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

时间：2022-08-21 00:17:15

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;

    }

    @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;

    }

}

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 queue

        while (!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 {

    @Override

    public 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;

    }

    @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();

                }

        }

    }

}

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 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();

    }

}

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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