线程池之ThreadPoolExecutor

所属包：

java.util.concurrent.ThreadPoolExecutor

 类关系：

public class ThreadPoolExecutor extends AbstractExecutorService

1. 继承关系

ThreadPoolExecutor 继承了一个抽象类：AbstractExecutorService

public abstract class AbstractExecutorService implements ExecutorService

而这个AbstractExecutorService实现了一个接口：ExecutorService

public interface ExecutorService extends Executor

这个ExecutorService接口又继承了一个类：Executor

public interface Executor

可以看出：

Executor是一个顶层接口，它的子接口ExecutorService继承了它（其实还有一个子接口： ScheduledExecutorService），抽象类AbstractExecutorService实现了这个子接口ExecutorService，最终ThreadPoolExecutor 继承了抽象类AbstractExecutorService并且同时实现了子接口ExecutorService。

2. 构造方法 

最简单的一个构造方法：

    /**
     * Creates a new {@code ThreadPoolExecutor} with the given initial
     * parameters and default thread factory and rejected execution handler.
     * It may be more convenient to use one of the {@link Executors} factory
     * methods instead of this general purpose constructor.
     *
     * @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.
     * @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} is null
     */
    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             Executors.defaultThreadFactory(), defaultHandler);
    }

有五个参数：

corePoolSize - 即使空闲时仍保留在池中的线程数，除非设置 allowCoreThreadTimeOut 
maximumPoolSize - 池中允许的最大线程数 
keepAliveTime - 当线程数大于核心时，这是多余的空闲线程在终止之前等待新任务的最大时间。 
unit - keepAliveTime参数的时间单位 
workQueue - 在执行任务之前用于保存任务的队列。 该队列将仅保存execute方法提交的Runnable任务

 实际上它调用了同类的另外一个构造方法，最后两个参数用的默认值

另外一个构造方法：

    /**
     * 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;
    }

参数：

corePoolSize - 即使空闲时仍保留在池中的线程数，除非设置 allowCoreThreadTimeOut 
maximumPoolSize - 池中允许的最大线程数 
keepAliveTime - 当线程数大于内核时，这是多余的空闲线程在终止前等待新任务的最大时间。 
unit - keepAliveTime参数的时间单位 
workQueue - 用于在执行任务之前使用的队列。 这个队列将仅保存execute方法提交的Runnable任务。 
threadFactory - 执行程序创建新线程时使用的工厂 
handler - 执行被阻止时使用的处理程序，因为达到线程限制和队列容量 （拒绝策略）

3. 如何使用

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class MyThreadPoolExecutor {

    /**
     * 使用有界队列：
     *  1、当线程数小于corePoolSize时，创建线程执行任务。
     *  2、当线程数大于等于corePoolSize并且workQueue没有满时，放入workQueue中
     *  3、线程数大于等于corePoolSize并且当workQueue满时，新任务新建线程运行，线程总数要小于maximumPoolSize
     *  4、当线程总数等于maximumPoolSize并且workQueue满了的时候执行handler的rejectedExecution。也就是拒绝策略。
     * 
     * ThreadPoolExecutor默认有四个拒绝策略：
     *  1、ThreadPoolExecutor.AbortPolicy()           直接抛出异常RejectedExecutionException
     *  2、ThreadPoolExecutor.CallerRunsPolicy()        直接调用run方法并且阻塞执行
     *  3、ThreadPoolExecutor.DiscardPolicy()           直接丢弃后来的任务
     *  4、ThreadPoolExecutor.DiscardOldestPolicy()    丢弃在队列中队首的任务
     *  
     * 
     *  
     */
    
    public static void main(String[] args) {
        
        ThreadPoolExecutor pool = new ThreadPoolExecutor(
                1,
                2,
                0L,
                TimeUnit.SECONDS,
                new ArrayBlockingQueue<>(3));
        
        Runnable[] runs = new Runnable[6];
        for (int i = 0; i < runs.length; i++) {
            runs[i] = new MyTask(i);
        }
        
        pool.execute(runs[0]);    //线程1个，队列0个
        pool.execute(runs[1]);    //线程1个，队列1个
        pool.execute(runs[2]);    //线程1个，队列2个
        pool.execute(runs[3]);    //线程1个，队列3个
        pool.execute(runs[4]);    //线程2个，队列3个
        pool.execute(runs[5]);    //线程2个，队列3个，拒绝第六个
        
        pool.shutdown();
        
    }
}

线程：

public class MyTask implements Runnable {

    private int id;
    
    public MyTask(int id) {
        this.id = id;
    }

    @Override
    public void run() {
        try {
            Thread.sleep(2000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("Task-" + id);
    }

}

这个就是最简单的一个使用方法了。ps：最后那个（线程1个，队列0个....）指的是，你仅仅执行runs[0]；runs[0]+runs[1]；runs[0]+runs[1]+runs[2]；....的时候，任务被放到哪里。

但是推荐使用这种方式创建线程池：

package cn.ying.thread.pool;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class MyTest {
    
    public static void main(String[] args) {
        ExecutorService pool = Executors.newFixedThreadPool(5);
        
        for (int i = 0; i < 10; i++) {
            pool.execute(new MyTask(i));
        }
        pool.shutdown();
    }

    public void note(){
        Executors.newCachedThreadPool();    //无界线程池，可以进行自动线程回收
//        public static ExecutorService newCachedThreadPool() {
//            return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
//                                          60L, TimeUnit.SECONDS,
//                                          new SynchronousQueue<Runnable>());//SynchronousQueue:长度为1的队列
//        }
        Executors.newFixedThreadPool(10);    //创建一个可重用固定线程数的线程池，以共享的无界队列方式来运行这些线程。
//        public static ExecutorService newFixedThreadPool(int nThreads) {
//            return new ThreadPoolExecutor(nThreads, nThreads,
//                                          0L, TimeUnit.MILLISECONDS,
//                                          new LinkedBlockingQueue<Runnable>());
//        }
        Executors.newScheduledThreadPool(10);    //创建一个单线程执行程序，它可安排在给定延迟后运行命令或者定期地执行
//        public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
//            return new ScheduledThreadPoolExecutor(corePoolSize);
//        }
//        public ScheduledThreadPoolExecutor(int corePoolSize) {
//            super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
//                  new DelayedWorkQueue());
//        }
        Executors.newSingleThreadExecutor();    //创建一个使用单个 worker 线程的 Executor，以无界队列方式来运行该线程
//        public static ExecutorService newSingleThreadExecutor() {
//            return new FinalizableDelegatedExecutorService
//                (new ThreadPoolExecutor(1, 1,
//                                        0L, TimeUnit.MILLISECONDS,
//                                        new LinkedBlockingQueue<Runnable>()));
//        }
        
    }
}

==============================更新

4. 拒绝策略

1. AbortPolicy（丢弃任务并抛出RejectedExecutionException异常）默认策略

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class Test {

    public static void main(String[] args) throws Exception {
        ThreadPoolExecutor pool = new ThreadPoolExecutor(
                1,
                2,
                0L,
                TimeUnit.SECONDS,
                new ArrayBlockingQueue<>(3),
                // 拒绝并抛出异常
                new ThreadPoolExecutor.AbortPolicy());

        for (int i = 0; i < 6; i++){
            pool.execute(new Task("任务" + i));
        }
        pool.shutdown();
    }

    private static class Task implements Runnable{
        private String name;

        Task(String name) {
            this.name = name;
        }

        @Override
        public void run() {
            System.out.println(Thread.currentThread().getName() + "===" + name);
        }
    }

}

运行：

2. DiscardPolicy（丢弃任务，但是不抛出异常）

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class Test {

    public static void main(String[] args) throws Exception {
        ThreadPoolExecutor pool = new ThreadPoolExecutor(
                1,
                2,
                0L,
                TimeUnit.SECONDS,
                new ArrayBlockingQueue<>(3),
                // 拒绝但不抛出异常
                new ThreadPoolExecutor.DiscardPolicy());

        for (int i = 0; i < 6; i++){
            pool.execute(new Task("任务" + i));
        }
        pool.shutdown();
    }

    private static class Task implements Runnable{
        private String name;

        Task(String name) {
            this.name = name;
        }

        @Override
        public void run() {
            System.out.println(Thread.currentThread().getName() + "===" + name);
        }
    }

}

运行：

 3. DiscardOldestPolicy（丢弃队列最前面的任务，然后重新提交被拒绝的任务）

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class Test {

    public static void main(String[] args) throws Exception {
        ThreadPoolExecutor pool = new ThreadPoolExecutor(
                1,
                2,
                0L,
                TimeUnit.SECONDS,
                new ArrayBlockingQueue<>(3),
                // 丢弃队列中最前面的任务，然后执行被拒绝的任务
                new ThreadPoolExecutor.DiscardOldestPolicy());

        for (int i = 0; i < 6; i++){
            pool.execute(new Task("任务" + i));
        }
        pool.shutdown();
    }

    private static class Task implements Runnable{
        private String name;

        Task(String name) {
            this.name = name;
        }

        @Override
        public void run() {
            System.out.println(Thread.currentThread().getName() + "===" + name);
        }
    }

}

运行：

4. CallerRunsPolicy（由提交任务的线程处理）

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class Test {

    public static void main(String[] args) throws Exception {
        ThreadPoolExecutor pool = new ThreadPoolExecutor(
                1,
                2,
                0L,
                TimeUnit.SECONDS,
                new ArrayBlockingQueue<>(3),
                // 由提交任务的线程直接执行此任务
                new ThreadPoolExecutor.CallerRunsPolicy());

        for (int i = 0; i < 6; i++){
            pool.execute(new Task("任务" + i));
        }
        pool.shutdown();
    }

    private static class Task implements Runnable{
        private String name;

        Task(String name) {
            this.name = name;
        }

        @Override
        public void run() {
            System.out.println(Thread.currentThread().getName() + "===" + name);
        }
    }

}

运行：

 5. 自己处理

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

public class Test {

    public static void main(String[] args) throws Exception {
        ThreadPoolExecutor pool = new ThreadPoolExecutor(
                1,
                2,
                0L,
                TimeUnit.SECONDS,
                new ArrayBlockingQueue<>(3),
                // 自定义处理
                (Runnable r, ThreadPoolExecutor executor) -> {
                    System.out.println("自定义处理");
                });

        for (int i = 0; i < 6; i++){
            pool.execute(new Task("任务" + i));
        }
        pool.shutdown();
    }

    private static class Task implements Runnable{
        private String name;

        Task(String name) {
            this.name = name;
        }

        @Override
        public void run() {
            System.out.println(Thread.currentThread().getName() + "===" + name);
        }
    }

}

运行：