Replicate deferred/async launch policies from C++ in Java

Future

First of all, we have to observe that std::async is a tool to execute a given task and return a std::future object that holds the result of the computation once its available.

For example we can call result.get() to block and wait for the result to arrive. Also, when the computation encountered an exception, it will be stored and rethrown to us as soon as we call result.get().

Java provides similar classes, the interface is Future and the most relevant implementation is CompletableFuture.

std::future#get translates roughly to Future#get. Even the exceptional behavior is very similar. While C++ rethrows the exception upon calling get, Java will throw a ExecutionException which has the original exception set as cause.

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How to obtain a Future?

In C++ you create your future object using std::async. In Java you could use one of the many static helper methods in CompletableFuture. In your case, the most relevant are

• CompletableFuture#runAsync, if the task does not return any result and
• CompletableFuture#supplyAsync, if the task will return a result upon completion

So in order to create a future that just prints Hello World!, you could for example do

CompletableFuture<Void> task = CompletableFuture.runAsync(() -> System.out.println("Hello World!"));
/*...*/
task.get();

Java not only has lambdas but also method references. Lets say you have a method that computes a heavy math task:

class MyMath {
    static int compute() {
        // Very heavy, duh
        return (int) Math.pow(2, 5);
    }
}

Then you could create a future that returns the result once its available as

CompletableFuture<Integer> task = CompletableFuture.runAsync(MyMath::compute);
/*...*/
Integer result = task.get();

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async vs deferred

In C++, you have the option to specify a launch policy which dictates the threading behavior for the task. Let us put the memory promises C++ makes aside, because in Java you do not have that much control over memory.

The differences are that async will immediately schedule creation of a thread and execute the task in that thread. The result will be available at some point and is computed while you can continue work in your main task. The exact details whether it is a new thread or a cached thread depend on the compiler and are not specified.

deferred behaves completely different to that. Basically nothing happens when you call std::async, no extra thread will be created and the task will not be computed yet. The result will not be made available in the meantime at all. However, as soon as you call get, the task will be computed in your current thread and return a result. Basically as if you would have called the method directly yourself, without any async utilities at all.

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std::launch::async in Java

That said, lets focus on how to translate this behavior to Java. Lets start with async.

This is the simple one, as it is basically the default and intended behavior offered in CompletableFuture. So you just do runAsync or supplyAsync, depending on whether your method returns a result or not. Let me show the previous examples again:

// without result
CompletableFuture<Void> task = CompletableFuture.runAsync(() -> System.out.println("Hello World!"));
/*...*/ // the task is computed in the meantime in a different thread
task.get();

// with result
CompletableFuture<Integer> task = CompletableFuture.supplyAsync(MyMath::compute);
/*...*/
Integer result = task.get();

Note that there are also overloads of the methods that except an Executor which can be used if you have your own thread pool and want CompletableFuture to use that instead of its own (see here for more details).

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std::launch::deferred in Java

I tried around a lot to mock this behavior with CompletableFuture but it does not seem to be possibly without creating your own implementation (please correct me if I am wrong though). No matter what, it either executes directly upon creation or not at all.

So I would just propose to use the underlying task interface that you gave to CompletableFuture, for example Runnable or Supplier, directly. In our case, we might also use IntSupplier to avoid the autoboxing.

Here are the two code examples again, but this time with deferred behavior:

// without result
Runnable task = () -> System.out.println("Hello World!");
/*...*/ // the task is not computed in the meantime, no threads involved
task.run(); // the task is computed now

// with result
IntSupplier task = MyMath::compute;
/*...*/
int result = task.getAsInt();

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Modern multithreading in Java

As a final note I would like to give you a better idea how multithreading is typically used in Java nowadays. The provided facilities are much richer than what C++ offers by default.

Ideally should design your system in a way that you do not have to care about such little threading details. You create an automatically managed dynamic thread pool using Executors and then launch your initial task against that (or use the default executor service provided by CompletableFuture). After that, you just setup an operation pipeline on the future object, similar to the Stream API and then just wait on the final future object.

For example, let us suppose you have a list of file names List<String> fileNames and you want to

1. read the file
2. validate its content, skip it if its invalid
3. compress the file
4. upload the file to some web server
5. check the response status code

and count how many where invalid, not successfull and successfull. Suppose you have some methods like

class FileUploader {
    static byte[] readFile(String name) { /*...*/ }
    static byte[] requireValid(byte[] content) throws IllegalStateException { /*...*/ }
    static byte[] compressContent(byte[] content) { /*...*/ }
    static int uploadContent(byte[] content) { /*...*/ }
}

then we can do so easily by

AtomicInteger successfull = new AtomicInteger();
AtomicInteger notSuccessfull = new AtomicInteger();
AtomicInteger invalid = new AtomicInteger();

// Setup the pipeline
List<CompletableFuture<Void>> tasks = fileNames.stream()
    .map(name -> CompletableFuture
        .completedFuture(name)
        .thenApplyAsync(FileUploader::readFile)
        .thenApplyAsync(FileUploader::requireValid)
        .thenApplyAsync(FileUploader::compressContent)
        .thenApplyAsync(FileUploader::uploadContent)
        .handleAsync((statusCode, exception) -> {
            AtomicInteger counter;
            if (exception == null) {
                counter = statusCode == 200 ? successfull : notSuccessfull;
            } else {
                counter = invalid;
            }
            counter.incrementAndGet();
        })
    ).collect(Collectors.toList());

// Wait until all tasks are done
tasks.forEach(CompletableFuture::join);

// Print the results
System.out.printf("Successfull %d, not successfull %d, invalid %d%n", successfull.get(), notSuccessfull.get(), invalid.get());

The huge benefit of this is that it will reach max throughput and use all hardware capacity offered by your system. All tasks are executed completely dynamic and independent, managed by an automatic pool of threads. And you just wait until everything is done.

To get something like a deferred thread, you might try running a thread at a reduced priority.

First, in Java it's often idiomatic to make a task using a Runnable first. You can also use the Callable<T> interface, which allows the thread to return a value (Runnable can't).

public class MyTask implements Runnable {
  @Override
  public void run() {
    System.out.println( "hello thread." );
  }
}

Then just create a thread. In Java threads normally wrap the task they execute.

MyTask myTask = new MyTask();
Thread t = new Tread( myTask );
t.setPriority( Thread.currentThread().getPriority()-1 );
t.start();

This should not run until there is a core available to do so, which means it shouldn't run until the current thread is blocked or run out of things to do. However you're at the mercy of the OS scheduler here, so the specific operation is not guaranteed. Most OSs will guarantee that all threads run eventually, so if the current thread takes a long time with out blocking the OSs will start it executing anyway.

setPriority() can throw a security exception if you're not allowed to set the priority of a thread (uncommon but possible). So just be aware of that minor inconvenience.

For an asynch task with a Future I would use an executor service. The helper methods in the class Executors are a convenient way to do this.

First make your task as before.

public class MyCallable implements Callable<String> {
  @Override
  public String call() {
    return "hello future thread.";
  }
}

Then use an executor service to run it:

MyCallable myCallable = new MyCallable();
ExecutorService es = Executors.newCachedThreadPool();
Future<String> f = es.submit( myCallable );

You can use the Future object to query the thread, determine its running status and get the value it returns. You will need to shutdown the executor to stop all of its threads before exiting the JVM.

es.shutdown();

I've tried to write this code as simply as possible, without the use of lambdas or clever use of generics. The above should show you what those lambdas are actually implementing. However it's usually considered better to be a bit more sophisticated when writing code (and a bit less verbose) so you should investigate other syntax once you feel you understand the above.

If I understood you correctly, may be something like this:

private static CompletableFuture<Void> deferred(Runnable run) {
   CompletableFuture<Void> future = new CompletableFuture<>();
   future.thenRun(run);
   return future;
}

private static CompletableFuture<Void> async(Runnable run) {
    return CompletableFuture.runAsync(run);
}

And then using them like:

public static void main(String[] args)  throws Exception {
    CompletableFuture<Void> def = deferred(() -> System.out.println("run"));
    def.complete(null);
    System.out.println(def.join());

    CompletableFuture<Void> async = async(() -> System.out.println("run async"));
    async.join();
}

For asynchronous launch of a thread, in modern Java prefer the use of a high-level java.util.concurrent.ExecutorService.

One way to obtain an ExecutorService is through java.util.concurrent.Executors. Different behaviors are available for ExecutorServices; the Executors class provides methods for some common cases.

Once you have an ExecutorService, you can submit Runnables and Callables to it.

Future<MyReturnValue> myFuture = myExecutorService.submit(myTask);