线程池实例：使用Executors和ThreadPoolExecutor

        线程池负责管理工作线程，包含一个等待执行的任务队列。线程池的任务队列是一个Runnable集合，工作线程负责从任务队列中取出并执行Runnable对象。

        java.util.concurrent.executors 提供了 java.util.concurrent.executor 接口的一个Java实现，可以创建线程池。下面是一个简单示例：

        首先创建一个Runable 类：

WorkerThread.java

package com.bijian.study;

public class WorkerThread implements Runnable {
	 
    private String command;
 
    public WorkerThread(String s){
        this.command=s;
    }
 
    @Override
    public 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();
        }
    }
 
    @Override
    public String toString(){
        return this.command;
    }
}

        下面是一个测试程序，从 Executors 框架中创建固定大小的线程池：

SimpleThreadPool.java

package com.bijian.study;

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();
        while (!executor.isTerminated()) {
        }
        System.out.println("Finished all threads");
    }
}

        在上面的程序中，我们创建了包含5个工作线程的固定大小线程池。然后，我们向线程池提交10个任务。由于线程池的大小是5，因此首先会启动5个工作线程，其他任务将进行等待。一旦有任务结束，工作线程会从等待队列中挑选下一个任务并开始执行。

        以上程序的输出结果如下：

pool-1-thread-1 Start. Command = 0
pool-1-thread-4 Start. Command = 3
pool-1-thread-5 Start. Command = 4
pool-1-thread-2 Start. Command = 1
pool-1-thread-3 Start. Command = 2
pool-1-thread-2 End.
pool-1-thread-4 End.
pool-1-thread-1 End.
pool-1-thread-5 End.
pool-1-thread-3 End.
pool-1-thread-5 Start. Command = 8
pool-1-thread-1 Start. Command = 7
pool-1-thread-2 Start. Command = 5
pool-1-thread-4 Start. Command = 6
pool-1-thread-3 Start. Command = 9
pool-1-thread-3 End.
pool-1-thread-2 End.
pool-1-thread-1 End.
pool-1-thread-4 End.
pool-1-thread-5 End.
Finished all threads

        从输出结果看，线程池中有五个名为“pool-1-thread-1”…“pool-1-thread-5”的工作线程负责执行提交的任务。

        Executors 类使用 ExecutorService  提供了一个 ThreadPoolExecutor 的简单实现，但ThreadPoolExecutor 提供的功能远不止这些。我们可以指定创建 ThreadPoolExecutor 实例时活跃的线程数，并且可以限制线程池的大小，还可以创建自己的 RejectedExecutionHandler 实现来处理不适合放在工作队列里的任务。

        下面是一个 RejectedExecutionHandler 接口的自定义实现：

RejectedExecutionHandlerImpl.java

package com.bijian.study;

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

        ThreadPoolExecutor 提供了一些方法，可以查看执行状态、线程池大小、活动线程数和任务数。所以，我通过一个监视线程在固定间隔输出执行信息。

MyMonitorThread.java

package com.bijian.study;

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;
    }
 
    @Override
    public 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();
                }
        }
 
    }
}

        下面是使用 ThreadPoolExecutor 的线程池实现示例：

WorkerPool.java

package com.bijian.study;

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 implementation
        RejectedExecutionHandlerImpl rejectionHandler = new RejectedExecutionHandlerImpl();
        //Get the ThreadFactory implementation to use
        ThreadFactory threadFactory = Executors.defaultThreadFactory();
        //creating the ThreadPoolExecutor
        ThreadPoolExecutor executorPool = new ThreadPoolExecutor(2, 4, 10, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(2), threadFactory, rejectionHandler);
        //start the monitoring thread
        MyMonitorThread monitor = new MyMonitorThread(executorPool, 3);
        Thread monitorThread = new Thread(monitor);
        monitorThread.start();
        //submit work to the thread pool
        for(int i=0; i<10; i++){
            executorPool.execute(new WorkerThread("cmd"+i));
        }
 
        Thread.sleep(30000);
        //shut down the pool
        executorPool.shutdown();
        //shut down the monitor thread
        Thread.sleep(5000);
        monitor.shutdown();
    }
}

        请注意：在初始化 ThreadPoolExecutor 时，初始线程池大小设为2、最大值设为4、工作队列大小设为2。所以，如果当前有4个任务正在运行，而此时又有新任务提交，工作队列将只存储2个任务，其他任务将交由RejectedExecutionHandlerImpl 处理。

        程序执行的结果如下，确认了上面的结论：

pool-1-thread-2 Start. Command = cmd1
cmd6 is rejected
cmd7 is rejected
cmd8 is rejected
cmd9 is rejected
pool-1-thread-4 Start. Command = cmd5
pool-1-thread-1 Start. Command = cmd0
pool-1-thread-3 Start. Command = cmd4
[monitor] [0/2] Active: 0, Completed: 0, Task: 6, isShutdown: false, isTerminated: false
[monitor] [4/2] Active: 4, Completed: 0, Task: 6, isShutdown: false, isTerminated: false
pool-1-thread-2 End.
pool-1-thread-4 End.
pool-1-thread-1 End.
pool-1-thread-3 End.
pool-1-thread-4 Start. Command = cmd3
pool-1-thread-2 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-2 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

        请注意活跃线程、已完成线程和任务完成总数的变化。我们可以调用 shutdown() 结束所有已提交任务并终止线程池。

        再修改WorkerPool.java，将工作队列大小设为3，如下所示：

//creating the ThreadPoolExecutor
ThreadPoolExecutor executorPool = new ThreadPoolExecutor(2, 4, 10, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(3), threadFactory, rejectionHandler);

        运行结果如下：

pool-1-thread-1 Start. Command = cmd0
pool-1-thread-2 Start. Command = cmd1
pool-1-thread-4 Start. Command = cmd6
cmd7 is rejected
pool-1-thread-3 Start. Command = cmd5
cmd8 is rejected
cmd9 is rejected
[monitor] [0/2] Active: 0, Completed: 0, Task: 6, isShutdown: false, isTerminated: false
[monitor] [4/2] Active: 4, Completed: 0, Task: 7, isShutdown: false, isTerminated: false
pool-1-thread-3 End.
pool-1-thread-2 End.
pool-1-thread-4 End.
pool-1-thread-1 End.
pool-1-thread-4 Start. Command = cmd4
pool-1-thread-2 Start. Command = cmd3
pool-1-thread-3 Start. Command = cmd2
[monitor] [4/2] Active: 3, Completed: 4, Task: 7, isShutdown: false, isTerminated: false
[monitor] [4/2] Active: 3, Completed: 4, Task: 7, isShutdown: false, isTerminated: false
pool-1-thread-2 End.
pool-1-thread-3 End.
pool-1-thread-4 End.
[monitor] [4/2] Active: 0, Completed: 7, Task: 7, isShutdown: false, isTerminated: false
[monitor] [3/2] Active: 0, Completed: 7, Task: 7, isShutdown: false, isTerminated: false
[monitor] [3/2] Active: 0, Completed: 7, Task: 7, isShutdown: false, isTerminated: false
[monitor] [2/2] Active: 0, Completed: 7, Task: 7, isShutdown: false, isTerminated: false
[monitor] [2/2] Active: 0, Completed: 7, Task: 7, isShutdown: false, isTerminated: false
[monitor] [2/2] Active: 0, Completed: 7, Task: 7, isShutdown: false, isTerminated: false
[monitor] [0/2] Active: 0, Completed: 7, Task: 7, isShutdown: true, isTerminated: true
[monitor] [0/2] Active: 0, Completed: 7, Task: 7, isShutdown: true, isTerminated: true

        在初始化 ThreadPoolExecutor 时，初始线程池大小设为2、最大值设为4、工作队列大小设为3。所以，如果当前有4个任务正在运行，而此时又有新任务提交，工作队列将只存储3个任务，其他任务将交由RejectedExecutionHandlerImpl 处理。

        进一步查看ThreadPoolExecutor的此构造方法源代码如下：

/**
 * Creates a new {@code ThreadPoolExecutor} with the given initial
 * parameters.
 *
 * @param corePoolSize the number of threads to keep in the pool, even
 *        if they are idle, unless {@code allowCoreThreadTimeOut} is set
 * @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 {@code keepAliveTime} argument
 * @param workQueue the queue to use for holding tasks before they are
 *        executed.  This queue will hold only the {@code Runnable}
 *        tasks submitted by the {@code 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 one of the following holds:<br>
 *         {@code corePoolSize < 0}<br>
 *         {@code keepAliveTime < 0}<br>
 *         {@code maximumPoolSize <= 0}<br>
 *         {@code maximumPoolSize < corePoolSize}
 * @throws NullPointerException if {@code workQueue}
 *         or {@code threadFactory} or {@code handler} is null
 */
public ThreadPoolExecutor(int corePoolSize,
						  int maximumPoolSize,
						  long keepAliveTime,
						  TimeUnit unit,
						  BlockingQueue<Runnable> workQueue,
						  ThreadFactory threadFactory,
						  RejectedExecutionHandler handler) {
	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;
}

        如果希望延迟执行或定期运行任务，那么可以使用 ScheduledThreadPoolExecutor 类。要了解更多，请参见 Java Schedule Thread Pool Executor。

 

文章来源：http://www.importnew.com/8542.html