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ThreadPoolExecutor.javaAPI DocJava SE 5 API55011Fri Aug 26 14:57:26 BST 2005java.util.concurrent

ThreadPoolExecutor

public class ThreadPoolExecutor extends AbstractExecutorService
An {@link ExecutorService} that executes each submitted task using one of possibly several pooled threads, normally configured using {@link Executors} factory methods.

Thread pools address two different problems: they usually provide improved performance when executing large numbers of asynchronous tasks, due to reduced per-task invocation overhead, and they provide a means of bounding and managing the resources, including threads, consumed when executing a collection of tasks. Each ThreadPoolExecutor also maintains some basic statistics, such as the number of completed tasks.

To be useful across a wide range of contexts, this class provides many adjustable parameters and extensibility hooks. However, programmers are urged to use the more convenient {@link Executors} factory methods {@link Executors#newCachedThreadPool} (unbounded thread pool, with automatic thread reclamation), {@link Executors#newFixedThreadPool} (fixed size thread pool) and {@link Executors#newSingleThreadExecutor} (single background thread), that preconfigure settings for the most common usage scenarios. Otherwise, use the following guide when manually configuring and tuning this class:

Core and maximum pool sizes
A ThreadPoolExecutor will automatically adjust the pool size (see {@link ThreadPoolExecutor#getPoolSize}) according to the bounds set by corePoolSize (see {@link ThreadPoolExecutor#getCorePoolSize}) and maximumPoolSize (see {@link ThreadPoolExecutor#getMaximumPoolSize}). When a new task is submitted in method {@link ThreadPoolExecutor#execute}, and fewer than corePoolSize threads are running, a new thread is created to handle the request, even if other worker threads are idle. If there are more than corePoolSize but less than maximumPoolSize threads running, a new thread will be created only if the queue is full. By setting corePoolSize and maximumPoolSize the same, you create a fixed-size thread pool. By setting maximumPoolSize to an essentially unbounded value such as Integer.MAX_VALUE, you allow the pool to accommodate an arbitrary number of concurrent tasks. Most typically, core and maximum pool sizes are set only upon construction, but they may also be changed dynamically using {@link ThreadPoolExecutor#setCorePoolSize} and {@link ThreadPoolExecutor#setMaximumPoolSize}.
On-demand construction
By default, even core threads are initially created and started only when needed by new tasks, but this can be overridden dynamically using method {@link ThreadPoolExecutor#prestartCoreThread} or {@link ThreadPoolExecutor#prestartAllCoreThreads}.
Creating new threads
New threads are created using a {@link java.util.concurrent.ThreadFactory}. If not otherwise specified, a {@link Executors#defaultThreadFactory} is used, that creates threads to all be in the same {@link ThreadGroup} and with the same NORM_PRIORITY priority and non-daemon status. By supplying a different ThreadFactory, you can alter the thread's name, thread group, priority, daemon status, etc. If a ThreadFactory fails to create a thread when asked by returning null from newThread, the executor will continue, but might not be able to execute any tasks.
Keep-alive times
If the pool currently has more than corePoolSize threads, excess threads will be terminated if they have been idle for more than the keepAliveTime (see {@link ThreadPoolExecutor#getKeepAliveTime}). This provides a means of reducing resource consumption when the pool is not being actively used. If the pool becomes more active later, new threads will be constructed. This parameter can also be changed dynamically using method {@link ThreadPoolExecutor#setKeepAliveTime}. Using a value of Long.MAX_VALUE {@link TimeUnit#NANOSECONDS} effectively disables idle threads from ever terminating prior to shut down.
Queuing
Any {@link BlockingQueue} may be used to transfer and hold submitted tasks. The use of this queue interacts with pool sizing:
  • If fewer than corePoolSize threads are running, the Executor always prefers adding a new thread rather than queuing.
  • If corePoolSize or more threads are running, the Executor always prefers queuing a request rather than adding a new thread.
  • If a request cannot be queued, a new thread is created unless this would exceed maximumPoolSize, in which case, the task will be rejected.
There are three general strategies for queuing:
  1. Direct handoffs. A good default choice for a work queue is a {@link SynchronousQueue} that hands off tasks to threads without otherwise holding them. Here, an attempt to queue a task will fail if no threads are immediately available to run it, so a new thread will be constructed. This policy avoids lockups when handling sets of requests that might have internal dependencies. Direct handoffs generally require unbounded maximumPoolSizes to avoid rejection of new submitted tasks. This in turn admits the possibility of unbounded thread growth when commands continue to arrive on average faster than they can be processed.
  2. Unbounded queues. Using an unbounded queue (for example a {@link LinkedBlockingQueue} without a predefined capacity) will cause new tasks to be queued in cases where all corePoolSize threads are busy. Thus, no more than corePoolSize threads will ever be created. (And the value of the maximumPoolSize therefore doesn't have any effect.) This may be appropriate when each task is completely independent of others, so tasks cannot affect each others execution; for example, in a web page server. While this style of queuing can be useful in smoothing out transient bursts of requests, it admits the possibility of unbounded work queue growth when commands continue to arrive on average faster than they can be processed.
  3. Bounded queues. A bounded queue (for example, an {@link ArrayBlockingQueue}) helps prevent resource exhaustion when used with finite maximumPoolSizes, but can be more difficult to tune and control. Queue sizes and maximum pool sizes may be traded off for each other: Using large queues and small pools minimizes CPU usage, OS resources, and context-switching overhead, but can lead to artificially low throughput. If tasks frequently block (for example if they are I/O bound), a system may be able to schedule time for more threads than you otherwise allow. Use of small queues generally requires larger pool sizes, which keeps CPUs busier but may encounter unacceptable scheduling overhead, which also decreases throughput.
Rejected tasks
New tasks submitted in method {@link ThreadPoolExecutor#execute} will be rejected when the Executor has been shut down, and also when the Executor uses finite bounds for both maximum threads and work queue capacity, and is saturated. In either case, the execute method invokes the {@link RejectedExecutionHandler#rejectedExecution} method of its {@link RejectedExecutionHandler}. Four predefined handler policies are provided:
  1. In the default {@link ThreadPoolExecutor.AbortPolicy}, the handler throws a runtime {@link RejectedExecutionException} upon rejection.
  2. In {@link ThreadPoolExecutor.CallerRunsPolicy}, the thread that invokes execute itself runs the task. This provides a simple feedback control mechanism that will slow down the rate that new tasks are submitted.
  3. In {@link ThreadPoolExecutor.DiscardPolicy}, a task that cannot be executed is simply dropped.
  4. In {@link ThreadPoolExecutor.DiscardOldestPolicy}, if the executor is not shut down, the task at the head of the work queue is dropped, and then execution is retried (which can fail again, causing this to be repeated.)
It is possible to define and use other kinds of {@link RejectedExecutionHandler} classes. Doing so requires some care especially when policies are designed to work only under particular capacity or queuing policies.
Hook methods
This class provides protected overridable {@link ThreadPoolExecutor#beforeExecute} and {@link ThreadPoolExecutor#afterExecute} methods that are called before and after execution of each task. These can be used to manipulate the execution environment; for example, reinitializing ThreadLocals, gathering statistics, or adding log entries. Additionally, method {@link ThreadPoolExecutor#terminated} can be overridden to perform any special processing that needs to be done once the Executor has fully terminated.

If hook or callback methods throw exceptions, internal worker threads may in turn fail and abruptly terminate.

Queue maintenance
Method {@link ThreadPoolExecutor#getQueue} allows access to the work queue for purposes of monitoring and debugging. Use of this method for any other purpose is strongly discouraged. Two supplied methods, {@link ThreadPoolExecutor#remove} and {@link ThreadPoolExecutor#purge} are available to assist in storage reclamation when large numbers of queued tasks become cancelled.

Extension example. Most extensions of this class override one or more of the protected hook methods. For example, here is a subclass that adds a simple pause/resume feature:

class PausableThreadPoolExecutor extends ThreadPoolExecutor {
private boolean isPaused;
private ReentrantLock pauseLock = new ReentrantLock();
private Condition unpaused = pauseLock.newCondition();

public PausableThreadPoolExecutor(...) { super(...); }

protected void beforeExecute(Thread t, Runnable r) {
super.beforeExecute(t, r);
pauseLock.lock();
try {
while (isPaused) unpaused.await();
} catch(InterruptedException ie) {
t.interrupt();
} finally {
pauseLock.unlock();
}
}

public void pause() {
pauseLock.lock();
try {
isPaused = true;
} finally {
pauseLock.unlock();
}
}

public void resume() {
pauseLock.lock();
try {
isPaused = false;
unpaused.signalAll();
} finally {
pauseLock.unlock();
}
}
}
since
1.5
author
Doug Lea

Fields Summary
private static final Runnable[]
EMPTY_RUNNABLE_ARRAY
Only used to force toArray() to produce a Runnable[].
private static final RuntimePermission
shutdownPerm
Permission for checking shutdown
private final BlockingQueue
workQueue
Queue used for holding tasks and handing off to worker threads.
private final ReentrantLock
mainLock
Lock held on updates to poolSize, corePoolSize, maximumPoolSize, and workers set.
private final Condition
termination
Wait condition to support awaitTermination
private final HashSet
workers
Set containing all worker threads in pool.
private volatile long
keepAliveTime
Timeout in nanoseconds for idle threads waiting for work. Threads use this timeout only when there are more than corePoolSize present. Otherwise they wait forever for new work.
private volatile int
corePoolSize
Core pool size, updated only while holding mainLock, but volatile to allow concurrent readability even during updates.
private volatile int
maximumPoolSize
Maximum pool size, updated only while holding mainLock but volatile to allow concurrent readability even during updates.
private volatile int
poolSize
Current pool size, updated only while holding mainLock but volatile to allow concurrent readability even during updates.
volatile int
runState
Lifecycle state
static final int
RUNNING
Normal, not-shutdown mode
static final int
SHUTDOWN
Controlled shutdown mode
static final int
STOP
Immediate shutdown mode
static final int
TERMINATED
Final state
private volatile RejectedExecutionHandler
handler
Handler called when saturated or shutdown in execute.
private volatile ThreadFactory
threadFactory
Factory for new threads.
private int
largestPoolSize
Tracks largest attained pool size.
private long
completedTaskCount
Counter for completed tasks. Updated only on termination of worker threads.
private static final RejectedExecutionHandler
defaultHandler
The default rejected execution handler
Constructors Summary
public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue workQueue, RejectedExecutionHandler handler)
Creates a new ThreadPoolExecutor with the given initial parameters.

param
corePoolSize the number of threads to keep in the pool, even if they are idle.
param
maximumPoolSize the maximum number of threads to allow in the pool.
param
keepAliveTime when the number of threads is greater than the core, this is the maximum time that excess idle threads will wait for new tasks before terminating.
param
unit the time unit for the keepAliveTime argument.
param
workQueue the queue to use for holding tasks before they are executed. This queue will hold only the Runnable tasks submitted by the execute method.
param
handler the handler to use when execution is blocked because the thread bounds and queue capacities are reached.
throws
IllegalArgumentException if corePoolSize, or keepAliveTime less than zero, or if maximumPoolSize less than or equal to zero, or if corePoolSize greater than maximumPoolSize.
throws
NullPointerException if workQueue or handler are null.

        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             Executors.defaultThreadFactory(), handler);
    
public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue workQueue, ThreadFactory threadFactory, RejectedExecutionHandler handler)
Creates a new ThreadPoolExecutor with the given initial parameters.

param
corePoolSize the number of threads to keep in the pool, even if they are idle.
param
maximumPoolSize the maximum number of threads to allow in the pool.
param
keepAliveTime when the number of threads is greater than the core, this is the maximum time that excess idle threads will wait for new tasks before terminating.
param
unit the time unit for the keepAliveTime argument.
param
workQueue the queue to use for holding tasks before they are executed. This queue will hold only the Runnable tasks submitted by the execute method.
param
threadFactory the factory to use when the executor creates a new thread.
param
handler the handler to use when execution is blocked because the thread bounds and queue capacities are reached.
throws
IllegalArgumentException if corePoolSize, or keepAliveTime less than zero, or if maximumPoolSize less than or equal to zero, or if corePoolSize greater than maximumPoolSize.
throws
NullPointerException if workQueue or threadFactory or handler are null.

        if (corePoolSize < 0 ||
            maximumPoolSize <= 0 ||
            maximumPoolSize < corePoolSize ||
            keepAliveTime < 0)
            throw new IllegalArgumentException();
        if (workQueue == null || threadFactory == null || handler == null)
            throw new NullPointerException();
        this.corePoolSize = corePoolSize;
        this.maximumPoolSize = maximumPoolSize;
        this.workQueue = workQueue;
        this.keepAliveTime = unit.toNanos(keepAliveTime);
        this.threadFactory = threadFactory;
        this.handler = handler;
    
public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue workQueue)
Creates a new ThreadPoolExecutor with the given initial parameters and default thread factory and handler. It may be more convenient to use one of the {@link Executors} factory methods instead of this general purpose constructor.

param
corePoolSize the number of threads to keep in the pool, even if they are idle.
param
maximumPoolSize the maximum number of threads to allow in the pool.
param
keepAliveTime when the number of threads is greater than the core, this is the maximum time that excess idle threads will wait for new tasks before terminating.
param
unit the time unit for the keepAliveTime argument.
param
workQueue the queue to use for holding tasks before they are executed. This queue will hold only the Runnable tasks submitted by the execute method.
throws
IllegalArgumentException if corePoolSize, or keepAliveTime less than zero, or if maximumPoolSize less than or equal to zero, or if corePoolSize greater than maximumPoolSize.
throws
NullPointerException if workQueue is null

        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             Executors.defaultThreadFactory(), defaultHandler);
    
public ThreadPoolExecutor(int corePoolSize, int maximumPoolSize, long keepAliveTime, TimeUnit unit, BlockingQueue workQueue, ThreadFactory threadFactory)
Creates a new ThreadPoolExecutor with the given initial parameters.

param
corePoolSize the number of threads to keep in the pool, even if they are idle.
param
maximumPoolSize the maximum number of threads to allow in the pool.
param
keepAliveTime when the number of threads is greater than the core, this is the maximum time that excess idle threads will wait for new tasks before terminating.
param
unit the time unit for the keepAliveTime argument.
param
workQueue the queue to use for holding tasks before they are executed. This queue will hold only the Runnable tasks submitted by the execute method.
param
threadFactory the factory to use when the executor creates a new thread.
throws
IllegalArgumentException if corePoolSize, or keepAliveTime less than zero, or if maximumPoolSize less than or equal to zero, or if corePoolSize greater than maximumPoolSize.
throws
NullPointerException if workQueue or threadFactory are null.

        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             threadFactory, defaultHandler);
    
Methods Summary
private booleanaddIfUnderCorePoolSize(java.lang.Runnable firstTask)
Create and start a new thread running firstTask as its first task, only if fewer than corePoolSize threads are running.

param
firstTask the task the new thread should run first (or null if none)
return
true if successful.

        Thread t = null;
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            if (poolSize < corePoolSize)
                t = addThread(firstTask);
        } finally {
            mainLock.unlock();
        }
        if (t == null)
            return false;
        t.start();
        return true;
    
private java.lang.RunnableaddIfUnderMaximumPoolSize(java.lang.Runnable firstTask)
Create and start a new thread only if fewer than maximumPoolSize threads are running. The new thread runs as its first task the next task in queue, or if there is none, the given task.

param
firstTask the task the new thread should run first (or null if none)
return
null on failure, else the first task to be run by new thread.

        Thread t = null;
        Runnable next = null;
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            if (poolSize < maximumPoolSize) {
                next = workQueue.poll();
                if (next == null)
                    next = firstTask;
                t = addThread(next);
            }
        } finally {
            mainLock.unlock();
        }
        if (t == null)
            return null;
        t.start();
        return next;
    
private java.lang.ThreadaddThread(java.lang.Runnable firstTask)
Create and return a new thread running firstTask as its first task. Call only while holding mainLock

param
firstTask the task the new thread should run first (or null if none)
return
the new thread, or null if threadFactory fails to create thread

        Worker w = new Worker(firstTask);
        Thread t = threadFactory.newThread(w);
        if (t != null) {
            w.thread = t;
            workers.add(w);
            int nt = ++poolSize;
            if (nt > largestPoolSize)
                largestPoolSize = nt;
        }
        return t;
    
protected voidafterExecute(java.lang.Runnable r, java.lang.Throwable t)
Method invoked upon completion of execution of the given Runnable. This method is invoked by the thread that executed the task. If non-null, the Throwable is the uncaught exception that caused execution to terminate abruptly. Note: To properly nest multiple overridings, subclasses should generally invoke super.afterExecute at the beginning of this method.

param
r the runnable that has completed.
param
t the exception that caused termination, or null if execution completed normally.

 
public booleanawaitTermination(long timeout, java.util.concurrent.TimeUnit unit)

        long nanos = unit.toNanos(timeout);
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            for (;;) {
                if (runState == TERMINATED) 
                    return true;
                if (nanos <= 0)
                    return false;
                nanos = termination.awaitNanos(nanos);
            }
        } finally {
            mainLock.unlock();
        }
    
protected voidbeforeExecute(java.lang.Thread t, java.lang.Runnable r)
Method invoked prior to executing the given Runnable in the given thread. This method is invoked by thread t that will execute task r, and may be used to re-initialize ThreadLocals, or to perform logging. Note: To properly nest multiple overridings, subclasses should generally invoke super.beforeExecute at the end of this method.

param
t the thread that will run task r.
param
r the task that will be executed.

 
public voidexecute(java.lang.Runnable command)
Executes the given task sometime in the future. The task may execute in a new thread or in an existing pooled thread. If the task cannot be submitted for execution, either because this executor has been shutdown or because its capacity has been reached, the task is handled by the current RejectedExecutionHandler.

param
command the task to execute
throws
RejectedExecutionException at discretion of RejectedExecutionHandler, if task cannot be accepted for execution
throws
NullPointerException if command is null

        if (command == null)
            throw new NullPointerException();
        for (;;) {
            if (runState != RUNNING) {
                reject(command);
                return;
            }
            if (poolSize < corePoolSize && addIfUnderCorePoolSize(command))
                return;
            if (workQueue.offer(command))
                return;
            Runnable r = addIfUnderMaximumPoolSize(command);
            if (r == command)
                return;
            if (r == null) {
                reject(command);
                return;
            }
            // else retry
        }
    
protected voidfinalize()
Invokes shutdown when this executor is no longer referenced.

        shutdown();
    
public intgetActiveCount()
Returns the approximate number of threads that are actively executing tasks.

return
the number of threads

        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            int n = 0;
            for (Worker w : workers) {
                if (w.isActive())
                    ++n;
            }
            return n;
        } finally {
            mainLock.unlock();
        }
    
public longgetCompletedTaskCount()
Returns the approximate total number of tasks that have completed execution. Because the states of tasks and threads may change dynamically during computation, the returned value is only an approximation, but one that does not ever decrease across successive calls.

return
the number of tasks

        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            long n = completedTaskCount;
            for (Worker w : workers)
                n += w.completedTasks;
            return n;
        } finally {
            mainLock.unlock();
        }
    
public intgetCorePoolSize()
Returns the core number of threads.

return
the core number of threads
see
#setCorePoolSize

        return corePoolSize;
    
public longgetKeepAliveTime(java.util.concurrent.TimeUnit unit)
Returns the thread keep-alive time, which is the amount of time which threads in excess of the core pool size may remain idle before being terminated.

param
unit the desired time unit of the result
return
the time limit
see
#setKeepAliveTime

        return unit.convert(keepAliveTime, TimeUnit.NANOSECONDS);
    
public intgetLargestPoolSize()
Returns the largest number of threads that have ever simultaneously been in the pool.

return
the number of threads

        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            return largestPoolSize;
        } finally {
            mainLock.unlock();
        }
    
public intgetMaximumPoolSize()
Returns the maximum allowed number of threads.

return
the maximum allowed number of threads
see
#setMaximumPoolSize

        return maximumPoolSize;
    
public intgetPoolSize()
Returns the current number of threads in the pool.

return
the number of threads

        return poolSize;
    
public java.util.concurrent.BlockingQueuegetQueue()
Returns the task queue used by this executor. Access to the task queue is intended primarily for debugging and monitoring. This queue may be in active use. Retrieving the task queue does not prevent queued tasks from executing.

return
the task queue

        return workQueue;
    
public java.util.concurrent.RejectedExecutionHandlergetRejectedExecutionHandler()
Returns the current handler for unexecutable tasks.

return
the current handler
see
#setRejectedExecutionHandler

        return handler;
    
java.lang.RunnablegetTask()
Get the next task for a worker thread to run.

return
the task
throws
InterruptedException if interrupted while waiting for task

        for (;;) {
            switch(runState) {
            case RUNNING: {
                if (poolSize <= corePoolSize)   // untimed wait if core
                    return workQueue.take();
                
                long timeout = keepAliveTime;
                if (timeout <= 0) // die immediately for 0 timeout
                    return null;
                Runnable r =  workQueue.poll(timeout, TimeUnit.NANOSECONDS);
                if (r != null)
                    return r;
                if (poolSize > corePoolSize) // timed out
                    return null;
                // else, after timeout, pool shrank so shouldn't die, so retry
                break;
            }

            case SHUTDOWN: {
                // Help drain queue 
                Runnable r = workQueue.poll();
                if (r != null)
                    return r;
                    
                // Check if can terminate
                if (workQueue.isEmpty()) {
                    interruptIdleWorkers();
                    return null;
                }

                // There could still be delayed tasks in queue.
                // Wait for one, re-checking state upon interruption
                try {
                    return workQueue.take();
                } catch(InterruptedException ignore) {}
                break;
            }

            case STOP:
                return null;
            default:
                assert false; 
            }
        }
    
public longgetTaskCount()
Returns the approximate total number of tasks that have been scheduled for execution. Because the states of tasks and threads may change dynamically during computation, the returned value is only an approximation, but one that does not ever decrease across successive calls.

return
the number of tasks

        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            long n = completedTaskCount;
            for (Worker w : workers) {
                n += w.completedTasks;
                if (w.isActive())
                    ++n;
            }
            return n + workQueue.size();
        } finally {
            mainLock.unlock();
        }
    
public java.util.concurrent.ThreadFactorygetThreadFactory()
Returns the thread factory used to create new threads.

return
the current thread factory
see
#setThreadFactory

        return threadFactory;
    
voidinterruptIdleWorkers()
Wake up all threads that might be waiting for tasks.

        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            for (Worker w : workers)
                w.interruptIfIdle();
        } finally {
            mainLock.unlock();
        }
    
public booleanisShutdown()

        return runState != RUNNING;
    
public booleanisTerminated()

        return runState == TERMINATED;
    
public booleanisTerminating()
Returns true if this executor is in the process of terminating after shutdown or shutdownNow but has not completely terminated. This method may be useful for debugging. A return of true reported a sufficient period after shutdown may indicate that submitted tasks have ignored or suppressed interruption, causing this executor not to properly terminate.

return
true if terminating but not yet terminated.

        return runState == STOP;
    
public intprestartAllCoreThreads()
Starts all core threads, causing them to idly wait for work. This overrides the default policy of starting core threads only when new tasks are executed.

return
the number of threads started.

        int n = 0;
        while (addIfUnderCorePoolSize(null))
            ++n;
        return n;
    
public booleanprestartCoreThread()
Starts a core thread, causing it to idly wait for work. This overrides the default policy of starting core threads only when new tasks are executed. This method will return false if all core threads have already been started.

return
true if a thread was started

        return addIfUnderCorePoolSize(null);
    
public voidpurge()
Tries to remove from the work queue all {@link Future} tasks that have been cancelled. This method can be useful as a storage reclamation operation, that has no other impact on functionality. Cancelled tasks are never executed, but may accumulate in work queues until worker threads can actively remove them. Invoking this method instead tries to remove them now. However, this method may fail to remove tasks in the presence of interference by other threads.

        // Fail if we encounter interference during traversal
        try {
            Iterator<Runnable> it = getQueue().iterator();
            while (it.hasNext()) {
                Runnable r = it.next();
                if (r instanceof Future<?>) {
                    Future<?> c = (Future<?>)r;
                    if (c.isCancelled())
                        it.remove();
                }
            }
        }
        catch(ConcurrentModificationException ex) {
            return; 
        }
    
voidreject(java.lang.Runnable command)
Invoke the rejected execution handler for the given command.


                  
       
        handler.rejectedExecution(command, this);
    
public booleanremove(java.lang.Runnable task)
Removes this task from the executor's internal queue if it is present, thus causing it not to be run if it has not already started.

This method may be useful as one part of a cancellation scheme. It may fail to remove tasks that have been converted into other forms before being placed on the internal queue. For example, a task entered using submit might be converted into a form that maintains Future status. However, in such cases, method {@link ThreadPoolExecutor#purge} may be used to remove those Futures that have been cancelled.

param
task the task to remove
return
true if the task was removed

        return getQueue().remove(task);
    
public voidsetCorePoolSize(int corePoolSize)
Sets the core number of threads. This overrides any value set in the constructor. If the new value is smaller than the current value, excess existing threads will be terminated when they next become idle. If larger, new threads will, if needed, be started to execute any queued tasks.

param
corePoolSize the new core size
throws
IllegalArgumentException if corePoolSize less than zero
see
#getCorePoolSize

        if (corePoolSize < 0)
            throw new IllegalArgumentException();
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            int extra = this.corePoolSize - corePoolSize;
            this.corePoolSize = corePoolSize;
            if (extra < 0) {
                int n = workQueue.size();
                // We have to create initially-idle threads here
                // because we otherwise have no recourse about
                // what to do with a dequeued task if addThread fails.
                while (extra++ < 0 && n-- > 0 && poolSize < corePoolSize ) {
                    Thread t = addThread(null);
                    if (t != null) 
                        t.start();
                    else
                        break;
                }
            }
            else if (extra > 0 && poolSize > corePoolSize) {
                Iterator<Worker> it = workers.iterator();
                while (it.hasNext() &&
                       extra-- > 0 &&
                       poolSize > corePoolSize &&
                       workQueue.remainingCapacity() == 0) 
                    it.next().interruptIfIdle();
            }
        } finally {
            mainLock.unlock();
        }
    
public voidsetKeepAliveTime(long time, java.util.concurrent.TimeUnit unit)
Sets the time limit for which threads may remain idle before being terminated. If there are more than the core number of threads currently in the pool, after waiting this amount of time without processing a task, excess threads will be terminated. This overrides any value set in the constructor.

param
time the time to wait. A time value of zero will cause excess threads to terminate immediately after executing tasks.
param
unit the time unit of the time argument
throws
IllegalArgumentException if time less than zero
see
#getKeepAliveTime

        if (time < 0)
            throw new IllegalArgumentException();
        this.keepAliveTime = unit.toNanos(time);
    
public voidsetMaximumPoolSize(int maximumPoolSize)
Sets the maximum allowed number of threads. This overrides any value set in the constructor. If the new value is smaller than the current value, excess existing threads will be terminated when they next become idle.

param
maximumPoolSize the new maximum
throws
IllegalArgumentException if maximumPoolSize less than zero or the {@link #getCorePoolSize core pool size}
see
#getMaximumPoolSize

        if (maximumPoolSize <= 0 || maximumPoolSize < corePoolSize)
            throw new IllegalArgumentException();
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            int extra = this.maximumPoolSize - maximumPoolSize;
            this.maximumPoolSize = maximumPoolSize;
            if (extra > 0 && poolSize > maximumPoolSize) {
                Iterator<Worker> it = workers.iterator();
                while (it.hasNext() &&
                       extra > 0 &&
                       poolSize > maximumPoolSize) {
                    it.next().interruptIfIdle();
                    --extra;
                }
            }
        } finally {
            mainLock.unlock();
        }
    
public voidsetRejectedExecutionHandler(java.util.concurrent.RejectedExecutionHandler handler)
Sets a new handler for unexecutable tasks.

param
handler the new handler
throws
NullPointerException if handler is null
see
#getRejectedExecutionHandler

        if (handler == null)
            throw new NullPointerException();
        this.handler = handler;
    
public voidsetThreadFactory(java.util.concurrent.ThreadFactory threadFactory)
Sets the thread factory used to create new threads.

param
threadFactory the new thread factory
throws
NullPointerException if threadFactory is null
see
#getThreadFactory

        if (threadFactory == null)
            throw new NullPointerException();
        this.threadFactory = threadFactory;
    
public voidshutdown()
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.

throws
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 {@link java.lang.RuntimePermission}("modifyThread"), or the security manager's checkAccess method denies access.

        // Fail if caller doesn't have modifyThread permission. We
        // explicitly check permissions directly because we can't trust
        // implementations of SecurityManager to correctly override
        // the "check access" methods such that our documented
        // security policy is implemented.
	SecurityManager security = System.getSecurityManager();
	if (security != null) 
            java.security.AccessController.checkPermission(shutdownPerm);

        boolean fullyTerminated = false;
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            if (workers.size() > 0) {
                // Check if caller can modify worker threads.  This
                // might not be true even if passed above check, if
                // the SecurityManager treats some threads specially.
                if (security != null) {
                    for (Worker w: workers)
                        security.checkAccess(w.thread);
                }

                int state = runState;
                if (state == RUNNING) // don't override shutdownNow
                    runState = SHUTDOWN;

                try {
                    for (Worker w: workers)
                        w.interruptIfIdle();
                } catch(SecurityException se) {
                    // If SecurityManager allows above checks, but
                    // then unexpectedly throws exception when
                    // interrupting threads (which it ought not do),
                    // back out as cleanly as we can. Some threads may
                    // have been killed but we remain in non-shutdown
                    // state.
                    runState = state; 
                    throw se;
                }
            }
            else { // If no workers, trigger full termination now
                fullyTerminated = true;
                runState = TERMINATED;
                termination.signalAll();
            }
        } finally {
            mainLock.unlock();
        }
        if (fullyTerminated)
            terminated();
    
public java.util.ListshutdownNow()
Attempts to stop all actively executing tasks, halts the processing of waiting tasks, and returns a list of the tasks that were awaiting execution.

This implementation cancels tasks via {@link Thread#interrupt}, so if any tasks mask or fail to respond to interrupts, they may never terminate.

return
list of tasks that never commenced execution
throws
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 {@link java.lang.RuntimePermission}("modifyThread"), or the security manager's checkAccess method denies access.

        // Almost the same code as shutdown()
	SecurityManager security = System.getSecurityManager();
	if (security != null) 
            java.security.AccessController.checkPermission(shutdownPerm);

        boolean fullyTerminated = false;
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            if (workers.size() > 0) {
                if (security != null) {
                    for (Worker w: workers)
                        security.checkAccess(w.thread);
                }

                int state = runState;
                if (state != TERMINATED)
                    runState = STOP;
                try {
                    for (Worker w : workers)
                        w.interruptNow();
                } catch(SecurityException se) {
                    runState = state; // back out;
                    throw se;
                }
            }
            else { // If no workers, trigger full termination now
                fullyTerminated = true;
                runState = TERMINATED;
                termination.signalAll();
            }
        } finally {
            mainLock.unlock();
        }
        if (fullyTerminated)
            terminated();
        return Arrays.asList(workQueue.toArray(EMPTY_RUNNABLE_ARRAY));
    
protected voidterminated()
Method invoked when the Executor has terminated. Default implementation does nothing. Note: To properly nest multiple overridings, subclasses should generally invoke super.terminated within this method.

 
voidworkerDone(java.util.concurrent.ThreadPoolExecutor$Worker w)
Perform bookkeeping for a terminated worker thread.

param
w the worker

        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            completedTaskCount += w.completedTasks;
            workers.remove(w);
            if (--poolSize > 0)
                return;

            // Else, this is the last thread. Deal with potential shutdown.

            int state = runState;
            assert state != TERMINATED;

            if (state != STOP) {
                // If there are queued tasks but no threads, create
                // replacement thread. We must create it initially
                // idle to avoid orphaned tasks in case addThread
                // fails.  This also handles case of delayed tasks
                // that will sometime later become runnable.
                if (!workQueue.isEmpty()) { 
                    Thread t = addThread(null);
                    if (t != null)
                        t.start();
                    return;
                }

                // Otherwise, we can exit without replacement
                if (state == RUNNING)
                    return;
            }

            // Either state is STOP, or state is SHUTDOWN and there is
            // no work to do. So we can terminate.
            termination.signalAll();
            runState = TERMINATED;
            // fall through to call terminate() outside of lock.
        } finally {
            mainLock.unlock();
        }

        assert runState == TERMINATED;
        terminated();