线程池中shutdown，awaitTermination，shutdownNow的作用和区别

package ThreadPools_5;

import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.TimeUnit;

/**
 * This is the implementation of {@link LockObjects_4.Worker} with threadPool
 * <br><br>
 * Codes with minor comments are from
 * <a href="http://www.caveofprogramming.com/youtube/">
 * <em>http://www.caveofprogramming.com/youtube/</em>
 * </a>
 * <br>
 * also freely available at
 * <a href="https://www.udemy.com/java-multithreading/?couponCode=FREE">
 *     <em>https://www.udemy.com/java-multithreading/?couponCode=FREE</em>
 * </a>
 *
 * @author Z.B. Celik <[email protected]>
 */
class Worker implements Runnable {

    private Random random = new Random();
    private final Object lock1 = new Object();
    private final Object lock2 = new Object();
    public List<Integer> list1 = new ArrayList<Integer>();
    public List<Integer> list2 = new ArrayList<Integer>();

    public void run() {
        process();
    }

    public void process() {
        for (int i = 0; i < 1000; i++) {
            stageOne();
            stageTwo();
        }
    }

    public void stageOne() {
        synchronized (lock1) {
            try {
                Thread.sleep(1);
            } catch (InterruptedException e) {
                //do your work here
                e.printStackTrace();
            }
            list1.add(random.nextInt(100));
        }
    }

    public void stageTwo() {
        synchronized (lock2) {
            try {
                Thread.sleep(1);
            } catch (InterruptedException e) {
                //do your work here
                e.printStackTrace();
            }
            list2.add(random.nextInt(100));
        }
    }
}

public class WorkerThreadPool {

    public static void main(String[] args) {
        ExecutorService executor = Executors.newFixedThreadPool(2);//two threads, try setting by 1 to observe time
        System.out.println("Starting ...");
        long start = System.currentTimeMillis();
        Worker worker = new Worker();
        for (int i = 0; i < 2; i++) {//worker.run is called 2 (threads started) times by two threads
            executor.submit(worker);
        }
        executor.shutdown(); //prevents new tasks from being accepted by the ExecutorService
        try {
            executor.awaitTermination(1, TimeUnit.DAYS);
            // How long should I wait for termination If I do not know exactly when threads are done with the tasks ?
            // Source:http://stackoverflow.com/questions/1250643/how-to-wait-for-all-threads-to-finish-using-executorservice
            // For a perpetually running batch kind of thing u need to submit jobs and wait for them to
            // finish before jumping ahead.
            // In Such a case a latch or a barrier makes more sense than a shutdown (see CountDownLatch_6.App).
        } catch (Exception ex) {
            System.out.println(ex.getMessage());
        }
        long end = System.currentTimeMillis();
        System.out.println("Time taken: " + (end - start));
        System.out.println("List1: " + worker.list1.size() + "; List2: " + worker.list2.size());
    }

}

上述代码执行后控制台打印

Starting ...
Time taken: 2183
List1: 2000; List2: 2000

shutdown方法：平滑的关闭ExecutorService，当此方法被调用时，ExecutorService停止接收新的任务并且等待已经提交的任务（包含提交正在执行和提交未执行）执行完成。当所有提交任务执行完毕，线程池即被关闭。
awaitTermination方法：接收人timeout和TimeUnit两个参数，用于设定超时时间及单位。当等待超过设定时间时，会监测ExecutorService是否已经关闭，若关闭则返回true，否则返回false。一般情况下会和shutdown方法组合使用。

3.shutdownNow方法的作用是向所有执行中的线程发出interrupted以中止线程的运行。这时，各个线程会抛出 InterruptedException异常（前提是
线程中运行了sleep等会抛出异常的方法）

在上述代码中 shutdown方法的意思是等待当前正在执行的线程执行完毕并且停止接受新的线程执行，如果新开线程的话会抛出RejectedExecutionException异常

awaitTermination的第一个参数是时间，第二个参数是时间单位，在这个地方监视线程是否都执行完毕，如果执行完毕后会执行后面的代码

一般情况下awaitTermination和shutdown配合使用，在上述代码中如果注释掉shutdown方法则awaitTermination不会监视到线程池关闭的信息所以在这个地方代码会堵塞，

如果注释掉awaitTermination方法则后面的代码不会得到线程执行过的结果

控制台结果

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Starting ...
Time taken: 2
List1: 0; List2: 0

参考资料点这里

线程池中shutdown，awaitTermination，shutdownNow的作用和区别

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