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Java Concurrency Util

1 java.util.concurrent - Java Concurrency Utilities
2 BlockingQueue
3 ArrayBlockingQueue
4 DelayQueue
5 LinkedBlockingQueue
6 PriorityBlockingQueue
7 SynchronousQueue
8 BlockingDeque
9 LinkedBlockingDeque
10 ConcurrentMap
11 ConcurrentNavigableMap
12 CountDownLatch
13 CyclicBarrier
14 Exchanger
15 Semaphore
16 ExecutorService
17 ThreadPoolExecutor
18 ScheduledExecutorService
19 Lock
20 ReadWriteLock
21 AtomicBoolean
22 AtomicInteger
23 AtomicLong
24 AtomicReference




ScheduledExecutorService


The java.util.concurrent.ScheduledExecutorService is an ExecutorService which can schedule tasks to run after a delay, or to execute repeatedly with a fixed interval of time in between each execution. Tasks are executed asynchronously by a worker thread, and not by the thread handing the task to the ScheduledExecutorService.

ScheduledExecutorService Example

Here is a simple ScheduledExecutorService example:

ScheduledExecutorService scheduledExecutorService =
        Executors.newScheduledThreadPool(5);

ScheduledFuture scheduledFuture =
    scheduledExecutorService.schedule(new Callable() {
        public Object call() throws Exception {
            System.out.println("Executed!");
            return "Called!";
        }
    },
    5,
    TimeUnit.SECONDS);

First a ScheduledExecutorService is created with 5 threads in. Then an anonymous implementation of the Callable interface is created and passed to the schedule() method. The two last parameters specify that the Callable should be executed after 5 seconds.

ScheduledExecutorService Implementations

Since ScheduledExecutorService is an interface, you will have to use its implementation in the java.util.concurrent package, in order to use it. ScheduledExecutorService as the following implementation:

  • ScheduledThreadPoolExecutor

Creating a ScheduledExecutorService

How you create an ScheduledExecutorService depends on the implementation you use. However, you can use the Executors factory class to create ScheduledExecutorService instances too. Here is an example:

ScheduledExecutorService scheduledExecutorService =

        Executors.newScheduledThreadPool(5);

ScheduledExecutorService Usage

Once you have created a ScheduledExecutorService you use it by calling one of its methods:

  • schedule (Callable task, long delay, TimeUnit timeunit)
  • schedule (Runnable task, long delay, TimeUnit timeunit)
  • scheduleAtFixedRate (Runnable, long initialDelay, long period, TimeUnit timeunit)
  • scheduleWithFixedDelay (Runnable, long initialDelay, long period, TimeUnit timeunit)

I will briefly cover each of these methods below.

schedule (Callable task, long delay, TimeUnit timeunit)

This method schedules the given Callable for execution after the given delay.

The method returns a ScheduledFuture which you can use to either cancel the task before it has started executing, or obtain the result once it is executed.

Here is an example:

ScheduledExecutorService scheduledExecutorService =
        Executors.newScheduledThreadPool(5);

ScheduledFuture scheduledFuture =
    scheduledExecutorService.schedule(new Callable() {
        public Object call() throws Exception {
            System.out.println("Executed!");
            return "Called!";
        }
    },
    5,
    TimeUnit.SECONDS);

System.out.println("result = " + scheduledFuture.get());

scheduledExecutorService.shutdown();

This example outputs:

Executed!
result = Called!

schedule (Runnable task, long delay, TimeUnit timeunit)

This method works like the method version taking a Callable as parameter, except a Runnable cannot return a value, so the ScheduledFuture.get() method returns null when the task is finished.

scheduleAtFixedRate (Runnable, long initialDelay, long period, TimeUnit timeunit)

This method schedules a task to be executed periodically. The task is executed the first time after the initialDelay, and then recurringly every time the period expires.

If any execution of the given task throws an exception, the task is no longer executed. If no exceptions are thrown, the task will continue to be executed until the ScheduledExecutorService is shut down.

If a task takes longer to execute than the period between its scheduled executions, the next execution will start after the current execution finishes. The scheduled task will not be executed by more than one thread at a time.

scheduleWithFixedDelay (Runnable, long initialDelay, long period, TimeUnit timeunit)

This method works very much like scheduleAtFixedRate() except that the period is interpreted differently.

In the scheduleAtFixedRate() method the period is interpreted as a delay between the start of the previous execution, until the start of the next execution.

In this method, however, the period is interpreted as the delay between the end of the previous execution, until the start of the next. The delay is thus between finished executions, not between the beginning of executions.

ScheduledExecutorService Shutdown

Just like an ExecutorService, the ScheduledExecutorService needs to be shut down when you are finished using it. If not, it will keep the JVM running, even when all other threads have been shut down.

You shut down a ScheduledExecutorService using the shutdown() or shutdownNow() methods which are inherited from the ExecutorService interface. See the ExecutorService Shutdown section for more information.



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