Java 多线程编程之九：使用 Executors 和 ThreadPoolExecutor 实现的 Java 线程池的例子

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转载地址：http://blog.csdn.net/defonds/article/details/9715455

 线程池用来管理工作线程的数量，它持有一个等待被执行的线程的队列。
        java.util.concurrent.Executors 提供了 java.util.concurrent.Executor 接口实现来创建 Java 里的线程池。我们写一个简单的程序来解释一下它的工作机制。
        首先我们需要有一个 Runnable 类。
        WorkerThread.java

package com.journaldev.threadpool;public class WorkerThread implements Runnable {private String command;public WorkerThread(String s){this.command=s;}@Overridepublic void run() {System.out.println(Thread.currentThread().getName()+" Start. Command = "+command);processCommand();System.out.println(Thread.currentThread().getName()+" End.");}private void processCommand() {try {Thread.sleep(5000);} catch (InterruptedException e) {e.printStackTrace();}}@Overridepublic String toString(){return this.command;}
}

这里是我们使用 Executors 框架创建了一个固定的线程池的测试程序。
SimpleThreadPool.java

package com.journaldev.threadpool;import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;public class SimpleThreadPool {public static void main(String[] args) {ExecutorService executor = Executors.newFixedThreadPool(5);for (int i = 0; i < 10; i++) {Runnable worker = new WorkerThread("" + i);executor.execute(worker);}executor.shutdown(); // This will make the executor accept no new threads and finish all existing threads in the queuewhile (!executor.isTerminated()) { // Wait until all threads are finish,and also you can use "executor.awaitTermination();" to wait}System.out.println("Finished all threads");}}

程序中我们创建了固定大小为五个工作线程的线程池。然后分配给线程池十个工作，因为线程池大小为五，它将启动五个工作线程先处理五个工作，其他的工作则处于等待状态，一旦有工作完成，空闲下来工作线程就会捡取等待队列里的其他工作进行执行。
        这里是以上程序的输出。
pool-1-thread-2 Start. Command = 1
pool-1-thread-4 Start. Command = 3
pool-1-thread-1 Start. Command = 0
pool-1-thread-3 Start. Command = 2
pool-1-thread-5 Start. Command = 4
pool-1-thread-4 End.
pool-1-thread-5 End.
pool-1-thread-1 End.
pool-1-thread-3 End.
pool-1-thread-3 Start. Command = 8
pool-1-thread-2 End.
pool-1-thread-2 Start. Command = 9
pool-1-thread-1 Start. Command = 7
pool-1-thread-5 Start. Command = 6
pool-1-thread-4 Start. Command = 5
pool-1-thread-2 End.
pool-1-thread-4 End.
pool-1-thread-3 End.
pool-1-thread-5 End.
pool-1-thread-1 End.
Finished all threads
        输出表明线程池中至始至终只有五个名为 "pool-1-thread-1" 到 "pool-1-thread-5" 的五个线程，这五个线程不随着工作的完成而消亡，会一直存在，并负责执行分配给线程池的任务，直到线程池消亡。
        Executors 类提供了使用了 ThreadPoolExecutor 的简单的 ExecutorService 实现，但是 ThreadPoolExecutor 提供的功能远不止于此。我们可以在创建 ThreadPoolExecutor 实例时指定活动线程的数量，我们也可以限制线程池的大小并且创建我们自己的 RejectedExecutionHandler 实现来处理不能适应工作队列的工作。
        这里是我们自定义的 RejectedExecutionHandler 接口的实现。
        RejectedExecutionHandlerImpl.java

package com.journaldev.threadpool;import java.util.concurrent.RejectedExecutionHandler;
import java.util.concurrent.ThreadPoolExecutor;public class RejectedExecutionHandlerImpl implements RejectedExecutionHandler {@Overridepublic void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {System.out.println(r.toString() + " is rejected");}}

ThreadPoolExecutor 提供了一些方法，我们可以使用这些方法来查询 executor 的当前状态，线程池大小，活动线程数量以及任务数量。因此我是用来一个监控线程在特定的时间间隔内打印 executor 信息。
        MyMonitorThread.java

package com.journaldev.threadpool;import java.util.concurrent.ThreadPoolExecutor;public class MyMonitorThread implements Runnable
{private ThreadPoolExecutor executor;private int seconds;private boolean run=true;public MyMonitorThread(ThreadPoolExecutor executor, int delay){this.executor = executor;this.seconds=delay;}public void shutdown(){this.run=false;}@Overridepublic void run(){while(run){System.out.println(String.format("[monitor] [%d/%d] Active: %d, Completed: %d, Task: %d, isShutdown: %s, isTerminated: %s",this.executor.getPoolSize(),this.executor.getCorePoolSize(),this.executor.getActiveCount(),this.executor.getCompletedTaskCount(),this.executor.getTaskCount(),this.executor.isShutdown(),this.executor.isTerminated()));try {Thread.sleep(seconds*1000);} catch (InterruptedException e) {e.printStackTrace();}}}
}

这里是使用 ThreadPoolExecutor 的线程池实现例子。
        WorkerPool.java

package com.journaldev.threadpool;import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.Executors;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;public class WorkerPool {public static void main(String args[]) throws InterruptedException{//RejectedExecutionHandler implementationRejectedExecutionHandlerImpl rejectionHandler = new RejectedExecutionHandlerImpl();//Get the ThreadFactory implementation to useThreadFactory threadFactory = Executors.defaultThreadFactory();//creating the ThreadPoolExecutorThreadPoolExecutor executorPool = new ThreadPoolExecutor(2, 4, 10, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(2), threadFactory, rejectionHandler);//start the monitoring threadMyMonitorThread monitor = new MyMonitorThread(executorPool, 3);Thread monitorThread = new Thread(monitor);monitorThread.start();//submit work to the thread poolfor(int i=0; i<10; i++){executorPool.execute(new WorkerThread("cmd"+i));}Thread.sleep(30000);//shut down the poolexecutorPool.shutdown();//shut down the monitor threadThread.sleep(5000);monitor.shutdown();}
}

注意在初始化 ThreadPoolExecutor 时，我们保持初始池大小为 2，最大池大小为 4 而工作队列大小为 2。因此如果已经有四个正在执行的任务而此时分配来更多任务的话，工作队列将仅仅保留他们(新任务)中的两个，其他的将会被 RejectedExecutionHandlerImpl 处理。
        上面程序的输出可以证实以上观点。
pool-1-thread-1 Start. Command = cmd0
pool-1-thread-4 Start. Command = cmd5
cmd6 is rejected
pool-1-thread-3 Start. Command = cmd4
pool-1-thread-2 Start. Command = cmd1
cmd7 is rejected
cmd8 is rejected
cmd9 is rejected
[monitor] [0/2] Active: 4, Completed: 0, Task: 6, isShutdown: false, isTerminated: false
[monitor] [4/2] Active: 4, Completed: 0, Task: 6, isShutdown: false, isTerminated: false
pool-1-thread-4 End.
pool-1-thread-1 End.
pool-1-thread-2 End.
pool-1-thread-3 End.
pool-1-thread-1 Start. Command = cmd3
pool-1-thread-4 Start. Command = cmd2
[monitor] [4/2] Active: 2, Completed: 4, Task: 6, isShutdown: false, isTerminated: false
[monitor] [4/2] Active: 2, Completed: 4, Task: 6, isShutdown: false, isTerminated: false
pool-1-thread-1 End.
pool-1-thread-4 End.
[monitor] [4/2] Active: 0, Completed: 6, Task: 6, isShutdown: false, isTerminated: false
[monitor] [2/2] Active: 0, Completed: 6, Task: 6, isShutdown: false, isTerminated: false
[monitor] [2/2] Active: 0, Completed: 6, Task: 6, isShutdown: false, isTerminated: false
[monitor] [2/2] Active: 0, Completed: 6, Task: 6, isShutdown: false, isTerminated: false
[monitor] [2/2] Active: 0, Completed: 6, Task: 6, isShutdown: false, isTerminated: false
[monitor] [2/2] Active: 0, Completed: 6, Task: 6, isShutdown: false, isTerminated: false
[monitor] [0/2] Active: 0, Completed: 6, Task: 6, isShutdown: true, isTerminated: true
[monitor] [0/2] Active: 0, Completed: 6, Task: 6, isShutdown: true, isTerminated: true
        注意 executor 的活动任务、完成任务以及所有完成任务，这些数量上的变化。我们可以调用 shutdown() 方法来结束所有提交的任务并终止线程池。
原文链接： http://www.journaldev.com/1069/java-thread-pool-example-using-executors-and-threadpoolexecutor

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