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 stop accepting new tasks. After being shut down, the executor
will eventually terminate, at which point no tasks are actively
executing, no tasks are 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 Example
Here is a sketch of a network service in which threads in a thread
pool service incoming requests. It uses the preconfigured
Executors.newFixedThreadPool
factory method:
class NetworkService {
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 serve() {
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
}
}
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()
.
Future submit
public Future submit(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
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.
task
- the task to submit
- a Future representing pending completion of the task
Future submit
public Future 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.
task
- the task to submitresult
- the result to return
- a Future representing pending completion of the task
List> invokeAll
public List> invokeAll(Callable> 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.
tasks
- the collection of tasks
- 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.
List> invokeAll
public List> invokeAll(Callable> 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.
tasks
- the collection of taskstimeout
- the maximum time to waitunit
- the time unit of the timeout argument
- 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.
T invokeAny
public T invokeAny(Callable> 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.
tasks
- the collection of tasks
- the result returned by one of the tasks
T invokeAny
public T invokeAny(Callable> 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.
tasks
- the collection of taskstimeout
- the maximum time to waitunit
- the time unit of the timeout argument
- the result returned by one of the tasks.
awaitTermination
public boolean 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 argument
- true if this executor terminated and
false if the timeout elapsed before termination
isShutdown
public boolean isShutdown()
Returns true if this executor has been shut down.
- true if this executor has been shut down
isTerminated
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.
- true if all tasks have completed following shut down
shutdown
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.
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.
shutdownNow
public 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
Thread.interrupt()
, so any
task that fails to respond to interrupts may never terminate.
- list of tasks that never commenced execution
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.
submit
public 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.
task
- the task to submit
- a Future representing pending completion of the task