Java Equivalent of C# async/await?

Question

No, there isn't any equivalent of async/await in Java - or even in C# before v5.

It's a fairly complex language feature to build a state machine behind the scenes.

There's relatively little language support for asynchrony/concurrency in Java, but the java.util.concurrent package contains a lot of useful classes around this. (Not quite equivalent to the Task Parallel Library, but the closest approximation to it.)

The await uses a continuation to execute additional code when the asynchronous operation completes (client.GetStringAsync(...)).

So, as the most close approximation I would use a CompletableFuture<T> (the Java 8 equivalent to .net Task<TResult>) based solution to process the Http request asynchronously.

UPDATED on 25-05-2016 to AsyncHttpClient v.2 released on Abril 13th of 2016:

So the Java 8 equivalent to the OP example of AccessTheWebAsync() is the following:

CompletableFuture<Integer> AccessTheWebAsync()
{
    AsyncHttpClient asyncHttpClient = new DefaultAsyncHttpClient();
    return asyncHttpClient
       .prepareGet("http://msdn.microsoft.com")
       .execute()
       .toCompletableFuture()
       .thenApply(Response::getResponseBody)
       .thenApply(String::length);
}

This usage was taken from the answer to How do I get a CompletableFuture from an Async Http Client request? and which is according to the new API provided in version 2 of AsyncHttpClient released on Abril 13th of 2016, that has already intrinsic support for CompletableFuture<T>.

Original answer using version 1 of AsyncHttpClient:

To that end we have two possible approaches:

• the first one uses non-blocking IO and I call it AccessTheWebAsyncNio. Yet, because the AsyncCompletionHandler is an abstract class (instead of a functional interface) we cannot pass a lambda as argument. So it incurs in inevitable verbosity due to the syntax of anonymous classes. However, this solution is the most close to the execution flow of the given C# example.

• the second one is slightly less verbose however it will submit a new Task that ultimately will block a thread on f.get() until the response is complete.

First approach, more verbose but non-blocking:

static CompletableFuture<Integer> AccessTheWebAsyncNio(){
    final AsyncHttpClient asyncHttpClient = new AsyncHttpClient();
    final CompletableFuture<Integer> promise = new CompletableFuture<>();
    asyncHttpClient
        .prepareGet("https://msdn.microsoft.com")
        .execute(new AsyncCompletionHandler<Response>(){
            @Override
            public Response onCompleted(Response resp) throws Exception {
                promise.complete(resp.getResponseBody().length());
                return resp;
            }
        });
    return promise;
}

Second approach less verbose but blocking a thread:

static CompletableFuture<Integer> AccessTheWebAsync(){
    try(AsyncHttpClient asyncHttpClient = new AsyncHttpClient()){
        Future<Response> f = asyncHttpClient
            .prepareGet("https://msdn.microsoft.com")
            .execute();
        return CompletableFuture.supplyAsync(
            () -> return f.join().getResponseBody().length());
    }
}

Check out ea-async which does Java bytecode rewriting to simulate async/await pretty nicely. Per their readme: "It is heavily inspired by Async-Await on the .NET CLR"

async and await are syntactic sugars. The essence of async and await is state machine. The compiler will transform your async/await code into a state machine.

At the same time, in order for async/await to be really practicable in real projects, we need to have lots of Async I/O library functions already in place. For C#, most original synchronized I/O functions has an alternative Async version. The reason we need these Async functions is because in most cases, your own async/await code will boil down to some library Async method.

The Async version library functions in C# is kind of like the AsynchronousChannel concept in Java. For example, we have AsynchronousFileChannel.read which can either return a Future or execute a callback after the read operation is done. But it’s not exactly the same. All C# Async functions return Tasks (similar to Future but more powerful than Future).

So let’s say Java do support async/await, and we write some code like this:

public static async Future<Byte> readFirstByteAsync(String filePath) {
    Path path = Paths.get(filePath);
    AsynchronousFileChannel channel = AsynchronousFileChannel.open(path);

    ByteBuffer buffer = ByteBuffer.allocate(100_000);
    await channel.read(buffer, 0, buffer, this);
    return buffer.get(0);
}

Then I would imagine the compiler will transform the original async/await code into something like this:

public static Future<Byte> readFirstByteAsync(String filePath) {

    CompletableFuture<Byte> result = new CompletableFuture<Byte>();

    AsyncHandler ah = new AsyncHandler(result, filePath);

    ah.completed(null, null);

    return result;
}

And here is the implementation for AsyncHandler:

class AsyncHandler implements CompletionHandler<Integer, ByteBuffer>
{
    CompletableFuture<Byte> future;
    int state;
    String filePath;

    public AsyncHandler(CompletableFuture<Byte> future, String filePath)
    {
        this.future = future;
        this.state = 0;
        this.filePath = filePath;
    }

    @Override
    public void completed(Integer arg0, ByteBuffer arg1) {
        try {
            if (state == 0) {
                state = 1;
                Path path = Paths.get(filePath);
                AsynchronousFileChannel channel = AsynchronousFileChannel.open(path);

                ByteBuffer buffer = ByteBuffer.allocate(100_000);
                channel.read(buffer, 0, buffer, this);
                return;
            } else {
                Byte ret = arg1.get(0);
                future.complete(ret);
            }

        } catch (Exception e) {
            future.completeExceptionally(e);
        }
    }

    @Override
    public void failed(Throwable arg0, ByteBuffer arg1) {
        future.completeExceptionally(arg0);
    }
}

There is no equivalent of C# async/await in Java at the language level. A concept known as Fibers aka cooperative threads aka lightweight threads could be an interesting alternative. You can find Java libraries providing support for fibers.

Java libraries implementing Fibers

• JetLang
• Kilim
• Quasar

You can read this article (from Quasar) for a nice introduction to fibers. It covers what threads are, how fibers can be implemented on the JVM and has some Quasar specific code.