Spring中的ThreadPoolTaskExecutor - Spring - Java - ITeye论坛

Spring中的ThreadPoolTaskExecutor - Spring - Java - ITeye论坛

• ThreadPoolExecutor (Java 2 Platform SE 5.0)
• ThreadpoolExecutor - Programming Examples
• ThreadPoolExecutor – basics « a cup of java

ThreadPoolExecutor – basics « a cup of java

The buzz words



1. Core & max pool size
This is the size of the core (minimam) and maximum possible thread that need to be in the thread pool every time. We can even specify the keep-alive time limit for the threads.

2. Work queue
Work queue is a BlockingQueue, which is used to hold the work/task which is being sent to the executor when all the threads in the pool are busy executing the tasks. Read more about the BlockingQueue in Sun Java API docs.

3. RejectedExecutionHandler
So we have a limited number of threads in the pool to execute the tasks and a fixed sized work queue to hold the additional tasks. But what if the work queue overflows! RejectedExecutionHandler can help us in such situations. Read the Sun Java API docs for more.

ThreadpoolExecutor - Programming Examples

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.[1]

ThreadPoolExecutor (Java 2 Platform SE 5.0)

Core and maximum pool sizes

A ThreadPoolExecutor will automatically adjust the pool size (see getPoolSize()) according to the bounds set by corePoolSize (see getCorePoolSize()) and maximumPoolSize (seegetMaximumPoolSize()). When a new task is submitted in method execute(java.lang.Runnable), 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 setCorePoolSize(int) and setMaximumPoolSize(int).

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 prestartCoreThread() or prestartAllCoreThreads().

Creating new threads

New threads are created using a ThreadFactory. If not otherwise specified, a Executors.defaultThreadFactory() is used, that creates threads to all be in the same ThreadGroup and with the sameNORM_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 (seegetKeepAliveTime(java.util.concurrent.TimeUnit)). 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 setKeepAliveTime(long, java.util.concurrent.TimeUnit). Using a value of Long.MAX_VALUETimeUnit.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 execute(java.lang.Runnable) 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(java.lang.Runnable, java.util.concurrent.ThreadPoolExecutor) 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 beforeExecute(java.lang.Thread, java.lang.Runnable) and afterExecute(java.lang.Runnable, java.lang.Throwable) 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, methodterminated() 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 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,remove(java.lang.Runnable) and purge() are available to assist in storage reclamation when large numbers of queued tasks become cancelled.

Spring中的ThreadPoolTaskExecutor - Spring - Java - ITeye论坛
RejectedExecutionHandler handler:
用来拒绝一个任务的执行，有两种情况会发生这种情况。

• 一是在execute方法中若addIfUnderMaximumPoolSize(command)为false，即线程池已经饱和； 
• 二是在execute方法中, 发现runState!=RUNNING || poolSize == 0,即已经shutdown,就调用ensureQueuedTaskHandled(Runnable command)，在该方法中有可能调用reject。 

Reject策略预定义有四种：

1. ThreadPoolExecutor.AbortPolicy策略，是默认的策略,处理程序遭到拒绝将抛出运行时 RejectedExecutionException。 
2. ThreadPoolExecutor.CallerRunsPolicy策略 ,调用者的线程会执行该任务,如果执行器已关闭,则丢弃. 
3. ThreadPoolExecutor.DiscardPolicy策略，不能执行的任务将被丢弃. 
4. ThreadPoolExecutor.DiscardOldestPolicy策略，如果执行程序尚未关闭，则位于工作队列头部的任务将被删除，然后重试执行程序（如果再次失败，则重复此过程）.

BEAN注入到容器中

1. <bean id="threadPoolTaskExecutor"  
2.     class="org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor">  
3.     <property name="corePoolSize" value="10" />  
4.     <property name="maxPoolSize" value="15" />  
5.     <property name="queueCapacity" value="1000" />  
6. </bean>