONTINUE;
    }

    @Override
    public Response onCompleted(Response response) throws Exception {
        return response;
    }
});

Response response=whenResponse.get();
// Thread will never exit if client is not closed
client.close();

You can get the bodyPart length and calculate the totoally received length

private int totalLength;

@Override
public State onBodyPartReceived(HttpResponseBodyPart bodyPart) throws Exception {
    os.getChannel().write(bodyPart.getBodyByteBuffer());
    receivedLength += bodyPart.length();
    System.out.println(receivedLength);
    if (bodyPart.isLast())
    {
        System.out.println("last");
        return State.ABORT;
    }
    return State.CONTINUE;
}

Range Requests, Partial Download

You can modify the request header to download part of the file, as long as the server supports range request.

Response DefaultHttpResponse(decodeResult: success, version: HTTP/1.1)
HTTP/1.1 200 OK
Server: openresty
Date: Sun, 04 Dec 2022 13:35:21 GMT
Content-Type: application/octet-stream
Last-Modified: Thu, 21 Apr 2022 17:16:39 GMT
Connection: keep-alive
ETag: "62619177-1b58d5"
Accept-Ranges: bytes           <--- this means the server supports range requests
content-length: 1792213

In Java code, create the request like

FileOutputStream os = new FileOutputStream(localFilePath, true);

Request request = Dsl.get(remoteUrl).setHeader(HttpHeaderNames.RANGE, "bytes="+offset+"-"+(offset + length - 1)).build();

ListenableFuture<FragmentResponse> whenResponse = client.executeRequest(request, new AsyncCompletionHandler<>() {
    //...
});

You will get the response header as

Response DefaultHttpResponse(decodeResult: success, version: HTTP/1.1)
HTTP/1.1 206 Partial Content
Server: openresty
Date: Sun, 04 Dec 2022 13:27:14 GMT
Content-Type: application/octet-stream
Last-Modified: Fri, 15 Oct 2021 12:25:23 GMT
Connection: keep-alive
ETag: "61697333-268f48e"
X-RateLimit-Byte-Rate: 67108864
Content-Range: bytes 38797312-39845887/40432782
content-length: 1048576

Range Boundary

According to Hypertext Transfer Protocol (HTTP/1.1): Range Requests, the range boundaries are inclusive - inclusive. Examples of byte-ranges-specifier values:

• The first 500 bytes (byte offsets 0-499, inclusive): bytes=0-499
• The second 500 bytes (byte offsets 500-999, inclusive): bytes=500-999

Example Code Of Download Speed Limit

This is an example of controlling download speed by splitting file into fragments -- and download them piece by piece.

public class FragmentableDownloader {

    Logger log = LoggerFactory.getLogger(FragmentableDownloader.class);

    public static final int CONN_TIMEOUT = 60000;
    public static final int REQ_TIMEOUT = 60000;

    private AsyncHttpClient client;
    /**
     * limit bytes per second
     */
    private long rateLimit;
    /**
     * size of each fragment
     */
    private long fragmentSize;

    private String remoteUrl;

    private String localFilePath;

    private long fileLength;

    private long timestamp;

    private long interval;

    public FragmentableDownloader(long rateLimit, long fragmentSize, String remoteUrl, String localFilePath) {
        this.rateLimit = rateLimit;
        this.fragmentSize = fragmentSize;
        this.remoteUrl = remoteUrl;
        this.localFilePath = localFilePath;

        AsyncHttpClientConfig config = Dsl.config()
                .setConnectTimeout(CONN_TIMEOUT)
                .setRequestTimeout(REQ_TIMEOUT)
                .setMaxRequestRetry(100)
                .build();
        this.client = Dsl.asyncHttpClient(config);

        interval = fragmentSize / rateLimit;
    }

    public FragmentResponse downloadFragment(long offset, long length) throws Exception {

        FileOutputStream os = new FileOutputStream(localFilePath, true);

        Request request = Dsl.get(remoteUrl).setHeader(HttpHeaderNames.RANGE, "bytes="+offset+"-"+(offset + length - 1)).build();

        ListenableFuture<FragmentResponse> whenResponse = client.executeRequest(request, new AsyncCompletionHandler<>() {
            private long totalLength;
            private int byteTransferred = 0;

            @Override
            public State onHeadersReceived(HttpHeaders headers) throws Exception {
                String length = headers.get(HttpHeaderNames.CONTENT_RANGE);
                int pos = length.lastIndexOf('/');
                length = length.substring(pos + 1);
                this.totalLength = Long.parseLong(length);
                return State.CONTINUE;
            }

            @Override
            public State onBodyPartReceived(HttpResponseBodyPart bodyPart) throws Exception {
                os.getChannel().write(bodyPart.getBodyByteBuffer());
                byteTransferred += bodyPart.length();
                //log.info("byteTransferred: {}", byteTransferred);
                return State.CONTINUE;
            }

            @Override
            public FragmentResponse onCompleted(Response response) throws Exception {
                log.info("complete");
                return new FragmentResponse(response.getStatusCode(), totalLength);
            }
        });
        return whenResponse.get();
    }

    public void download() throws Exception {
        Files.deleteIfExists(Path.of(localFilePath));
        long offset = 0;
        FragmentResponse response = downloadFragment(0, fragmentSize);
        offset += fragmentSize;
        this.fileLength = response.getTotalLength();
        if (this.fileLength <= fragmentSize) {
            return;
        }
        while (offset < fileLength - 1) {
            log.info("Offset: {}", offset);
            timestamp = System.currentTimeMillis();
            response = downloadFragment(offset, fragmentSize);
            offset += fragmentSize;
            long duration = System.currentTimeMillis() - timestamp;
            log.info("file:{}, offset:{}, response: {}, speed:{}", fileLength, offset, response.getStatus(), fragmentSize / duration);
            long wait = interval * 1000L - duration;
            if (wait > 0) {
                log.info("Sleep {} milliseconds for rate limit", wait);
                Thread.sleep(wait);
            }
        }
        log.info("Download finished");
        client.close();
    }

    public static void main(String[] args) throws Exception {
        String url = "https://mirrors.ustc.edu.cn/ubuntu/dists/jammy/main/signed/linux-amd64/5.13.0-19.19/signed.tar.gz";
        String path = "/home/milton/Downloads/signed.tar.gz";
        long rateLimit = 500 * 1024L;
        long fragmentSize = 10 * 1024 * 1024L;
        FragmentableDownloader downloader = new FragmentableDownloader(rateLimit, fragmentSize, url, path);
        downloader.download();
    }

    public static class FragmentResponse {
        private int status;
        private long totalLength;

        public FragmentResponse(int status, long totalLength) {
            this.status = status;
            this.totalLength = totalLength;
        }

        public int getStatus() {
            return status;
        }

        public long getTotalLength() {
            return totalLength;
        }
    }
}

Ref

• https://stackoverflow.com/questions/56397766/java-asynchttpclient-broken-file-while-writing-from-lazyresponsebodypart-to-asy