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(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.
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().
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.
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.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.
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.boolean isShutdown()
Returns true if this executor has been shut down.
true if this executor has been shut downboolean 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.
true if all tasks have completed following shut downboolean 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.
timeout - the maximum time to waitunit - the time unit of the timeout argumenttrue if this executor terminated and false if the timeout elapsed before terminationInterruptedException - if interrupted while waiting<T> 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, PrivilegedAction to Callable form so they can be submitted.
T - the type of the task's resulttask - the task to submitRejectedExecutionException - if the task cannot be scheduled for executionNullPointerException - if the task is null<T> 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.
T - the type of the resulttask - the task to submitresult - the result to returnRejectedExecutionException - if the task cannot be scheduled for executionNullPointerException - if the task is nullFuture<?> 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.
task - the task to submitRejectedExecutionException - if the task cannot be scheduled for executionNullPointerException - if the task is null<T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> 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.
T - the type of the values returned from the taskstasks - the collection of tasksInterruptedException - if interrupted while waiting, in which case unfinished tasks are cancelledNullPointerException - if tasks or any of its elements are null
RejectedExecutionException - if any task cannot be scheduled for execution<T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> 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.
T - the type of the values returned from the taskstasks - the collection of taskstimeout - the maximum time to waitunit - the time unit of the timeout argumentInterruptedException - if interrupted while waiting, in which case unfinished tasks are cancelledNullPointerException - if tasks, any of its elements, or unit are null
RejectedExecutionException - if any task cannot be scheduled for execution<T> T invokeAny(Collection<? extends Callable<T>> 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.
T - the type of the values returned from the taskstasks - the collection of tasksInterruptedException - if interrupted while waitingNullPointerException - if tasks or any element task subject to execution is null
IllegalArgumentException - if tasks is emptyExecutionException - if no task successfully completesRejectedExecutionException - if tasks cannot be scheduled for execution<T> T invokeAny(Collection<? extends Callable<T>> 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.
T - the type of the values returned from the taskstasks - the collection of taskstimeout - the maximum time to waitunit - the time unit of the timeout argumentInterruptedException - if interrupted while waitingNullPointerException - if tasks, or unit, or any element task subject to execution is null
TimeoutException - if the given timeout elapses before any task successfully completesExecutionException - if no task successfully completesRejectedExecutionException - if tasks cannot be scheduled for execution
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