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org.apache.geronimo.concurrent.harmony
public class: ThreadPoolExecutor [javadoc | source]
java.lang.Object
   java.util.concurrent.AbstractExecutorService
      org.apache.geronimo.concurrent.harmony.ThreadPoolExecutor

All Implemented Interfaces:
    ExecutorService

Direct Known Subclasses:
    AbstractManagedScheduledExecutorService, ComponentManagedScheduledExecutorService, AbstractManagedExecutorService, ServerManagedScheduledExecutorService, ServerManagedExecutorService, ComponentManagedExecutorService

An ExecutorService that executes each submitted task using one of possibly several pooled threads, normally configured using 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 Executors factory methods Executors#newCachedThreadPool (unbounded thread pool, with automatic thread reclamation), Executors#newFixedThreadPool (fixed size thread pool) and 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 ThreadPoolExecutor#getPoolSize ) according to the bounds set by corePoolSize (see ThreadPoolExecutor#getCorePoolSize ) and maximumPoolSize (see ThreadPoolExecutor#getMaximumPoolSize ). When a new task is submitted in method 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 ThreadPoolExecutor#setCorePoolSize and 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 ThreadPoolExecutor#prestartCoreThread or ThreadPoolExecutor#prestartAllCoreThreads .
Creating new threads
New threads are created using a java.util.concurrent.ThreadFactory . If not otherwise specified, a Executors#defaultThreadFactory is used, that creates threads to all be in the same 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.
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 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 ThreadPoolExecutor#setKeepAliveTime . Using a value of Long.MAX_VALUE TimeUnit#NANOSECONDS effectively disables idle threads from ever terminating prior to shut down.
Queuing
Any 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 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 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 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 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 RejectedExecutionHandler#rejectedExecution method of its RejectedExecutionHandler . Four predefined handler policies are provided:
  1. In the default ThreadPoolExecutor.AbortPolicy , the handler throws a runtime RejectedExecutionException upon rejection.
  2. In 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 ThreadPoolExecutor.DiscardPolicy , a task that cannot be executed is simply dropped.
  4. In 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 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 ThreadPoolExecutor#beforeExecute and 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 ThreadPoolExecutor#terminated can be overridden to perform any special processing that needs to be done once the Executor has fully terminated.
Queue maintenance
Method 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, ThreadPoolExecutor#remove and 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();
    }
  }
}
Nested Class Summary:
public static class  ThreadPoolExecutor.CallerRunsPolicy  A handler for rejected tasks that runs the rejected task directly in the calling thread of the execute method, unless the executor has been shut down, in which case the task is discarded. 
public static class  ThreadPoolExecutor.AbortPolicy  A handler for rejected tasks that throws a RejectedExecutionException
public static class  ThreadPoolExecutor.DiscardPolicy  A handler for rejected tasks that silently discards the rejected task. 
public static class  ThreadPoolExecutor.DiscardOldestPolicy  A handler for rejected tasks that discards the oldest unhandled request and then retries execute, unless the executor is shut down, in which case the task is discarded. 
Field Summary
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 
Fields inherited from java.util.concurrent.AbstractExecutorService:
$assertionsDisabled
Constructor:
 public ThreadPoolExecutor(int corePoolSize,
    int maximumPoolSize,
    long keepAliveTime,
    TimeUnit unit,
    BlockingQueue<Runnable> 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 Executors factory methods instead of this general purpose constructor.
    Parameters:
    corePoolSize - the number of threads to keep in the pool, even if they are idle.
    maximumPoolSize - the maximum number of threads to allow in the pool.
    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.
    unit - the time unit for the keepAliveTime argument.
    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.
    NullPointerException - if workQueue is null
 public ThreadPoolExecutor(int corePoolSize,
    int maximumPoolSize,
    long keepAliveTime,
    TimeUnit unit,
    BlockingQueue<Runnable> workQueue,
    ThreadFactory threadFactory) 
    Creates a new ThreadPoolExecutor with the given initial parameters.
    Parameters:
    corePoolSize - the number of threads to keep in the pool, even if they are idle.
    maximumPoolSize - the maximum number of threads to allow in the pool.
    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.
    unit - the time unit for the keepAliveTime argument.
    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.
    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.
    NullPointerException - if workQueue or threadFactory are null.
 public ThreadPoolExecutor(int corePoolSize,
    int maximumPoolSize,
    long keepAliveTime,
    TimeUnit unit,
    BlockingQueue<Runnable> workQueue,
    RejectedExecutionHandler handler) 
    Creates a new ThreadPoolExecutor with the given initial parameters.
    Parameters:
    corePoolSize - the number of threads to keep in the pool, even if they are idle.
    maximumPoolSize - the maximum number of threads to allow in the pool.
    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.
    unit - the time unit for the keepAliveTime argument.
    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.
    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.
    NullPointerException - if workQueue or handler are null.
 public ThreadPoolExecutor(int corePoolSize,
    int maximumPoolSize,
    long keepAliveTime,
    TimeUnit unit,
    BlockingQueue<Runnable> workQueue,
    ThreadFactory threadFactory,
    RejectedExecutionHandler handler) 
    Creates a new ThreadPoolExecutor with the given initial parameters.
    Parameters:
    corePoolSize - the number of threads to keep in the pool, even if they are idle.
    maximumPoolSize - the maximum number of threads to allow in the pool.
    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.
    unit - the time unit for the keepAliveTime argument.
    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.
    threadFactory - the factory to use when the executor creates a new thread.
    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.
    NullPointerException - if workQueue or threadFactory or handler are null.
Method from org.apache.geronimo.concurrent.harmony.ThreadPoolExecutor Summary:
afterExecute,   awaitTermination,   beforeExecute,   execute,   finalize,   getActiveCount,   getCompletedTaskCount,   getCorePoolSize,   getKeepAliveTime,   getLargestPoolSize,   getMaximumPoolSize,   getPoolSize,   getQueue,   getRejectedExecutionHandler,   getTask,   getTaskCount,   getThreadFactory,   interruptIdleWorkers,   isShutdown,   isTerminated,   isTerminating,   prestartAllCoreThreads,   prestartCoreThread,   purge,   reject,   remove,   setCorePoolSize,   setKeepAliveTime,   setMaximumPoolSize,   setRejectedExecutionHandler,   setThreadFactory,   shutdown,   shutdownNow,   terminated,   workerDone
Methods from java.util.concurrent.AbstractExecutorService:
invokeAll,   invokeAll,   invokeAny,   invokeAny,   newTaskFor,   newTaskFor,   submit,   submit,   submit
Methods from java.lang.Object:
clone,   equals,   finalize,   getClass,   hashCode,   notify,   notifyAll,   toString,   wait,   wait,   wait
Method from org.apache.geronimo.concurrent.harmony.ThreadPoolExecutor Detail:
 protected  void afterExecute(Runnable r,
    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.
 public boolean awaitTermination(long timeout,
    TimeUnit unit) throws InterruptedException 
 protected  void beforeExecute(Thread t,
    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.
 public  void execute(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.
 protected  void finalize() 
    Invokes shutdown when this executor is no longer referenced.
 public int getActiveCount() 
    Returns the approximate number of threads that are actively executing tasks.
 public long getCompletedTaskCount() 
    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.
 public int getCorePoolSize() 
    Returns the core number of threads.
 public long getKeepAliveTime(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.
 public int getLargestPoolSize() 
    Returns the largest number of threads that have ever simultaneously been in the pool.
 public int getMaximumPoolSize() 
    Returns the maximum allowed number of threads.
 public int getPoolSize() 
    Returns the current number of threads in the pool.
 public BlockingQueue<Runnable> getQueue() 
    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.
 public RejectedExecutionHandler getRejectedExecutionHandler() 
    Returns the current handler for unexecutable tasks.
 Runnable getTask() throws InterruptedException 
    Get the next task for a worker thread to run.
 public long getTaskCount() 
    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.
 public ThreadFactory getThreadFactory() 
    Returns the thread factory used to create new threads.
 protected  void interruptIdleWorkers() 
    Wake up all threads that might be waiting for tasks.
 public boolean isShutdown() 
 public boolean isTerminated() 
 public boolean isTerminating() 
    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.
 public int prestartAllCoreThreads() 
    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.
 public boolean prestartCoreThread() 
    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.
 public  void purge() 
    Tries to remove from the work queue all 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.
 protected  void reject(Runnable command) 
    Invoke the rejected execution handler for the given command.
 public boolean remove(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 ThreadPoolExecutor#purge may be used to remove those Futures that have been cancelled.

 public  void setCorePoolSize(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.
 public  void setKeepAliveTime(long time,
    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.
 public  void setMaximumPoolSize(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.
 public  void setRejectedExecutionHandler(RejectedExecutionHandler handler) 
    Sets a new handler for unexecutable tasks.
 public  void setThreadFactory(ThreadFactory threadFactory) 
    Sets the thread factory used to create new threads.
 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.
 public List<Runnable> 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.

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

 protected  void terminated() 
    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.
  void workerDone(Worker w) 
    Perform bookkeeping for a terminated worker thread.