ExecutorServicepublic interface ExecutorService implements Executor| An {@link Executor} that provides methods to manage termination and
methods that can produce a {@link 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 {@link #shutdown}
method will allow previously submitted tasks to execute before
terminating, while the {@link #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 {@link
Executor#execute} by creating and returning a {@link 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 {@link ExecutorCompletionService} can be used to
write customized variants of these methods.)
The {@link 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 preconfigured {@link
Executors#newFixedThreadPool} 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 {@code Runnable} or {@code Callable} task to an
{@code ExecutorService}
happen-before
any actions taken by that task, which in turn happen-before the
result is retrieved via {@code Future.get()}. |
| Methods Summary |
|---|
| public boolean | awaitTermination(long timeout, java.util.concurrent.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.
| | public java.util.List | invokeAll(java.util.Collection tasks, long timeout, java.util.concurrent.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.
{@link 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.
| | public java.util.List | invokeAll(java.util.Collection tasks)Executes the given tasks, returning a list of Futures holding
their status and results when all complete.
{@link 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.
| | public T | invokeAny(java.util.Collection 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.
| | public T | invokeAny(java.util.Collection tasks, long timeout, java.util.concurrent.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.
| | public boolean | isShutdown()Returns true if this executor has been shut down.
| | 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.
| | 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.
| | public java.util.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 {@link Thread#interrupt}, so any
task that fails to respond to interrupts may never terminate.
| | public java.util.concurrent.Future | submit(java.util.concurrent.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 {@link Executors} class includes a set of methods
that can convert some other common closure-like objects,
for example, {@link java.security.PrivilegedAction} to
{@link Callable} form so they can be submitted.
| | public java.util.concurrent.Future | submit(java.lang.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.
| | public java.util.concurrent.Future | submit(java.lang.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.
|
|
