Prerequisite: Future and callable
Future:
A Future interface provides methods to check if the computation is complete, to wait for its completion and to retrieve the results of the computation. The result is retrieved using Future’s get() method when the computation has completed, and it blocks until it is completed.
Future and FutureTask both are available in java.util.concurrent package from Java 1.5.
FutureTask:
- FutureTask is a concrete implementation of the Future, Runnable, and RunnableFuture interfaces and therefore can be submitted to an ExecutorService instance for execution.
- When calling ExecutorService.submit() on a Callable or Runnable instance, the ExecutorService returns a Future representing the task. and one can create it manually also.
- FutureTask acts similar to a CountDownLatch when calling get() in that it waits for the task to complete or error out.
- Behaviour of the parameterless get() method depends on the state of the task. If tasks are not completed, get() method blocks until the task is completed. Once the task complete, it returns the result or throws an ExecutionException.
- An overloaded variant of get() allows passing a timeout parameter to limit the amount of time the thread waits for a result.
Example: When submitting a FutureTask instance to a thread pool (ExecutorService instance) , it returns a Future object immediately. This Future object can be used for task completion and getting result of computation asynchronously.
Examples: Create two task. After one is completely executed, then after waiting 2000 millisecond, second task is being executed
Note: Online IDE does not work properly on sleep() method.
Java
// Java program do two FutureTask// using Runnable Interfaceimport java.util.concurrent.*;import java.util.logging.Level;import java.util.logging.Logger;class MyRunnable implements Runnable { private final long waitTime; public MyRunnable(int timeInMillis) { this.waitTime = timeInMillis; } @Override public void run() { try { // sleep for user given millisecond // before checking again Thread.sleep(waitTime); // return current thread name System.out.println(Thread .currentThread() .getName()); } catch (InterruptedException ex) { Logger .getLogger(MyRunnable.class.getName()) .log(Level.SEVERE, null, ex); } }}// Class FutureTaskExample execute two future taskclass FutureTaskExample { public static void main(String[] args) { // create two object of MyRunnable class // for FutureTask and sleep 1000, 2000 // millisecond before checking again MyRunnable myrunnableobject1 = new MyRunnable(1000); MyRunnable myrunnableobject2 = new MyRunnable(2000); FutureTask<String> futureTask1 = new FutureTask<>(myrunnableobject1, "FutureTask1 is complete"); FutureTask<String> futureTask2 = new FutureTask<>(myrunnableobject2, "FutureTask2 is complete"); // create thread pool of 2 size for ExecutorService ExecutorService executor = Executors.newFixedThreadPool(2); // submit futureTask1 to ExecutorService executor.submit(futureTask1); // submit futureTask2 to ExecutorService executor.submit(futureTask2); while (true) { try { // if both future task complete if (futureTask1.isDone() && futureTask2.isDone()) { System.out.println("Both FutureTask Complete"); // shut down executor service executor.shutdown(); return; } if (!futureTask1.isDone()) { // wait indefinitely for future // task to complete System.out.println("FutureTask1 output = " + futureTask1.get()); } System.out.println("Waiting for FutureTask2 to complete"); // Wait if necessary for the computation to complete, // and then retrieves its result String s = futureTask2.get(250, TimeUnit.MILLISECONDS); if (s != null) { System.out.println("FutureTask2 output=" + s); } } catch (Exception e) { System.out.println("Exception: " + e); } } }} |
Output
pool-1-thread-1 FutureTask1 output = FutureTask1 is complete Waiting for FutureTask2 to complete Exception: java.util.concurrent.TimeoutException Waiting for FutureTask2 to complete Exception: java.util.concurrent.TimeoutException Waiting for FutureTask2 to complete Exception: java.util.concurrent.TimeoutException Waiting for FutureTask2 to complete pool-1-thread-2 FutureTask2 output=FutureTask2 is complete Both FutureTask Complete
Reference:
- https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/Future.html
- https://docs.oracle.com/javase/7/docs/api/java/util/concurrent/FutureTask.html
