java.util.concurrent.ExecutorService

java.util.concurrent

Interface ExecutorService

• All Superinterfaces:

  Executor

  All Known Subinterfaces:

  ScheduledExecutorService

  All Known Implementing Classes:

  AbstractExecutorService,  ForkJoinPool,  ScheduledThreadPoolExecutor,  ThreadPoolExecutor

  ---

  
  

  public interface ExecutorService
  extends Executor

  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(java.lang.Runnable) 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(int) 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 Runnable or Callable task to an ExecutorService happen-before any actions taken by that task, which in turn happen-before the result is retrieved via Future.get().

  Since:

  1.5

• • Method Summary

    Methods  

    Modifier and Type	Method and Description
    boolean	awaitTermination(long timeout, TimeUnit unit) Blocks until all tasks have completed execution after a shutdown request, or the timeout occurs, or the current thread is interrupted, whichever happens first.
    List>	invokeAll(Collection> tasks) Executes the given tasks, returning a list of Futures holding their status and results when all complete.
    List>	invokeAll(Collection> tasks, long timeout, TimeUnit unit) Executes the given tasks, returning a list of Futures holding their status and results when all complete or the timeout expires, whichever happens first.
    T	invokeAny(Collection> tasks) Executes the given tasks, returning the result of one that has completed successfully (i.e., without throwing an exception), if any do.
    T	invokeAny(Collection> tasks, long timeout, TimeUnit unit) 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.
    boolean	isShutdown() Returns  true if this executor has been shut down.
    boolean	isTerminated() Returns  true if all tasks have completed following shut down.
    void	shutdown() Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted.
    List	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.
    Future	submit(Callable task) Submits a value-returning task for execution and returns a Future representing the pending results of the task.
    Future	submit(Runnable task) Submits a Runnable task for execution and returns a Future representing that task.
    Future	submit(Runnable task, T result) Submits a Runnable task for execution and returns a Future representing that task.

    • Methods inherited from interface java.util.concurrent.Executor

      execute

• • Method Detail

    • shutdown

      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.

      Throws:

      SecurityException - if a security manager exists and shutting down this ExecutorService may manipulate threads that the caller is not permitted to modify because it does not hold  RuntimePermission ("modifyThread"), or the security manager's  checkAccess method denies access.

    • shutdownNow

      List 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.

      Returns:

      list of tasks that never commenced execution

      Throws:

      SecurityException - if a security manager exists and shutting down this ExecutorService may manipulate threads that the caller is not permitted to modify because it does not hold  RuntimePermission ("modifyThread"), or the security manager's  checkAccess method denies access.

    • isShutdown

      boolean isShutdown()

      Returns  true if this executor has been shut down.

      Returns:

      true if this executor has been shut down

    • isTerminated

      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.

      Returns:

      true if all tasks have completed following shut down

    • awaitTermination

      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.

      Parameters:

      timeout - the maximum time to wait

      unit - the time unit of the timeout argument

      Returns:

      true if this executor terminated and  false if the timeout elapsed before termination

      Throws:

      InterruptedException - if interrupted while waiting

    • submit

       Future submit(Callable 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, PrivilegedAction to Callable form so they can be submitted.

      Parameters:

      task - the task to submit

      Returns:

      a Future representing pending completion of the task

      Throws:

      RejectedExecutionException - if the task cannot be scheduled for execution

      NullPointerException - if the task is null

    • submit

       Future 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.

      Parameters:

      task - the task to submit

      result - the result to return

      Returns:

      a Future representing pending completion of the task

      Throws:

      RejectedExecutionException - if the task cannot be scheduled for execution

      NullPointerException - if the task is null

    • submit

      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.

      Parameters:

      task - the task to submit

      Returns:

      a Future representing pending completion of the task

      Throws:

      RejectedExecutionException - if the task cannot be scheduled for execution

      NullPointerException - if the task is null

    • invokeAll

       List> invokeAll(Collection> 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.

      Parameters:

      tasks - the collection of tasks

      Returns:

      A list of Futures representing the tasks, in the same sequential order as produced by the iterator for the given task list, each of which has completed.

      Throws:

      InterruptedException - if interrupted while waiting, in which case unfinished tasks are cancelled.

      NullPointerException - if tasks or any of its elements are  null

      RejectedExecutionException - if any task cannot be scheduled for execution

    • invokeAll

       List> invokeAll(Collection> 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.

      Parameters:

      tasks - the collection of tasks

      timeout - the maximum time to wait

      unit - the time unit of the timeout argument

      Returns:

      a list of Futures representing the tasks, in the same sequential order as produced by the iterator for the given task list. If the operation did not time out, each task will have completed. If it did time out, some of these tasks will not have completed.

      Throws:

      InterruptedException - if interrupted while waiting, in which case unfinished tasks are cancelled

      NullPointerException - if tasks, any of its elements, or unit are  null

      RejectedExecutionException - if any task cannot be scheduled for execution

    • invokeAny

       T invokeAny(Collection> 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.

      Parameters:

      tasks - the collection of tasks

      Returns:

      the result returned by one of the tasks

      Throws:

      InterruptedException - if interrupted while waiting

      NullPointerException - if tasks or any element task subject to execution is  null

      IllegalArgumentException - if tasks is empty

      ExecutionException - if no task successfully completes

      RejectedExecutionException - if tasks cannot be scheduled for execution

    • invokeAny

       T invokeAny(Collection> 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.

      Parameters:

      tasks - the collection of tasks

      timeout - the maximum time to wait

      unit - the time unit of the timeout argument

      Returns:

      the result returned by one of the tasks.

      Throws:

      InterruptedException - if interrupted while waiting

      NullPointerException - if tasks, or unit, or any element task subject to execution is  null

      TimeoutException - if the given timeout elapses before any task successfully completes

      ExecutionException - if no task successfully completes

      RejectedExecutionException - if tasks cannot be scheduled for execution