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java.util.concurrent
public interface: ExecutorService [javadoc | source]

All Implemented Interfaces:
    Executor

All Known Implementing Classes:
    ThreadPoolExecutor, DelegatedExecutorService, ScheduledThreadPoolExecutor, FinalizableDelegatedExecutorService, ForkJoinPool, AbstractExecutorService, DelegatedScheduledExecutorService, ScheduledExecutorService

An Executor that provides methods to manage termination and methods that can produce a Future for tracking progress of one or more asynchronous tasks.

An ExecutorService can be shut down, which will cause it to reject new tasks. Two different methods are provided for shutting down an ExecutorService. The #shutdown method will allow previously submitted tasks to execute before terminating, while the #shutdownNow method prevents waiting tasks from starting and attempts to stop currently executing tasks. Upon termination, an executor has no tasks actively executing, no tasks awaiting execution, and no new tasks can be submitted. An unused ExecutorService should be shut down to allow reclamation of its resources.

Method submit extends base method Executor#execute by creating and returning a Future that can be used to cancel execution and/or wait for completion. Methods invokeAny and invokeAll perform the most commonly useful forms of bulk execution, executing a collection of tasks and then waiting for at least one, or all, to complete. (Class ExecutorCompletionService can be used to write customized variants of these methods.)

The Executors class provides factory methods for the executor services provided in this package.

Usage Examples

Here is a sketch of a network service in which threads in a thread pool service incoming requests. It uses the preconfigured Executors#newFixedThreadPool factory method:
class NetworkService implements Runnable {
  private final ServerSocket serverSocket;
  private final ExecutorService pool;

  public NetworkService(int port, int poolSize)
      throws IOException {
    serverSocket = new ServerSocket(port);
    pool = Executors.newFixedThreadPool(poolSize);
  }

  public void run() { // run the service
    try {
      for (;;) {
        pool.execute(new Handler(serverSocket.accept()));
      }
    } catch (IOException ex) {
      pool.shutdown();
    }
  }
}

class Handler implements Runnable {
  private final Socket socket;
  Handler(Socket socket) { this.socket = socket; }
  public void run() {
    // read and service request on socket
  }
}
The following method shuts down an ExecutorService in two phases, first by calling shutdown to reject incoming tasks, and then calling shutdownNow, if necessary, to cancel any lingering tasks:
void shutdownAndAwaitTermination(ExecutorService pool) {
  pool.shutdown(); // Disable new tasks from being submitted
  try {
    // Wait a while for existing tasks to terminate
    if (!pool.awaitTermination(60, TimeUnit.SECONDS)) {
      pool.shutdownNow(); // Cancel currently executing tasks
      // Wait a while for tasks to respond to being cancelled
      if (!pool.awaitTermination(60, TimeUnit.SECONDS))
          System.err.println("Pool did not terminate");
    }
  } catch (InterruptedException ie) {
    // (Re-)Cancel if current thread also interrupted
    pool.shutdownNow();
    // Preserve interrupt status
    Thread.currentThread().interrupt();
  }
}

Memory consistency effects: Actions in a thread prior to the submission of a {@code Runnable} or {@code Callable} task to an {@code ExecutorService} happen-before any actions taken by that task, which in turn happen-before the result is retrieved via {@code Future.get()}.

Method from java.util.concurrent.ExecutorService Summary:
awaitTermination,   invokeAll,   invokeAll,   invokeAny,   invokeAny,   isShutdown,   isTerminated,   shutdown,   shutdownNow,   submit,   submit,   submit
Method from java.util.concurrent.ExecutorService Detail:
 public boolean awaitTermination(long timeout,
    TimeUnit unit) throws InterruptedException
    Blocks until all tasks have completed execution after a shutdown request, or the timeout occurs, or the current thread is interrupted, whichever happens first.
 public List<T> invokeAll(Collection<Callable> tasks) throws InterruptedException
    Executes the given tasks, returning a list of Futures holding their status and results when all complete. Future#isDone is true for each element of the returned list. Note that a completed task could have terminated either normally or by throwing an exception. The results of this method are undefined if the given collection is modified while this operation is in progress.
 public List<T> invokeAll(Collection<Callable> tasks,
    long timeout,
    TimeUnit unit) throws InterruptedException
    Executes the given tasks, returning a list of Futures holding their status and results when all complete or the timeout expires, whichever happens first. Future#isDone is true for each element of the returned list. Upon return, tasks that have not completed are cancelled. Note that a completed task could have terminated either normally or by throwing an exception. The results of this method are undefined if the given collection is modified while this operation is in progress.
 public T invokeAny(Collection<Callable> tasks) throws InterruptedException, ExecutionException
    Executes the given tasks, returning the result of one that has completed successfully (i.e., without throwing an exception), if any do. Upon normal or exceptional return, tasks that have not completed are cancelled. The results of this method are undefined if the given collection is modified while this operation is in progress.
 public T invokeAny(Collection<Callable> tasks,
    long timeout,
    TimeUnit unit) throws InterruptedException, ExecutionException, TimeoutException
    Executes the given tasks, returning the result of one that has completed successfully (i.e., without throwing an exception), if any do before the given timeout elapses. Upon normal or exceptional return, tasks that have not completed are cancelled. The results of this method are undefined if the given collection is modified while this operation is in progress.
 public boolean isShutdown()
    Returns true if this executor has been shut down.
 public boolean isTerminated()
    Returns true if all tasks have completed following shut down. Note that isTerminated is never true unless either shutdown or shutdownNow was called first.
 public  void shutdown()
    Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted. Invocation has no additional effect if already shut down.

    This method does not wait for previously submitted tasks to complete execution. Use awaitTermination to do that.

 public List<Runnable> shutdownNow()
    Attempts to stop all actively executing tasks, halts the processing of waiting tasks, and returns a list of the tasks that were awaiting execution.

    This method does not wait for actively executing tasks to terminate. Use awaitTermination to do that.

    There are no guarantees beyond best-effort attempts to stop processing actively executing tasks. For example, typical implementations will cancel via Thread#interrupt , so any task that fails to respond to interrupts may never terminate.

 public Future<T> submit(Callable<T> task)
    Submits a value-returning task for execution and returns a Future representing the pending results of the task. The Future's get method will return the task's result upon successful completion.

    If you would like to immediately block waiting for a task, you can use constructions of the form result = exec.submit(aCallable).get();

    Note: The Executors class includes a set of methods that can convert some other common closure-like objects, for example, java.security.PrivilegedAction to Callable form so they can be submitted.

 public Future<?> submit(Runnable task)
    Submits a Runnable task for execution and returns a Future representing that task. The Future's get method will return null upon successful completion.
 public Future<T> submit(Runnable task,
    T result)
    Submits a Runnable task for execution and returns a Future representing that task. The Future's get method will return the given result upon successful completion.