【Java多线程系列七】ExecutorService

java.util.concurrent.ExecutorService接口提供了许多线程管理的方法

Method	说明
shutdown	拒绝接收新的任务，待已提交的任务执行后关闭，且宿主线程不阻塞，若需要阻塞可借助awaitTermination实现
shutdownNow	停止所有正在执行的任务，挂起未执行的任务并关闭，且宿主线程不阻塞，若需要阻塞可借助awaitTermination实现
awaitTermination	当发生shutdown时，阻塞宿主线程直到约定的时间已过或者所有任务完成
submit	提交任务Callable/Runnable，可利用Future的get()方法使宿主线程阻塞直到任务结束后返回结果

有了以上方法，便可以基于此接口实现线程池的各种功能(例如java.util.concurrent.ThreadPoolExecutor/java.util.concurrent.ScheduledThreadPoolExecutor)，以java.util.concurrent.ThreadPoolExecutor为例，其参数的详解

Name	Type	说明
corePoolSize	int	线程池中最小的线程数
maximumPoolSize	int	线程池中最大的线程数
keepAliveTime	long	线程空闲时间，若线程数大于corePoolSize，空闲时间超过该值的线程将被终止回收
unit	TimeUnit	keepAliveTime的时间单位
workQueue	BlockingQueue<Runnable>	已提交但未执行的任务队列
threadFactory	ThreadFactory	创建新线程的工厂
handler	RejectedExecutionHandler	当线程池或队列达到上限拒绝新任务抛出异常时的处理类

同时，java.util.concurrent.Executors类提供的常用方法有

Method	说明	基类
newFixedThreadPool	线程池中含固定数量的线程	基于java.util.concurrent.ThreadPoolExecutor类
newSingleThreadExecutor	线程池中仅含一个工作线程
newCachedThreadPool	按需创建线程，若线程池中无可用线程，则创建新的线程并加入，直到线程数达到上限值(Integer.MAX_VALUE)
newWorkStealingPool	按照可用CPU数创建线程池	基于java.util.concurrent.ForkJoinPool类

java.util.concurrent.ForkJoinPool类是Fork/Join框架的实现类，Fork/Join框架是Java7提供了的一个用于并行执行任务的框架， 是一个把大任务分割成若干个小任务，最终汇总每个小任务结果后得到大任务结果的框架，该类在有递归实现的场景有更优异的表现。

package com.concurrent.test;

import java.util.Arrays;
import java.util.Date;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.ForkJoinPool;
import java.util.concurrent.Future;
import java.util.concurrent.TimeUnit;

import org.junit.Assert;
import org.junit.Test;

/**
 * 测试ExecutorService
 */
public class ThreadExecutorServiceTest {
    private static final String THIS_IS_SHUTDOWN_WITH_AWAIT_TERMINATION = "This is shutdownWithAwaitTermination";
    private static final int RESULT = 111;

    private static boolean submitRunable() throws InterruptedException, ExecutionException {
        ExecutorService executorService = Executors.newSingleThreadExecutor();
        Future<?> future = executorService.submit(new Runnable() {

            @Override
            public void run() {
                System.out.println("This is submitRunnable");
            }
        });

        return future.get() == null;
    }

    private static Integer submitRunnableWithResult() throws InterruptedException, ExecutionException {
        ExecutorService executorService = Executors.newSingleThreadExecutor();
        Future<Integer> future = executorService.submit(new Runnable() {

            @Override
            public void run() {
                System.out.println("This is submitRunnableWithResult");
            }
        }, RESULT);

        return future.get();
    }

    private static Integer submitBlockCallable() throws InterruptedException, ExecutionException {
        ExecutorService executorService = Executors.newSingleThreadExecutor();
        Future<Integer> future = executorService.submit(new Callable<Integer>() {
            @Override
            public Integer call() throws Exception {
                System.out.println("This is submitBlockCallable");
                return RESULT;
            }
        });
        return future.get(); //阻塞
    }

    private static boolean submitNonBlockCallable() {
        ExecutorService executorService = Executors.newSingleThreadExecutor();
        Future<Integer> future = executorService.submit(new Callable<Integer>() {
            @Override
            public Integer call() throws Exception {
                System.out.println("This is submitNonBlockCallable");
                return RESULT;
            }
        });

        while (!future.isDone()) {// 非阻塞
            System.out.println(new Date());
        }

        return future.isDone();
    }

    private static String shutdown() {
        ExecutorService executorService = Executors.newSingleThreadExecutor();
        final StringBuilder sb = new StringBuilder();
        executorService.submit(new Callable<Integer>() {
            @Override
            public Integer call() throws Exception {
                Thread.sleep(10000);
                sb.append("This is shutdown");
                return RESULT;
            }
        });
        executorService.shutdown();
        return sb.toString();
    }

    private static String shutdownWithAwaitTermination() throws InterruptedException {
        ExecutorService executorService = Executors.newSingleThreadExecutor();
        final StringBuilder sb = new StringBuilder();
        executorService.submit(new Callable<Integer>() {
            @Override
            public Integer call() throws Exception {
                Thread.sleep(10000);
                sb.append(THIS_IS_SHUTDOWN_WITH_AWAIT_TERMINATION);
                return RESULT;
            }
        });

        executorService.shutdown();
        executorService.awaitTermination(Integer.MAX_VALUE, TimeUnit.DAYS);
        return sb.toString();
    }

    private static int testForkJoinPool(List<Integer> list) throws InterruptedException, ExecutionException {
        ForkJoinPool forkJoinPool = new ForkJoinPool(8);
        Future<Integer> future = forkJoinPool.submit(new SumTask(list));
        return future.get();
    }

    @Test
    public void test() throws InterruptedException, ExecutionException {
        Assert.assertTrue(submitRunable());
        Assert.assertEquals(RESULT, submitRunnableWithResult().intValue());
        Assert.assertEquals(RESULT, submitBlockCallable().intValue());
        Assert.assertTrue(submitNonBlockCallable());
        Assert.assertTrue(shutdown().isEmpty());
        Assert.assertEquals(THIS_IS_SHUTDOWN_WITH_AWAIT_TERMINATION, shutdownWithAwaitTermination());

        Assert.assertEquals(10, testForkJoinPool(Arrays.asList(new Integer[] { 1, 2, 3, 4 })));
        Assert.assertEquals(49, testForkJoinPool(Arrays.asList(new Integer[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 })));
        Assert.assertEquals(60, testForkJoinPool(Arrays.asList(new Integer[] { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11 })));

    }
}

SumTask类如下：

package com.concurrent.test;

import java.util.List;
import java.util.concurrent.RecursiveTask;

public class SumTask extends RecursiveTask<Integer> {

    /**
     *
     */
    private static final long serialVersionUID = -5468389855825594053L;

    private List<Integer> list;

    public SumTask(List<Integer> list) {
        this.list = list;
    }

    /*
     * Ensure it is necessary to divide the job to parts and finish them separately
     */
    @Override
    protected Integer compute() {
        int rtn , size = list.size();
        if (size < 10) {
            rtn = sum(list);
        }
        else{
            SumTask subTask1 = new SumTask(list.subList(0, size /2));
            SumTask subTask2 = new SumTask(list.subList(size /2 + 1, size));
            subTask1.fork();
            subTask2.fork();
            rtn = subTask1.join() + subTask2.join();
        }
        return rtn;

    }

    private int sum(List<Integer> list) {
        int sum = 0;
        for (Integer integer : list) {
            sum += integer;
        }
        return sum;
    }

}