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 ExecutorService.shutdown()
method will allow previously submitted tasks to execute before
terminating, while the ExecutorService.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 preconfigured
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()
.