- All Superinterfaces:
Executor
- All Known Subinterfaces:
ScheduledExecutorService
- All Known Implementing Classes:
AbstractExecutorService,ForkJoinPool,ScheduledThreadPoolExecutor,ThreadPoolExecutor
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.
Usage Examples
Here is a sketch of a network service in which threads in a thread pool service incoming requests. It uses the preconfiguredExecutors.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 ex) {
// (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
Modifier and TypeMethodDescriptionbooleanawaitTermination(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.invokeAll(Collection<? extends Callable<T>> tasks) Executes the given tasks, returning a list of Futures holding their status and results when all complete.invokeAll(Collection<? extends Callable<T>> 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> TinvokeAny(Collection<? extends Callable<T>> tasks) Executes the given tasks, returning the result of one that has completed successfully (i.e., without throwing an exception), if any do.<T> TinvokeAny(Collection<? extends Callable<T>> 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.booleanReturnstrueif this executor has been shut down.booleanReturnstrueif all tasks have completed following shut down.voidshutdown()Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted.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<?>Submits a Runnable task for execution and returns a Future representing that task.<T> Future<T>Submits a Runnable task for execution and returns a Future representing that task.<T> Future<T>Submits a value-returning task for execution and returns a Future representing the pending results of the task.
-
Method Details
-
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
awaitTerminationto 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 holdRuntimePermission("modifyThread"), or the security manager'scheckAccessmethod denies access.
-
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
awaitTerminationto 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 holdRuntimePermission("modifyThread"), or the security manager'scheckAccessmethod denies access.
-
isShutdown
boolean isShutdown()Returnstrueif this executor has been shut down.- Returns:
trueif this executor has been shut down
-
isTerminated
boolean isTerminated()Returnstrueif all tasks have completed following shut down. Note thatisTerminatedis nevertrueunless eithershutdownorshutdownNowwas called first.- Returns:
trueif all tasks have completed following shut down
-
awaitTermination
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 waitunit- the time unit of the timeout argument- Returns:
trueif this executor terminated andfalseif the timeout elapsed before termination- Throws:
InterruptedException- if interrupted while waiting
-
submit
Submits a value-returning task for execution and returns a Future representing the pending results of the task. The Future'sgetmethod 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
Executorsclass includes a set of methods that can convert some other common closure-like objects, for example,PrivilegedActiontoCallableform so they can be submitted.- Type Parameters:
T- the type of the task's result- Parameters:
task- the task to submit- Returns:
- a Future representing pending completion of the task
- Throws:
RejectedExecutionException- if the task cannot be scheduled for executionNullPointerException- if the task is null
-
submit
Submits a Runnable task for execution and returns a Future representing that task. The Future'sgetmethod will return the given result upon successful completion.- Type Parameters:
T- the type of the result- Parameters:
task- the task to submitresult- the result to return- Returns:
- a Future representing pending completion of the task
- Throws:
RejectedExecutionException- if the task cannot be scheduled for executionNullPointerException- if the task is null
-
submit
Submits a Runnable task for execution and returns a Future representing that task. The Future'sgetmethod will returnnullupon 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 executionNullPointerException- if the task is null
-
invokeAll
Executes the given tasks, returning a list of Futures holding their status and results when all complete.Future.isDone()istruefor 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.- Type Parameters:
T- the type of the values returned from the tasks- 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 cancelledNullPointerException- if tasks or any of its elements arenullRejectedExecutionException- if any task cannot be scheduled for execution
-
invokeAll
<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()istruefor 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.- Type Parameters:
T- the type of the values returned from the tasks- Parameters:
tasks- the collection of taskstimeout- the maximum time to waitunit- 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 cancelledNullPointerException- if tasks, any of its elements, or unit arenullRejectedExecutionException- if any task cannot be scheduled for execution
-
invokeAny
<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.- Type Parameters:
T- the type of the values returned from the tasks- Parameters:
tasks- the collection of tasks- Returns:
- the result returned by one of the tasks
- Throws:
InterruptedException- if interrupted while waitingNullPointerException- if tasks or any element task subject to execution isnullIllegalArgumentException- if tasks is emptyExecutionException- if no task successfully completesRejectedExecutionException- if tasks cannot be scheduled for execution
-
invokeAny
<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.- Type Parameters:
T- the type of the values returned from the tasks- Parameters:
tasks- the collection of taskstimeout- the maximum time to waitunit- the time unit of the timeout argument- Returns:
- the result returned by one of the tasks
- Throws:
InterruptedException- if interrupted while waitingNullPointerException- if tasks, or unit, or any element task subject to execution isnullTimeoutException- if the given timeout elapses before any task successfully completesExecutionException- if no task successfully completesRejectedExecutionException- if tasks cannot be scheduled for execution
-