EDU.oswego.cs.dl.util.concurrent

Class QueuedExecutor

Implemented Interfaces:
Executor

public class QueuedExecutor
extends ThreadFactoryUser
implements Executor

An implementation of Executor that queues incoming requests until they can be processed by a single background thread.

The thread is not actually started until the first execute request is encountered. Also, if the thread is stopped for any reason (for example, after hitting an unrecoverable exception in an executing task), one is started upon encountering a new request, or if restart() is invoked.

Beware that, especially in situations where command objects themselves invoke execute, queuing can sometimes lead to lockups, since commands that might allow other threads to terminate do not run at all when they are in the queue.

[ Introduction to this package. ]

Nested Class Summary

protected class
QueuedExecutor.RunLoop
The runloop is isolated in its own Runnable class just so that the main class need not implement Runnable, which would allow others to directly invoke run, which would never make sense here.

Nested classes/interfaces inherited from class EDU.oswego.cs.dl.util.concurrent.ThreadFactoryUser

ThreadFactoryUser.DefaultThreadFactory

Field Summary

protected static Runnable
ENDTASK
Special queue element to signal termination *
protected Channel
queue_
The queue *
protected QueuedExecutor.RunLoop
runLoop_
protected boolean
shutdown_
true if thread should shut down after processing current task *
protected Thread
thread_
The thread used to process commands *

Fields inherited from class EDU.oswego.cs.dl.util.concurrent.ThreadFactoryUser

threadFactory_

Constructor Summary

QueuedExecutor()
Construct a new QueuedExecutor that uses a BoundedLinkedQueue with the current DefaultChannelCapacity as its queue.
QueuedExecutor(Channel queue)
Construct a new QueuedExecutor that uses the supplied Channel as its queue.

Method Summary

protected void
clearThread()
set thread_ to null to indicate termination *
void
execute(Runnable command)
Arrange for execution of the command in the background thread by adding it to the queue.
Thread
getThread()
Return the thread being used to process commands, or null if there is no such thread.
void
restart()
Start (or restart) the background thread to process commands.
void
shutdownAfterProcessingCurrentTask()
Terminate background thread after it processes the current task, removing other queued tasks and leaving them unprocessed.
void
shutdownAfterProcessingCurrentlyQueuedTasks()
Terminate background thread after it processes all elements currently in queue.
void
shutdownNow()
Terminate background thread even if it is currently processing a task.

Methods inherited from class EDU.oswego.cs.dl.util.concurrent.ThreadFactoryUser

getThreadFactory, setThreadFactory

Field Details

ENDTASK

protected static Runnable ENDTASK
Special queue element to signal termination *

queue_

protected final Channel queue_
The queue *

runLoop_

protected final QueuedExecutor.RunLoop runLoop_

shutdown_

protected boolean shutdown_
true if thread should shut down after processing current task *

thread_

protected Thread thread_
The thread used to process commands *

Constructor Details

QueuedExecutor

public QueuedExecutor()
Construct a new QueuedExecutor that uses a BoundedLinkedQueue with the current DefaultChannelCapacity as its queue.

QueuedExecutor

public QueuedExecutor(Channel queue)
Construct a new QueuedExecutor that uses the supplied Channel as its queue.

This class does not support any methods that reveal this queue. If you need to access it independently (for example to invoke any special status monitoring operations), you should record a reference to it separately.

Method Details

clearThread

protected void clearThread()
set thread_ to null to indicate termination *

execute

public void execute(Runnable command)
            throws InterruptedException
Arrange for execution of the command in the background thread by adding it to the queue. The method may block if the channel's put operation blocks.

If the background thread does not exist, it is created and started.

Specified by:
execute in interface Executor

getThread

public Thread getThread()
Return the thread being used to process commands, or null if there is no such thread. You can use this to invoke any special methods on the thread, for example, to interrupt it.

restart

public void restart()
Start (or restart) the background thread to process commands. It has no effect if a thread is already running. This method can be invoked if the background thread crashed due to an unrecoverable exception.

shutdownAfterProcessingCurrentTask

public void shutdownAfterProcessingCurrentTask()
Terminate background thread after it processes the current task, removing other queued tasks and leaving them unprocessed. A shut down thread cannot be restarted.

shutdownAfterProcessingCurrentlyQueuedTasks

public void shutdownAfterProcessingCurrentlyQueuedTasks()
Terminate background thread after it processes all elements currently in queue. Any tasks entered after this point will not be processed. A shut down thread cannot be restarted. This method may block if the task queue is finite and full. Also, this method does not in general apply (and may lead to comparator-based exceptions) if the task queue is a priority queue.

shutdownNow

public void shutdownNow()
Terminate background thread even if it is currently processing a task. This method uses Thread.interrupt, so relies on tasks themselves responding appropriately to interruption. If the current tasks does not terminate on interruption, then the thread will not terminate until processing current task. A shut down thread cannot be restarted.