ExecutorService
public
interface
ExecutorService
implements
AutoCloseable,
Executor
| java.util.concurrent.ExecutorService |
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 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
}
}
An ExecutorService may also be established and closed
(shutdown, blocking until terminated) as follows; illustrating with
a different Executors factory method:
try (ExecutorService e = Executors.newWorkStealingPool()) {
// submit or execute many tasks with e ...
}
Further customization is also possible. For example, 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().
Summary
Public methods | |
|---|---|
abstract
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. |
default
void
|
close()
Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted. |
abstract
<T>
List<Future<T>>
|
invokeAll(Collection<? extends Callable<T>> tasks)
Executes the given tasks, returning a list of Futures holding their status and results when all complete. |
abstract
<T>
List<Future<T>>
|
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. |
abstract
<T>
T
|
invokeAny(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. |
abstract
<T>
T
|
invokeAny(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. |
abstract
boolean
|
isShutdown()
Returns |
abstract
boolean
|
isTerminated()
Returns |
abstract
void
|
shutdown()
Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted. |
abstract
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. |
abstract
Future<?>
|
submit(Runnable task)
Submits a Runnable task for execution and returns a Future representing that task. |
abstract
<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. |
abstract
<T>
Future<T>
|
submit(Runnable task, T result)
Submits a Runnable task for execution and returns a Future representing that task. |
Inherited methods | |
|---|---|
Public methods
awaitTermination
public abstract 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.
| Parameters | |
|---|---|
timeout |
long: the maximum time to wait |
unit |
TimeUnit: the time unit of the timeout argument |
| Returns | |
|---|---|
boolean |
true if this executor terminated and
false if the timeout elapsed before termination |
| Throws | |
|---|---|
InterruptedException |
if interrupted while waiting |
close
public void close ()
Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted. This method waits until all tasks have completed execution and the executor has terminated.
If interrupted while waiting, this method stops all executing tasks as
if by invoking shutdownNow(). It then continues to wait until all
actively executing tasks have completed. Tasks that were awaiting
execution are not executed. The interrupt status will be re-asserted
before this method returns.
If already terminated, invoking this method has no effect.
Implementation Requirements:
- The default implementation invokes
shutdown()and waits for tasks to complete execution withawaitTermination.
| 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. |
invokeAll
public abstract List<Future<T>> invokeAll (Collection<? extends Callable<T>> tasks)
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 |
Collection: the collection of tasks |
| Returns | |
|---|---|
List<Future<T>> |
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
public abstract List<Future<T>> 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.
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 |
Collection: the collection of tasks |
timeout |
long: the maximum time to wait |
unit |
TimeUnit: the time unit of the timeout argument |
| Returns | |
|---|---|
List<Future<T>> |
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
public abstract T invokeAny (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. 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 |
Collection: the collection of tasks |
| Returns | |
|---|---|
T |
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
public abstract T invokeAny (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. 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 |
Collection: the collection of tasks |
timeout |
long: the maximum time to wait |
unit |
TimeUnit: the time unit of the timeout argument |
| Returns | |
|---|---|
T |
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 |
isShutdown
public abstract boolean isShutdown ()
Returns true if this executor has been shut down.
| Returns | |
|---|---|
boolean |
true if this executor has been shut down |
isTerminated
public abstract 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 | |
|---|---|
boolean |
true if all tasks have completed following shut down |
shutdown
public abstract 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.
shutdownNow
public abstract 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.
| Returns | |
|---|---|
List<Runnable> |
list of tasks that never commenced execution |
submit
public abstract 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 |
Runnable: the task to submit |
| Returns | |
|---|---|
Future<?> |
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
public abstract 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.
| Parameters | |
|---|---|
task |
Callable: the task to submit |
| Returns | |
|---|---|
Future<T> |
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
public abstract 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.
| Parameters | |
|---|---|
task |
Runnable: the task to submit |
result |
T: the result to return |
| Returns | |
|---|---|
Future<T> |
a Future representing pending completion of the task |
| Throws | |
|---|---|
RejectedExecutionException |
if the task cannot be scheduled for execution |
NullPointerException |
if the task is null |