edu.emory.mathcs.backport.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().

Author:
Doug Lea
Since:
1.5

Method Summary

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.
Object
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.
Object
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(Runnable task)
Submits a Runnable task for execution and returns a Future representing that task.
Future
submit(Runnable task, Object result)
Submits a Runnable task for execution and returns a Future representing that task.
Future
submit(Callable task)
Submits a value-returning task for execution and returns a Future representing the pending results of the task.

Methods inherited from interface edu.emory.mathcs.backport.java.util.concurrent.Executor

execute

Method Details

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

invokeAll

public 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.

invokeAll

public 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.

invokeAny

public Object 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:
ExecutionException - if no task successfully completes

invokeAny

public Object 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:
TimeoutException - if the given timeout elapses before any task successfully completes
ExecutionException - if no task successfully completes

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.

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

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

submit

public Future submit(Runnable task,
                     Object 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

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, java.security.PrivilegedAction to Callable form so they can be submitted.

Parameters:
task - the task to submit
Returns:
a Future representing pending completion of the task