How to implement non blocking Futures in Java

Question

The Java Future object is used to get the result of asynchronous computation which is performed by a parallel thread(Executors). We call Future.get() method and wait until result is ready. This example shows a non blocking way for retrieving result from Future. java-implement-java-non-blocking-futures.

NonBlockingExecutor executor = new NonBlockingExecutor(Executors.newSingleThreadExecutor());

NonBlockingFuture<Integer> future = executor.submitNonBlocking(new Callable<Integer>() {

            @Override
            public Integer call() throws Exception {
                String threadName = Thread.currentThread().getName();
                System.out.println(threadName);
                //print -> pool-1-thread-1
                return 1;
            }
});

future.setHandler(new FutureHandler<Integer>() {

       @Override
       public void onSuccess(Integer value) {
            String threadName = Thread.currentThread().getName();
            System.out.println(threadName);
            //print -> pool-1-thread-1
       }

       @Override
       public void onFailure(Throwable e) {
            System.out.println(e.getMessage());
       }
 });

 Thread.sleep(50000);

In this onSuccess() method is called after parallel execution is finish. The problem is onSuccess() method is not running on the main thread. I want to perform onSuccess() method on the main thread. How can i fix this. Thanks

Answer 1

This is supported with CompletableFutures.

    CompletableFuture.runAsync(() -> {
        String threadName = Thread.currentThread().getName();
        System.out.println(threadName);
        //print -> pool-1-thread-1
    }).whenComplete((task, throwable) -> {
        if(throwable != null) {
           System.out.println(e.getMessage());
        } else {
            String threadName = Thread.currentThread().getName();
            System.out.println(threadName);
            //print -> pool-1-thread-1
        }
    });

The caveat here is that the future would run the whenComplete task on the executing thread and not the submitting thread.