This article is an example of how to build a simple asynchronous Http client using Java Nio. This example will make use of thehttpbin service for much of it’s test cases, which can also be verified via postman or curl. Although the examples work, this is by no means a production ready. The exhaustive Http client implementation was merely an exercise in attempting to implement an Http client using Java Nio in an asynchronous manner. This example does not support redirect instructions (3.xx). For production ready implementations of Http clients, I recommendApache’s Asynchronous Http client or if your’e patient Java 9 has something in the works.
 
 
 
 

1. Introduction

So how does an Http Client make a request to a server and what is involved?

The client opens a connection to the server and sends a request. Most of the time this is done via a browser, obviously in our case this custom client is the culprit. The request consists of:

• Method (GET, PUT, POST, DELETE)
• URI (/index.html)
• Protocol version (HTTP/1.0)

Header line 1

GET / HTTP/1.1

A series of headers (meta information) is following, describing to the server what is to come:

Headers

Host: httpbin.orgConnection: keep-aliveUpgrade-Insecure-Requests: 1User-Agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/59.0.3071.104 Safari/537.36Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/webp,image/apng,*/*;q=0.8Accept-Encoding: gzip, deflate, brAccept-Language: en-US,en;q=0.8,nl;q=0.6Cookie: _gauges_unique_month=1; _gauges_unique_year=1; _gauges_unique=1; _gauges_unique_hour=1; _gauges_unique_day=1

Following the headers (terminated by\r\n\r\n) comes the body, if any.

2. Technologies used

The example code in this article was built and run using:

• Java 1.8.101 (1.8.x will do fine)
• Maven 3.3.9 (3.3.x will do fine)
• Spring source tool suite 4.6.3 (Any Java IDE would work)
• Ubuntu 16.04 (Windows, Mac or Linux will do fine)

3. Overview

The sample program is a very simple asynchronous implementation of an Http client that uses Java Nio. The functionality of the client is tested via test cases which make requests against httpbin which simply echoes back what our request was. In the event of a bad request (400) it will respond accordingly. For theputandpost requests the body content is hard coded to betext/plain.

4. The program

NioAsyncHttpClient

public final class NioAsyncHttpClient implements AutoCloseable {    private static final int PORT = 80;    private AsynchronousChannelGroup httpChannelGroup;    public static NioAsyncHttpClient create(final AsynchronousChannelGroup httpChannelGroup) {        return new NioAsyncHttpClient(httpChannelGroup);    }    private NioAsyncHttpClient(final AsynchronousChannelGroup httpChannelGroup) {        Objects.requireNonNull(httpChannelGroup);        this.httpChannelGroup = httpChannelGroup;    }    public void get(final String url, final String headers, final Consumer<? super ByteBuffer> success, final Consumer<? super Exception> failure)            throws URISyntaxException, IOException {        Objects.requireNonNull(url);        Objects.requireNonNull(headers);        Objects.requireNonNull(success);        Objects.requireNonNull(failure);        process(url, Optional.<ByteBuffer>empty(), headers, success, failure);    }    public void post(final String url, String data, final String headers, final Consumer<? super ByteBuffer> success, final Consumer<? super Exception> failure)            throws URISyntaxException, IOException {        Objects.requireNonNull(data);        Objects.requireNonNull(url);        Objects.requireNonNull(headers);        Objects.requireNonNull(success);        Objects.requireNonNull(failure);        process(url, Optional.of(ByteBuffer.wrap(data.getBytes())), headers, success, failure);    }    @Override    public void close() throws Exception {        this.httpChannelGroup.shutdown();    }    private void process(final String url, final Optional<ByteBuffer> data, final String headers, final Consumer<? super ByteBuffer> success,            final Consumer<? super Exception> failure) throws IOException, URISyntaxException {        assert StringUtils.isNotEmpty(url) && !Objects.isNull(data) && StringUtils.isNotEmpty(headers) && !Objects.isNull(success) && !Objects.isNull(failure);        final URI uri = new URI(url);        final SocketAddress serverAddress = new InetSocketAddress(getHostName(uri), PORT);        final RequestHandler handler = new RequestHandler(AsynchronousSocketChannel.open(this.httpChannelGroup), success, failure);        doConnect(uri, handler, serverAddress, ByteBuffer.wrap(createRequestHeaders(headers, uri).getBytes()), data);    }    private void doConnect(final URI uri, final RequestHandler handler, final SocketAddress address, final ByteBuffer headers, final Optional<ByteBuffer> body) {        assert !Objects.isNull(uri) && !Objects.isNull(handler) && !Objects.isNull(address) && !Objects.isNull(headers);        handler.getChannel().connect(address, null, new CompletionHandler<Void, Void>() {            @Override            public void completed(final Void result, final Void attachment) {                handler.headers(headers, body);            }            @Override            public void failed(final Throwable exc, final Void attachment) {                handler.getFailure().accept(new Exception(exc));            }        });    }    private String createRequestHeaders(final String headers, final URI uri) {        assert StringUtils.isNotEmpty(headers) && !Objects.isNull(uri);        return headers + "Host: " + getHostName(uri) + "\r\n\r\n";    }    private String getHostName(final URI uri) {        assert !Objects.isNull(uri);        return uri.getHost();    }}

• line 57-68: calls connect on theAsynchronousSocketChannel and passes aCompletionHandler to it. We make use of a customRequestHandlerto handle success and failure as well as to provide the reading and writing semantics for the headers, body and response.
• line 74: the\r\n\r\n sequence of characters signal to the server the end of the headers section meaning anything that follows should be body content and should also correspond in length to theContent-Lengthheader attribute value

RequestHandler

final class RequestHandler {    private final AsynchronousSocketChannel channel;    private final Consumer<? super ByteBuffer> success;    private final Consumer<? super Exception> failure;    RequestHandler(final AsynchronousSocketChannel channel, final Consumer<? super ByteBuffer> success, final Consumer<? super Exception> failure) {        assert !Objects.isNull(channel) && !Objects.isNull(success) && !Objects.isNull(failure);        this.channel = channel;        this.success = success;        this.failure = failure;    }    AsynchronousSocketChannel getChannel() {        return this.channel;    }    Consumer<? super ByteBuffer> getSuccess() {        return this.success;    }    Consumer<? super Exception> getFailure() {        return this.failure;    }    void closeChannel() {        try {            this.channel.close();        } catch (IOException e) {            throw new RuntimeException(e);        }    }    void headers(final ByteBuffer headers, final Optional<ByteBuffer> body) {        assert !Objects.isNull(headers);        this.channel.write(headers, this, new CompletionHandler<Integer, RequestHandler>() {            @Override            public void completed(final Integer result, final RequestHandler handler) {                if (headers.hasRemaining()) {                    RequestHandler.this.channel.write(headers, handler, this);                } else if (body.isPresent()) {                    RequestHandler.this.body(body.get(), handler);                } else {                    RequestHandler.this.response();                }            }            @Override            public void failed(final Throwable exc, final RequestHandler handler) {                handler.getFailure().accept(new Exception(exc));                RequestHandler.this.closeChannel();            }        });    }    void body(final ByteBuffer body, final RequestHandler handler) {        assert !Objects.isNull(body) && !Objects.isNull(handler);        this.channel.write(body, handler, new CompletionHandler<Integer, RequestHandler>() {            @Override            public void completed(final Integer result, final RequestHandler handler) {                if (body.hasRemaining()) {                    RequestHandler.this.channel.write(body, handler, this);                } else {                    RequestHandler.this.response();                }            }            @Override            public void failed(final Throwable exc, final RequestHandler handler) {                handler.getFailure().accept(new Exception(exc));                RequestHandler.this.closeChannel();            }        });    }    void response() {        final ByteBuffer buffer = ByteBuffer.allocate(2048);        this.channel.read(buffer, this, new CompletionHandler<Integer, RequestHandler>() {            @Override            public void completed(final Integer result, final RequestHandler handler) {                if (result > 0) {                    handler.getSuccess().accept(buffer);                    buffer.clear();                    RequestHandler.this.channel.read(buffer, handler, this);                } else if (result < 0) {                    RequestHandler.this.closeChannel();                } else {                    RequestHandler.this.channel.read(buffer, handler, this);                }            }            @Override            public void failed(final Throwable exc, final RequestHandler handler) {                handler.getFailure().accept(new Exception(exc));                RequestHandler.this.closeChannel();            }        });    }}

TheRequestHandleris responsible for executing the reading and writing of headers, body and responses. It is injected with 2Consumercallbacks, one for success and the other for failure. The successConsumercallback simply console logs the output and the failureConsumercallback will print the stacktrace accordingly.

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Snippet of test case

@Testpublic void get() throws Exception {    doGet(() -> "https://httpbin.org/get", () -> String.format(HEADERS_TEMPLATE, "GET", "get", "application/json", String.valueOf(0)));} private void doGet(final Supplier<? extends String> url, final Supplier<? extends String> headers) throws Exception {        final WritableByteChannel target = Channels.newChannel(System.out);        final AtomicBoolean pass = new AtomicBoolean(true);        final CountDownLatch latch = new CountDownLatch(1);        try (NioAsyncHttpClient client = NioAsyncHttpClient.create(this.asynchronousChannelGroup)) {            client.get(url.get(), headers.get(), (buffer) -> {                try {                    buffer.flip();                    while (buffer.hasRemaining()) {                        target.write(buffer);                    }                } catch (IOException e) {                    pass.set(false);                } finally {                    latch.countDown();                }            }, (exc) -> {                exc.printStackTrace();                pass.set(false);                latch.countDown();            });        }        latch.await();        assertTrue("Test failed", pass.get());    }

• line 13-29: we invoke get in this test case supplying the url and the headers. A successConsumerand failureConsumer callback are supplied when the response is read from the server or when an exception occurs during processing.

Test case output

HTTP/1.1 200 OKConnection: keep-aliveServer: meinheld/0.6.1Date: Tue, 20 Jun 2017 18:36:56 GMTContent-Type: application/jsonAccess-Control-Allow-Origin: *Access-Control-Allow-Credentials: trueX-Powered-By: FlaskX-Processed-Time: 0.00129985809326Content-Length: 228Via: 1.1 vegur{  "args": {},   "headers": {    "Accept": "application/json",     "Connection": "close",     "Content-Type": "text/plain",     "Host": "httpbin.org"  },   "origin": "105.27.116.66",   "url": "http://httpbin.org/get"}

The output is the response from thehttpbin service which is console logged by our successConsumer callback.

5. Summary

In this example we briefly discussed what’s involved with an Http request and then demonstrated an asynchronous http client built using Java Nio. We made a use of a 3rd party servicehttpbin to verify our client’s calls.

6. Download the source code

This was a Java Nio Async HTTP Client Example.

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