EDU.oswego.cs.dl.util.concurrent

Class QueuedExecutor

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 classQueuedExecutor.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.
Field Summary
protected static RunnableENDTASK
Special queue element to signal termination *
protected Channelqueue_
The queue *
protected QueuedExecutor.RunLooprunLoop_
protected booleanshutdown_
true if thread should shut down after processing current task *
protected Threadthread_
The thread used to process commands *
Constructor Summary
QueuedExecutor(Channel queue)
Construct a new QueuedExecutor that uses the supplied Channel as its queue.
QueuedExecutor()
Construct a new QueuedExecutor that uses a BoundedLinkedQueue with the current DefaultChannelCapacity as its queue.
Method Summary
protected voidclearThread()
set thread_ to null to indicate termination *
voidexecute(Runnable command)
Arrange for execution of the command in the background thread by adding it to the queue.
ThreadgetThread()
Return the thread being used to process commands, or null if there is no such thread.
voidrestart()
Start (or restart) the background thread to process commands.
voidshutdownAfterProcessingCurrentlyQueuedTasks()
Terminate background thread after it processes all elements currently in queue.
voidshutdownAfterProcessingCurrentTask()
Terminate background thread after it processes the current task, removing other queued tasks and leaving them unprocessed.
voidshutdownNow()
Terminate background thread even if it is currently processing a task.

Field Detail

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

thread_

protected Thread thread_
The thread used to process commands *

Constructor Detail

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.

QueuedExecutor

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

Method Detail

clearThread

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

execute

public void execute(Runnable command)
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.

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.

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.

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.

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.