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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.
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 or task to an happen-before any actions taken by that task, which in turn happen-before the result is retrieved via .
Method Summary | |
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. |
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. |
java.util.List |
invokeAll(java.util.Collection 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. |
java.lang.Object |
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. |
java.lang.Object |
invokeAny(java.util.Collection 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. |
boolean |
isShutdown()
Returns true if this executor has been shut down. |
boolean |
isTerminated()
Returns true if all tasks have completed following shut down. |
void |
shutdown()
Initiates an orderly shutdown in which previously submitted tasks are executed, but no new tasks will be accepted. |
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. |
Future |
submit(Callable task)
Submits a value-returning task for execution and returns a Future representing the pending results of the task. |
Future |
submit(java.lang.Runnable task)
Submits a Runnable task for execution and returns a Future representing that task. |
Future |
submit(java.lang.Runnable task,
java.lang.Object result)
Submits a Runnable task for execution and returns a Future representing that task. |
Methods inherited from interface edu.emory.mathcs.backport.java.util.concurrent.Executor |
execute |
Method Detail |
public void shutdown()
java.lang.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.public java.util.List shutdownNow()
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.
java.lang.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.public boolean isShutdown()
public boolean isTerminated()
public boolean awaitTermination(long timeout, TimeUnit unit) throws java.lang.InterruptedException
timeout
- the maximum time to waitunit
- the time unit of the timeout argument
java.lang.InterruptedException
- if interrupted while waitingpublic Future submit(Callable task)
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
RejectedExecutionException
- if the task cannot be
scheduled for execution
java.lang.NullPointerException
- if the task is nullpublic Future submit(java.lang.Runnable task, java.lang.Object result)
task
- the task to submitresult
- the result to return
RejectedExecutionException
- if the task cannot be
scheduled for execution
java.lang.NullPointerException
- if the task is nullpublic Future submit(java.lang.Runnable task)
task
- the task to submit
RejectedExecutionException
- if the task cannot be
scheduled for execution
java.lang.NullPointerException
- if the task is nullpublic java.util.List invokeAll(java.util.Collection tasks) throws java.lang.InterruptedException
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
java.lang.InterruptedException
- if interrupted while waiting, in
which case unfinished tasks are cancelled.
java.lang.NullPointerException
- if tasks or any of its elements are null
RejectedExecutionException
- if any task cannot be
scheduled for executionpublic java.util.List invokeAll(java.util.Collection tasks, long timeout, TimeUnit unit) throws java.lang.InterruptedException
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
java.lang.InterruptedException
- if interrupted while waiting, in
which case unfinished tasks are cancelled
java.lang.NullPointerException
- if tasks, any of its elements, or
unit are null
RejectedExecutionException
- if any task cannot be scheduled
for executionpublic java.lang.Object invokeAny(java.util.Collection tasks) throws java.lang.InterruptedException, ExecutionException
tasks
- the collection of tasks
java.lang.InterruptedException
- if interrupted while waiting
java.lang.NullPointerException
- if tasks or any of its elements
are null
java.lang.IllegalArgumentException
- if tasks is empty
ExecutionException
- if no task successfully completes
RejectedExecutionException
- if tasks cannot be scheduled
for executionpublic java.lang.Object invokeAny(java.util.Collection tasks, long timeout, TimeUnit unit) throws java.lang.InterruptedException, ExecutionException, TimeoutException
tasks
- the collection of taskstimeout
- the maximum time to waitunit
- the time unit of the timeout argument
java.lang.InterruptedException
- if interrupted while waiting
java.lang.NullPointerException
- if tasks, any of its elements, or
unit are 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
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