java.util.concurrent

Interface ExecutorService

All Superinterfaces:
Executor
Known Subinterfaces:
ScheduledExecutorService
Known Implementing Classes:
AbstractExecutorService, 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 stop accepting new tasks. After being shut down, the executor will eventually terminate, at which point no tasks are actively executing, no tasks are 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(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 Example

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 {
   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 serve() {
     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
   }
 }
 

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

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, T result)
Submits a Runnable task for execution and returns a Future representing that task.
List> invokeAll(Callable> tasks)
Executes the given tasks, returning a list of Futures holding their status and results when all complete.
List> invokeAll(Callable> 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(Callable> 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(Callable> 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
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.
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(Runnable task)
Submits a Runnable task for execution and returns a Future representing that task.

Methods inherited from interface java.util.concurrent.Executor

execute

Method Details

Future submit

public  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

Future submit

public  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

List> invokeAll

public  List> invokeAll(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.
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

List> invokeAll

public  List> invokeAll(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.
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

T invokeAny

public  T invokeAny(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.
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 of its elements are null
IllegalArgumentException - if tasks is empty
ExecutionException - if no task successfully completes
RejectedExecutionException - if tasks cannot be scheduled for execution

T invokeAny

public  T invokeAny(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.
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, any of its elements, or unit are 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

awaitTermination

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.
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

isShutdown

public boolean isShutdown()
Returns true if this executor has been shut down.
Returns:
true if this executor has been shut down

isTerminated

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.
Returns:
true if all tasks have completed following shut down

shutdown

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.
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

public 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.

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.

submit

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.
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