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.journaldev.threadpool;

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;

    }

}

package com.journaldev.threadpool;

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.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();

        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-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

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.journaldev.threadpool;

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.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;

    }

    @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.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 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个任务和其他任务将交由RejectedExecutionHandlerImpl 处理。

程序执行的结果如下，确认了上面的结论：

pool-1-thread-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

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。

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。

原文链接： journaldev    翻译： ImportNew.com - 彭秦进 译文链接： http://www.importnew.com/8542.html