Java Equivalent of C# async/await?

Question

I am a normal C# developer but occasionally I develop application in Java. I'm wondering if there is any Java equivalent of C# async/await? In simple words what is the java equivalent of:

async Task<int> AccessTheWebAsync()
{ 
    HttpClient client = new HttpClient();
    var urlContents = await client.GetStringAsync("http://msdn.microsoft.com");
    return urlContents.Length;
}

Answer 1

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.)

Answer 2

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());
    }
}

Answer 3

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.

Answer 4

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);
    }
}

Answer 5

As it was mentioned, there is no direct equivalent, but very close approximation could be created with Java bytecode modifications (for both async/await-like instructions and underlying continuations implementation).

I'm working right now on a project that implements async/await on top of JavaFlow continuation library, please check https://github.com/vsilaev/java-async-await

No Maven mojo is created yet, but you may run examples with supplied Java agent. Here is how async/await code looks like:

public class AsyncAwaitNioFileChannelDemo {

public static void main(final String[] argv) throws Exception {

    ...
    final AsyncAwaitNioFileChannelDemo demo = new AsyncAwaitNioFileChannelDemo();
    final CompletionStage<String> result = demo.processFile("./.project");
    System.out.println("Returned to caller " + LocalTime.now());
    ...
}


public @async CompletionStage<String> processFile(final String fileName) throws IOException {
    final Path path = Paths.get(new File(fileName).toURI());
    try (
            final AsyncFileChannel file = new AsyncFileChannel(
                path, Collections.singleton(StandardOpenOption.READ), null
            );              
            final FileLock lock = await(file.lockAll(true))
        ) {

        System.out.println("In process, shared lock: " + lock);
        final ByteBuffer buffer = ByteBuffer.allocateDirect((int)file.size());

        await( file.read(buffer, 0L) );
        System.out.println("In process, bytes read: " + buffer);
        buffer.rewind();

        final String result = processBytes(buffer);

        return asyncResult(result);

    } catch (final IOException ex) {
        ex.printStackTrace(System.out);
        throw ex;
    }
}

@async is the annotation that flags a method as asynchronously executable, await() is a function that waits on CompletableFuture using continuations and a call to "return asyncResult(someValue)" is what finalizes associated CompletableFuture/Continuation

As with C#, control flow is preserved and exception handling may be done in regular manner (try/catch like in sequentially executed code)

Answer 6

Java itself has no equivalent features, but third-party libraries exist which offer similar functionality, e.g.Kilim.

Answer 7

Java doesn't have direct equivalent of C# language feature called async/await, however there's a different approach to the problem that async/await tries to solve. It's called project Loom, which will provide virtual threads for high-throughput concurrency. It will be available in some future version of OpenJDK.

This approach also solves "colored function problem" that async/await has.

Similar feature can be also found in Golang (goroutines).

Answer 8

First, understand what async/await is. It is a way for a single-threaded GUI application or an efficient server to run multiple "fibers" or "co-routines" or "lightweight threads" on a single thread. Of note is that it uses the cooperative form of multi-threading, as opposed to pre-emptive. It is meant to solve one of two, separate issues. The first is that the framework developer found it too hard to do concurrent programming (typical of GUI frameworks). The second is that OS threads have too much overhead, particularly memory usage (typical of server frameworks).

In case you don't have either problem, then just use regular OS multi-threading. This is often easier and faster than asynchronous programming. To launch threads, consider using ExecutorService.submit and Future.get or the upcoming StructuredTaskScope. Inside of those threads, use ordinary synchronous, blocking calls.

In case your problem is scalability and OS thread overhead, then know that Java 21 offers "virtual" threads. These are a lot like OS threads, but use much less memory. Go has a similar feature called goroutines.

For older versions of Java, you'll need to look to your framework, which has likely implemented some sort of "asynchronous" optimization. Many frameworks have done this, including Servlet 3.0. These don't have the elegance of language features, though. They work through ordinary callbacks.

In case your problem is concurrency, then you likewise need to look at your framework. For example, JavaFX offers javafx.concurrent.Task.

Answer 9

There is an "equivalent" of await developed by EA: https://github.com/electronicarts/ea-async. Refer to the Java example code:

import static com.ea.async.Async.await;
import static java.util.concurrent.CompletableFuture.completedFuture;

public class Store
{
    public CompletableFuture<Boolean> buyItem(String itemTypeId, int cost)
    {
        if(!await(bank.decrement(cost))) {
            return completedFuture(false);
        }
        await(inventory.giveItem(itemTypeId));
        return completedFuture(true);
    }
}

Answer 10

There isn't anything native to java that lets you do this like async/await keywords, but what you can do if you really want to is use a CountDownLatch. You could then imitate async/await by passing this around (at least in Java7). This is a common practice in Android unit testing where we have to make an async call (usually a runnable posted by a handler), and then await for the result (count down).

Using this however inside your application as opposed to your test is NOT what I am recommending. That would be extremely shoddy as CountDownLatch depends on you effectively counting down the right number of times and in the right places.

Answer 11

I make and released Java async/await library. https://github.com/stofu1234/kamaitachi

This library don't need compiler extension, and realize stackless IO processing in Java.

    async Task<int> AccessTheWebAsync(){ 
        HttpClient client= new HttpClient();
        var urlContents= await client.GetStringAsync("http://msdn.microsoft.com");
  　    return urlContents.Length;
    }

　　　↓

    //LikeWebApplicationTester.java
    BlockingQueue<Integer> AccessTheWebAsync() {
       HttpClient client = new HttpClient();
       return awaiter.await(
            () -> client.GetStringAsync("http://msdn.microsoft.com"),
            urlContents -> {
                return urlContents.length();
            });
    }
    public void doget(){
        BlockingQueue<Integer> lengthQueue=AccessTheWebAsync();
        awaiter.awaitVoid(()->lengthQueue.take(),
            length->{
                System.out.println("Length:"+length);
            }
            );
    }

Answer 12

Java has unfortunately no equivalent of async/await. The closest you can get is probably with ListenableFuture from Guava and listener chaining, but it would be still very cumbersome to write for cases involving multiple asynchronous calls, as the nesting level would very quickly grow.

If you're ok with using a different language on top of JVM, fortunately there is async/await in Scala which is a direct C# async/await equivalent with an almost identical syntax and semantics: https://github.com/scala/async/

Note that although this functionality needed a pretty advanced compiler support in C#, in Scala it could be added as a library thanks to a very powerful macro system in Scala and therefore can be added even to older versions of Scala like 2.10. Additionally Scala is class-compatible with Java, so you can write the async code in Scala and then call it from Java.

There is also another similar project called Akka Dataflow http://doc.akka.io/docs/akka/2.3-M1/scala/dataflow.html which uses different wording but conceptually is very similar, however implemented using delimited continuations, not macros (so it works with even older Scala versions like 2.9).

Answer 13

If you're just after clean code which simulates the same effect as async/await in java and don't mind blocking the thread it is called on until it is finished, such as in a test, you could use something like this code:

interface Async {
    void run(Runnable handler);
}

static void await(Async async) throws InterruptedException {

    final CountDownLatch countDownLatch = new CountDownLatch(1);
    async.run(new Runnable() {

        @Override
        public void run() {
            countDownLatch.countDown();
        }
    });
    countDownLatch.await(YOUR_TIMEOUT_VALUE_IN_SECONDS, TimeUnit.SECONDS);
}

    await(new Async() {
        @Override
        public void run(final Runnable handler) {
            yourAsyncMethod(new CompletionHandler() {

                @Override
                public void completion() {
                    handler.run();
                }
            });
        }
    });

Answer 14

I have developed a library JAsync to do this. It is just released today. It makes the developer's asynchronous programming experience as close as possible to the usual synchronous programming, including code style and debugging. Here is the example.

@RestController
@RequestMapping("/employees")
public class MyRestController {
    @Inject
    private EmployeeRepository employeeRepository;
    @Inject
    private SalaryRepository salaryRepository;

    // The standard JAsync async method must be annotated with the Async annotation, and return a Promise object.
    @Async()
    private Promise<Double> _getEmployeeTotalSalaryByDepartment(String department) {
        double money = 0.0;
        // A Mono object can be transformed to the Promise object. So we get a Mono object first.
        Mono<List<Employee>> empsMono = employeeRepository.findEmployeeByDepartment(department);
        // Transformed the Mono object to the Promise object.
        Promise<List<Employee>> empsPromise = JAsync.from(empsMono);
        // Use await just like es and c# to get the value of the Promise without blocking the current thread.
        for (Employee employee : empsPromise.await()) {
            // The method findSalaryByEmployee also return a Mono object. We transform it to the Promise just like above. And then await to get the result.
            Salary salary = JAsync.from(salaryRepository.findSalaryByEmployee(employee.id)).await();
            money += salary.total;
        }
        // The async method must return a Promise object, so we use just method to wrap the result to a Promise.
        return JAsync.just(money);
    }

    // This is a normal webflux method.
    @GetMapping("/{department}/salary")
    public Mono<Double> getEmployeeTotalSalaryByDepartment(@PathVariable String department) { 
        // Use unwrap method to transform the Promise object back to the Mono object.
        return _getEmployeeTotalSalaryByDepartment(department).unwrap(Mono.class);
    }
}

And in debug mode, you can see all the variable just like the synchronous code.

The other great thing about this project is that it's one of the few projects of its kind that's still active right now. It's just been released, so it has a lot of potential