Class PooledExecutor
- java.lang.Object
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- EDU.oswego.cs.dl.util.concurrent.ThreadFactoryUser
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- EDU.oswego.cs.dl.util.concurrent.PooledExecutor
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- All Implemented Interfaces:
Executor
public class PooledExecutor extends ThreadFactoryUser implements Executor
A tunable, extensible thread pool class. The main supported public method isexecute(Runnable command)
, which can be called instead of directly creating threads to execute commands.Thread pools can be useful for several, usually intertwined reasons:
- To bound resource use. A limit can be placed on the maximum number of simultaneously executing threads.
- To manage concurrency levels. A targeted number of threads can be allowed to execute simultaneously.
- To manage a set of threads performing related tasks.
- To minimize overhead, by reusing previously constructed Thread objects rather than creating new ones. (Note however that pools are hardly ever cure-alls for performance problems associated with thread construction, especially on JVMs that themselves internally pool or recycle threads.)
- Queueing
- By default, this pool uses queueless synchronous channels to
to hand off work to threads. This is a safe, conservative policy
that avoids lockups when handling sets of requests that might
have internal dependencies. (In these cases, queuing one task
could lock up another that would be able to continue if the
queued task were to run.) If you are sure that this cannot
happen, then you can instead supply a queue of some sort (for
example, a BoundedBuffer or LinkedQueue) in the constructor.
This will cause new commands to be queued in cases where all
MaximumPoolSize threads are busy. Queues are sometimes
appropriate when each task is completely independent of others,
so tasks cannot affect each others execution. For example, in an
http server.
When given a choice, this pool always prefers adding a new thread rather than queueing if there are currently fewer than the current getMinimumPoolSize threads running, but otherwise always prefers queuing a request rather than adding a new thread. Thus, if you use an unbounded buffer, you will never have more than getMinimumPoolSize threads running. (Since the default minimumPoolSize is one, you will probably want to explicitly setMinimumPoolSize.)
While queuing can be useful in smoothing out transient bursts of requests, especially in socket-based services, it is not very well behaved when commands continue to arrive on average faster than they can be processed. Using bounds for both the queue and the pool size, along with run-when-blocked policy is often a reasonable response to such possibilities.
Queue sizes and maximum pool sizes can often 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 artifically low throughput. Especially if tasks frequently block (for example if they are I/O bound), a JVM and underlying OS may be able to schedule time for more threads than you otherwise allow. Use of small queues or queueless handoffs generally requires larger pool sizes, which keeps CPUs busier but may encounter unacceptable scheduling overhead, which also decreases throughput.
- Maximum Pool size
- The maximum number of threads to use, when needed. The pool
does not by default preallocate threads. Instead, a thread is
created, if necessary and if there are fewer than the maximum,
only when an
execute
request arrives. The default value is (for all practical purposes) infinite --Integer.MAX_VALUE
, so should be set in the constructor or the set method unless you are just using the pool to minimize construction overhead. Because task handoffs to idle worker threads require synchronization that in turn relies on JVM scheduling policies to ensure progress, it is possible that a new thread will be created even though an existing worker thread has just become idle but has not progressed to the point at which it can accept a new task. This phenomenon tends to occur on some JVMs when bursts of short tasks are executed. - Minimum Pool size
- The minimum number of threads to use, when needed (default
1). When a new request is received, and fewer than the minimum
number of threads are running, a new thread is always created to
handle the request even if other worker threads are idly waiting
for work. Otherwise, a new thread is created only if there are
fewer than the maximum and the request cannot immediately be
queued.
- Preallocation
- You can override lazy thread construction policies via
method createThreads, which establishes a given number of warm
threads. Be aware that these preallocated threads will time out
and die (and later be replaced with others if needed) if not used
within the keep-alive time window. If you use preallocation, you
probably want to increase the keepalive time. The difference
between setMinimumPoolSize and createThreads is that
createThreads immediately establishes threads, while setting the
minimum pool size waits until requests arrive.
- Keep-alive time
- If the pool maintained references to a fixed set of threads
in the pool, then it would impede garbage collection of otherwise
idle threads. This would defeat the resource-management aspects
of pools. One solution would be to use weak references. However,
this would impose costly and difficult synchronization issues.
Instead, threads are simply allowed to terminate and thus be
GCable if they have been idle for the given keep-alive time. The
value of this parameter represents a trade-off between GCability
and construction time. In most current Java VMs, thread
construction and cleanup overhead is on the order of
milliseconds. The default keep-alive value is one minute, which
means that the time needed to construct and then GC a thread is
expended at most once per minute.
To establish worker threads permanently, use a negative argument to setKeepAliveTime.
- Blocked execution policy
- If the maximum pool size or queue size is bounded, then it
is possible for incoming
execute
requests to block. There are four supported policies for handling this problem, and mechanics (based on the Strategy Object pattern) to allow others in subclasses:- Run (the default)
- The thread making the
execute
request runs the task itself. This policy helps guard against lockup. - Wait
- Wait until a thread becomes available. This policy should, in general, not be used if the minimum number of of threads is zero, in which case a thread may never become available.
- Abort
- Throw a RuntimeException
- Discard
- Throw away the current request and return.
- DiscardOldest
- Throw away the oldest request and return.
These cases can never occur if the maximum pool size is unbounded or the queue is unbounded. In these cases you instead face potential resource exhaustion.) The execute method does not throw any checked exceptions in any of these cases since any errors associated with them must normally be dealt with via handlers or callbacks. (Although in some cases, these might be associated with throwing unchecked exceptions.) You may wish to add special implementations even if you choose one of the listed policies. For example, the supplied Discard policy does not inform the caller of the drop. You could add your own version that does so. Since choice of policies is normally a system-wide decision, selecting a policy affects all calls to
execute
. If for some reason you would instead like to make per-call decisions, you could add variant versions of theexecute
method (for example,executeIfWouldNotBlock
) in subclasses. - Thread construction parameters
- A settable ThreadFactory establishes each new thread. By
default, it merely generates a new instance of class Thread, but
can be changed to use a Thread subclass, to set priorities,
ThreadLocals, etc.
- Interruption policy
- Worker threads check for interruption after processing each
command, and terminate upon interruption. Fresh threads will
replace them if needed. Thus, new tasks will not start out in an
interrupted state due to an uncleared interruption in a previous
task. Also, unprocessed commands are never dropped upon
interruption. It would conceptually suffice simply to clear
interruption between tasks, but implementation characteristics of
interruption-based methods are uncertain enough to warrant this
conservative strategy. It is a good idea to be equally
conservative in your code for the tasks running within pools.
- Shutdown policy
- The interruptAll method interrupts, but does not disable the
pool. Two different shutdown methods are supported for use when
you do want to (permanently) stop processing tasks. Method
shutdownAfterProcessingCurrentlyQueuedTasks waits until all
current tasks are finished. The shutDownNow method interrupts
current threads and leaves other queued requests unprocessed.
- Handling requests after shutdown
- When the pool is shutdown, new incoming requests are handled
by the blockedExecutionHandler. By default, the handler is set to
discard new requests, but this can be set with an optional
argument to method
shutdownAfterProcessingCurrentlyQueuedTasks.
Also, if you are using some form of queuing, you may wish to call method drain() to remove (and return) unprocessed commands from the queue after shutting down the pool and its clients. If you need to be sure these commands are processed, you can then run() each of the commands in the list returned by drain().
Usage examples.
Probably the most common use of pools is in statics or singletons accessible from a number of classes in a package; for example:
class MyPool { // initialize to use a maximum of 8 threads. static PooledExecutor pool = new PooledExecutor(8); }
Here are some sample variants in initialization:- Using a bounded buffer of 10 tasks, at least 4 threads (started only
when needed due to incoming requests), but allowing
up to 100 threads if the buffer gets full.
pool = new PooledExecutor(new BoundedBuffer(10), 100); pool.setMinimumPoolSize(4);
- Same as (1), except pre-start 9 threads, allowing them to
die if they are not used for five minutes.
pool = new PooledExecutor(new BoundedBuffer(10), 100); pool.setMinimumPoolSize(4); pool.setKeepAliveTime(1000 * 60 * 5); pool.createThreads(9);
- Same as (2) except clients abort if both the buffer is full and
all 100 threads are busy:
pool = new PooledExecutor(new BoundedBuffer(10), 100); pool.setMinimumPoolSize(4); pool.setKeepAliveTime(1000 * 60 * 5); pool.abortWhenBlocked(); pool.createThreads(9);
- An unbounded queue serviced by exactly 5 threads:
pool = new PooledExecutor(new LinkedQueue()); pool.setKeepAliveTime(-1); // live forever pool.createThreads(5);
Usage notes.
Pools do not mesh well with using thread-specific storage via java.lang.ThreadLocal. ThreadLocal relies on the identity of a thread executing a particular task. Pools use the same thread to perform different tasks.
If you need a policy not handled by the parameters in this class consider writing a subclass.
Version note: Previous versions of this class relied on ThreadGroups for aggregate control. This has been removed, and the method interruptAll added, to avoid differences in behavior across JVMs.
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Nested Class Summary
Nested Classes Modifier and Type Class Description protected class
PooledExecutor.AbortWhenBlocked
Class defining Abort action.static interface
PooledExecutor.BlockedExecutionHandler
Class for actions to take when execute() blocks.protected class
PooledExecutor.DiscardOldestWhenBlocked
Class defining DiscardOldest action.protected class
PooledExecutor.DiscardWhenBlocked
Class defining Discard action.protected class
PooledExecutor.RunWhenBlocked
Class defining Run action.protected class
PooledExecutor.WaitWhenBlocked
Class defining Wait action.protected class
PooledExecutor.Worker
Class defining the basic run loop for pooled threads.-
Nested classes/interfaces inherited from class EDU.oswego.cs.dl.util.concurrent.ThreadFactoryUser
ThreadFactoryUser.DefaultThreadFactory
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Field Summary
Fields Modifier and Type Field Description protected PooledExecutor.BlockedExecutionHandler
blockedExecutionHandler_
The current handler for unserviceable requests.static long
DEFAULT_KEEPALIVETIME
The maximum time to keep worker threads alive waiting for new tasks; used if not otherwise specified.static int
DEFAULT_MAXIMUMPOOLSIZE
The maximum pool size; used if not otherwise specified.static int
DEFAULT_MINIMUMPOOLSIZE
The minimum pool size; used if not otherwise specified.protected Channel
handOff_
The channel used to hand off the command to a thread in the pool.protected long
keepAliveTime_
The maximum time for an idle thread to wait for new task.protected int
maximumPoolSize_
The maximum number of threads allowed in pool.protected int
minimumPoolSize_
The minumum number of threads to maintain in pool.protected int
poolSize_
Current pool size.protected boolean
shutdown_
Shutdown flag - latches true when a shutdown method is called in order to disable queuing/handoffs of new tasks.protected java.util.Map
threads_
The set of active threads, declared as a map from workers to their threads.-
Fields inherited from class EDU.oswego.cs.dl.util.concurrent.ThreadFactoryUser
threadFactory_
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Constructor Summary
Constructors Constructor Description PooledExecutor()
Create a new pool with all default settingsPooledExecutor(int maxPoolSize)
Create a new pool with all default settings except for maximum pool size.PooledExecutor(Channel channel)
Create a new pool that uses the supplied Channel for queuing, and with all default parameter settings.PooledExecutor(Channel channel, int maxPoolSize)
Create a new pool that uses the supplied Channel for queuing, and with all default parameter settings except for maximum pool size.
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Method Summary
All Methods Instance Methods Concrete Methods Modifier and Type Method Description void
abortWhenBlocked()
Set the policy for blocked execution to be to throw a RuntimeException.protected void
addThread(java.lang.Runnable command)
Create and start a thread to handle a new command.void
awaitTerminationAfterShutdown()
Wait for a shutdown pool to fully terminate.boolean
awaitTerminationAfterShutdown(long maxWaitTime)
Wait for a shutdown pool to fully terminate, or until the timeout has expired.int
createThreads(int numberOfThreads)
Create and start up to numberOfThreads threads in the pool.void
discardOldestWhenBlocked()
Set the policy for blocked execution to be to discard the oldest unhandled requestvoid
discardWhenBlocked()
Set the policy for blocked execution to be to return without executing the request.java.util.List
drain()
Remove all unprocessed tasks from pool queue, and return them in a java.util.List.void
execute(java.lang.Runnable command)
Arrange for the given command to be executed by a thread in this pool.PooledExecutor.BlockedExecutionHandler
getBlockedExecutionHandler()
Get the handler for blocked executionlong
getKeepAliveTime()
Return the number of milliseconds to keep threads alive waiting for new commands.int
getMaximumPoolSize()
Return the maximum number of threads to simultaneously execute New unqueued requests will be handled according to the current blocking policy once this limit is exceeded.int
getMinimumPoolSize()
Return the minimum number of threads to simultaneously execute.int
getPoolSize()
Return the current number of active threads in the pool.protected java.lang.Runnable
getTask()
Get a task from the handoff queue, or null if shutting down.void
interruptAll()
Interrupt all threads in the pool, causing them all to terminate.boolean
isTerminatedAfterShutdown()
Return true if a shutDown method has succeeded in terminating all threads.void
runWhenBlocked()
Set the policy for blocked execution to be that the current thread executes the command if there are no available threads in the pool.void
setBlockedExecutionHandler(PooledExecutor.BlockedExecutionHandler h)
Set the handler for blocked executionvoid
setKeepAliveTime(long msecs)
Set the number of milliseconds to keep threads alive waiting for new commands.void
setMaximumPoolSize(int newMaximum)
Set the maximum number of threads to use.void
setMinimumPoolSize(int newMinimum)
Set the minimum number of threads to use.void
shutdownAfterProcessingCurrentlyQueuedTasks()
Terminate threads after processing all elements currently in queue.void
shutdownAfterProcessingCurrentlyQueuedTasks(PooledExecutor.BlockedExecutionHandler handler)
Terminate threads after processing all elements currently in queue.void
shutdownNow()
Interrupt all threads and disable construction of new threads.void
shutdownNow(PooledExecutor.BlockedExecutionHandler handler)
Interrupt all threads and disable construction of new threads.void
waitWhenBlocked()
Set the policy for blocked execution to be to wait until a thread is available, unless the pool has been shut down, in which case the action is discarded.protected void
workerDone(PooledExecutor.Worker w)
Cleanup method called upon termination of worker thread.-
Methods inherited from class EDU.oswego.cs.dl.util.concurrent.ThreadFactoryUser
getThreadFactory, setThreadFactory
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Field Detail
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DEFAULT_MAXIMUMPOOLSIZE
public static final int DEFAULT_MAXIMUMPOOLSIZE
The maximum pool size; used if not otherwise specified. Default value is essentially infinite (Integer.MAX_VALUE)- See Also:
- Constant Field Values
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DEFAULT_MINIMUMPOOLSIZE
public static final int DEFAULT_MINIMUMPOOLSIZE
The minimum pool size; used if not otherwise specified. Default value is 1.- See Also:
- Constant Field Values
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DEFAULT_KEEPALIVETIME
public static final long DEFAULT_KEEPALIVETIME
The maximum time to keep worker threads alive waiting for new tasks; used if not otherwise specified. Default value is one minute (60000 milliseconds).- See Also:
- Constant Field Values
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maximumPoolSize_
protected int maximumPoolSize_
The maximum number of threads allowed in pool.
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minimumPoolSize_
protected int minimumPoolSize_
The minumum number of threads to maintain in pool.
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poolSize_
protected int poolSize_
Current pool size.
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keepAliveTime_
protected long keepAliveTime_
The maximum time for an idle thread to wait for new task.
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shutdown_
protected boolean shutdown_
Shutdown flag - latches true when a shutdown method is called in order to disable queuing/handoffs of new tasks.
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handOff_
protected final Channel handOff_
The channel used to hand off the command to a thread in the pool.
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threads_
protected final java.util.Map threads_
The set of active threads, declared as a map from workers to their threads. This is needed by the interruptAll method. It may also be useful in subclasses that need to perform other thread management chores.
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blockedExecutionHandler_
protected PooledExecutor.BlockedExecutionHandler blockedExecutionHandler_
The current handler for unserviceable requests.
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Constructor Detail
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PooledExecutor
public PooledExecutor()
Create a new pool with all default settings
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PooledExecutor
public PooledExecutor(int maxPoolSize)
Create a new pool with all default settings except for maximum pool size.
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PooledExecutor
public PooledExecutor(Channel channel)
Create a new pool that uses the supplied Channel for queuing, and with all default parameter settings.
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PooledExecutor
public PooledExecutor(Channel channel, int maxPoolSize)
Create a new pool that uses the supplied Channel for queuing, and with all default parameter settings except for maximum pool size.
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Method Detail
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getMaximumPoolSize
public int getMaximumPoolSize()
Return the maximum number of threads to simultaneously execute New unqueued requests will be handled according to the current blocking policy once this limit is exceeded.
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setMaximumPoolSize
public void setMaximumPoolSize(int newMaximum)
Set the maximum number of threads to use. Decreasing the pool size will not immediately kill existing threads, but they may later die when idle.- Throws:
java.lang.IllegalArgumentException
- if less or equal to zero. (It is not considered an error to set the maximum to be less than than the minimum. However, in this case there are no guarantees about behavior.)
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getMinimumPoolSize
public int getMinimumPoolSize()
Return the minimum number of threads to simultaneously execute. (Default value is 1). If fewer than the mininum number are running upon reception of a new request, a new thread is started to handle this request.
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setMinimumPoolSize
public void setMinimumPoolSize(int newMinimum)
Set the minimum number of threads to use.- Throws:
java.lang.IllegalArgumentException
- if less than zero. (It is not considered an error to set the minimum to be greater than the maximum. However, in this case there are no guarantees about behavior.)
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getPoolSize
public int getPoolSize()
Return the current number of active threads in the pool. This number is just a snaphot, and may change immediately upon returning
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getKeepAliveTime
public long getKeepAliveTime()
Return the number of milliseconds to keep threads alive waiting for new commands. A negative value means to wait forever. A zero value means not to wait at all.
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setKeepAliveTime
public void setKeepAliveTime(long msecs)
Set the number of milliseconds to keep threads alive waiting for new commands. A negative value means to wait forever. A zero value means not to wait at all.
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getBlockedExecutionHandler
public PooledExecutor.BlockedExecutionHandler getBlockedExecutionHandler()
Get the handler for blocked execution
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setBlockedExecutionHandler
public void setBlockedExecutionHandler(PooledExecutor.BlockedExecutionHandler h)
Set the handler for blocked execution
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addThread
protected void addThread(java.lang.Runnable command)
Create and start a thread to handle a new command. Call only when holding lock.
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createThreads
public int createThreads(int numberOfThreads)
Create and start up to numberOfThreads threads in the pool. Return the number created. This may be less than the number requested if creating more would exceed maximum pool size bound.
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interruptAll
public void interruptAll()
Interrupt all threads in the pool, causing them all to terminate. Assuming that executed tasks do not disable (clear) interruptions, each thread will terminate after processing its current task. Threads will terminate sooner if the executed tasks themselves respond to interrupts.
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shutdownNow
public void shutdownNow()
Interrupt all threads and disable construction of new threads. Any tasks entered after this point will be discarded. A shut down pool cannot be restarted.
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shutdownNow
public void shutdownNow(PooledExecutor.BlockedExecutionHandler handler)
Interrupt all threads and disable construction of new threads. Any tasks entered after this point will be handled by the given BlockedExecutionHandler. A shut down pool cannot be restarted.
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shutdownAfterProcessingCurrentlyQueuedTasks
public void shutdownAfterProcessingCurrentlyQueuedTasks()
Terminate threads after processing all elements currently in queue. Any tasks entered after this point will be discarded. A shut down pool cannot be restarted.
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shutdownAfterProcessingCurrentlyQueuedTasks
public void shutdownAfterProcessingCurrentlyQueuedTasks(PooledExecutor.BlockedExecutionHandler handler)
Terminate threads after processing all elements currently in queue. Any tasks entered after this point will be handled by the given BlockedExecutionHandler. A shut down pool cannot be restarted.
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isTerminatedAfterShutdown
public boolean isTerminatedAfterShutdown()
Return true if a shutDown method has succeeded in terminating all threads.
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awaitTerminationAfterShutdown
public boolean awaitTerminationAfterShutdown(long maxWaitTime) throws java.lang.InterruptedException
Wait for a shutdown pool to fully terminate, or until the timeout has expired. This method may only be called after invoking shutdownNow or shutdownAfterProcessingCurrentlyQueuedTasks.- Parameters:
maxWaitTime
- the maximum time in milliseconds to wait- Returns:
- true if the pool has terminated within the max wait period
- Throws:
java.lang.IllegalStateException
- if shutdown has not been requestedjava.lang.InterruptedException
- if the current thread has been interrupted in the course of waiting
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awaitTerminationAfterShutdown
public void awaitTerminationAfterShutdown() throws java.lang.InterruptedException
Wait for a shutdown pool to fully terminate. This method may only be called after invoking shutdownNow or shutdownAfterProcessingCurrentlyQueuedTasks.- Throws:
java.lang.IllegalStateException
- if shutdown has not been requestedjava.lang.InterruptedException
- if the current thread has been interrupted in the course of waiting
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drain
public java.util.List drain()
Remove all unprocessed tasks from pool queue, and return them in a java.util.List. Thsi method should be used only when there are not any active clients of the pool. Otherwise you face the possibility that the method will loop pulling out tasks as clients are putting them in. This method can be useful after shutting down a pool (via shutdownNow) to determine whether there are any pending tasks that were not processed. You can then, for example execute all unprocessed commands via code along the lines of:List tasks = pool.drain(); for (Iterator it = tasks.iterator(); it.hasNext();) ( (Runnable)(it.next()) ).run();
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workerDone
protected void workerDone(PooledExecutor.Worker w)
Cleanup method called upon termination of worker thread.
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getTask
protected java.lang.Runnable getTask() throws java.lang.InterruptedException
Get a task from the handoff queue, or null if shutting down.- Throws:
java.lang.InterruptedException
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runWhenBlocked
public void runWhenBlocked()
Set the policy for blocked execution to be that the current thread executes the command if there are no available threads in the pool.
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waitWhenBlocked
public void waitWhenBlocked()
Set the policy for blocked execution to be to wait until a thread is available, unless the pool has been shut down, in which case the action is discarded.
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discardWhenBlocked
public void discardWhenBlocked()
Set the policy for blocked execution to be to return without executing the request.
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abortWhenBlocked
public void abortWhenBlocked()
Set the policy for blocked execution to be to throw a RuntimeException.
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discardOldestWhenBlocked
public void discardOldestWhenBlocked()
Set the policy for blocked execution to be to discard the oldest unhandled request
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execute
public void execute(java.lang.Runnable command) throws java.lang.InterruptedException
Arrange for the given command to be executed by a thread in this pool. The method normally returns when the command has been handed off for (possibly later) execution.
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