从头认识java-18.2 主要的线程机制（2）-Executors的使用

在前面的章节我们都是直接对Thread进行管理，我们这里解释一下还有一个管理Thread的类Executors。

1.样例：

package com.ray.ch17;

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

public class Test {

	public static void main(String[] args) throws InterruptedException {
		long startTime = System.currentTimeMillis();
		ExecutorService executorService = Executors.newCachedThreadPool();
		CountDownLatch countDownLatch = new CountDownLatch(5);
		for (int i = 5; i < 10; i++) {
			DoneMission doneMission = new DoneMission(i, countDownLatch);
			executorService.execute(doneMission);
		}
		executorService.shutdown();
		countDownLatch.await();
		long endTime = System.currentTimeMillis();
		System.out.println();
		System.out.println(endTime - startTime);
	}
}

class DoneMission implements Runnable {
	private final int id = index++;
	private int count = 0;
	private static int index = 0;
	private CountDownLatch countDownLatch;

	public DoneMission(int count, CountDownLatch countDownLatch) {
		this.count = count;
		this.countDownLatch = countDownLatch;
	}

	public String leftMission() {
		return "#" + id + "(" + count + ") ";
	}

	@Override
	public void run() {
		while (count-- > 0) {
			System.out.print(leftMission());
			try {
				Thread.sleep(100);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
			Thread.yield();
		}
		countDownLatch.countDown();
	}
}

解释：

（1）运行任务的类跟前面的基本一致

（2）使用Executors.newCachedThreadPool()生成线程池。当须要线程驱动的时候，我们能够到里面拿，它生成的线程数是有系统控制的。

2.对线程的数量做出控制

package com.ray.ch17;

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

public class Test {

	public static void main(String[] args) throws InterruptedException {
		long startTime = System.currentTimeMillis();
		ExecutorService executorService = Executors.newFixedThreadPool(5);//控制线程的数量
		CountDownLatch countDownLatch = new CountDownLatch(5);
		for (int i = 5; i < 10; i++) {
			DoneMission doneMission = new DoneMission(i, countDownLatch);
			executorService.execute(doneMission);
		}
		executorService.shutdown();
		countDownLatch.await();
		long endTime = System.currentTimeMillis();
		System.out.println();
		System.out.println(endTime - startTime);
	}
}

class DoneMission implements Runnable {
	private final int id = index++;
	private int count = 0;
	private static int index = 0;
	private CountDownLatch countDownLatch;

	public DoneMission(int count, CountDownLatch countDownLatch) {
		this.count = count;
		this.countDownLatch = countDownLatch;
	}

	public String leftMission() {
		return "#" + id + "(" + count + ") ";
	}

	@Override
	public void run() {
		while (count-- > 0) {
			System.out.print(leftMission());
			try {
				Thread.sleep(100);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
			Thread.yield();
		}
		countDownLatch.countDown();
	}
}

上面的代码仅仅是替换了一句，就能够控制线程的数量，可是我们一般还是建议使用cache的那个。由于它对线程池做出来优化。

特别是对于短的异步任务它具有明显优势。

3.測试不同线程运行的时间：

package com.ray.ch17;

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

public class Test {

	public static void main(String[] args) throws InterruptedException {
		long startTime = System.currentTimeMillis();
		ExecutorService executorService = Executors.newCachedThreadPool();
		CountDownLatch countDownLatch = new CountDownLatch(5);
		for (int i = 5; i < 10; i++) {
			DoneMission doneMission = new DoneMission(i, countDownLatch);
			executorService.execute(doneMission);
		}
		executorService.shutdown();
		countDownLatch.await();
		long endTime = System.currentTimeMillis();
		System.out.println();
		System.out.println("time:"+(endTime - startTime));
	}
}

class DoneMission implements Runnable {
	private final int id = index++;
	private int count = 0;
	private static int index = 0;
	private CountDownLatch countDownLatch;

	public DoneMission(int count, CountDownLatch countDownLatch) {
		this.count = count;
		this.countDownLatch = countDownLatch;
	}

	public String leftMission() {
		return "#" + id + "(" + count + ") ";
	}

	@Override
	public void run() {
		while (count-- > 0) {
			System.out.print(leftMission());
			try {
				Thread.sleep(100);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
			Thread.yield();
		}
		countDownLatch.countDown();
	}
}

输出：

#0(4) #1(5) #2(6) #3(7) #4(8) #0(3) #1(4) #4(7) #3(6) #2(5) #0(2) #1(3) #3(5) #2(4) #4(6) #0(1) #1(2) #4(5) #3(4) #2(3) #0(0) #1(1) #2(2) #4(4) #3(3) #1(0) #2(1) #3(2) #4(3) #4(2) #3(1) #2(0) #3(0) #4(1) #4(0) 
904

控制为3个线程：

package com.ray.ch17;

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

public class Test {

	public static void main(String[] args) throws InterruptedException {
		long startTime = System.currentTimeMillis();
		ExecutorService executorService = Executors.newFixedThreadPool(3);
		CountDownLatch countDownLatch = new CountDownLatch(5);
		for (int i = 5; i < 10; i++) {
			DoneMission doneMission = new DoneMission(i, countDownLatch);
			executorService.execute(doneMission);
		}
		executorService.shutdown();
		countDownLatch.await();
		long endTime = System.currentTimeMillis();
		System.out.println();
		System.out.println(endTime - startTime);
	}
}

class DoneMission implements Runnable {
	private final int id = index++;
	private int count = 0;
	private static int index = 0;
	private CountDownLatch countDownLatch;

	public DoneMission(int count, CountDownLatch countDownLatch) {
		this.count = count;
		this.countDownLatch = countDownLatch;
	}

	public String leftMission() {
		return "#" + id + "(" + count + ") ";
	}

	@Override
	public void run() {
		while (count-- > 0) {
			System.out.print(leftMission());
			try {
				Thread.sleep(100);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
			Thread.yield();
		}
		countDownLatch.countDown();
	}
}

输出：

#0(4) #2(6) #1(5) #1(4) #0(3) #2(5) #0(2) #2(4) #1(3) #1(2) #2(3) #0(1) #0(0) #1(1) #2(2) #1(0) #2(1) #3(7) #3(6) #4(8) #2(0) #3(5) #4(7) #3(4) #4(6) #3(3) #4(5) #4(4) #3(2) #4(3) #3(1) #4(2) #3(0) #4(1) #4(0) 
1504

控制为单线程：

package com.ray.ch17;

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

public class Test {

	public static void main(String[] args) throws InterruptedException {
		long startTime = System.currentTimeMillis();
		ExecutorService executorService = Executors.newSingleThreadExecutor();
		CountDownLatch countDownLatch = new CountDownLatch(5);
		for (int i = 5; i < 10; i++) {
			DoneMission doneMission = new DoneMission(i, countDownLatch);
			executorService.execute(doneMission);
		}
		executorService.shutdown();
		countDownLatch.await();
		long endTime = System.currentTimeMillis();
		System.out.println();
		System.out.println(endTime - startTime);
	}
}

class DoneMission implements Runnable {
	private final int id = index++;
	private int count = 0;
	private static int index = 0;
	private CountDownLatch countDownLatch;

	public DoneMission(int count, CountDownLatch countDownLatch) {
		this.count = count;
		this.countDownLatch = countDownLatch;
	}

	public String leftMission() {
		return "#" + id + "(" + count + ") ";
	}

	@Override
	public void run() {
		while (count-- > 0) {
			System.out.print(leftMission());
			try {
				Thread.sleep(100);
			} catch (InterruptedException e) {
				e.printStackTrace();
			}
			Thread.yield();
		}
		countDownLatch.countDown();
	}
}

输出：

#0(4) #0(3) #0(2) #0(1) #0(0) #1(5) #1(4) #1(3) #1(2) #1(1) #1(0) #2(6) #2(5) #2(4) #2(3) #2(2) #2(1) #2(0) #3(7) #3(6) #3(5) #3(4) #3(3) #3(2) #3(1) #3(0) #4(8) #4(7) #4(6) #4(5) #4(4) #4(3) #4(2) #4(1) #4(0) 
3503

单线程说白了就像直接在main方法里面使用for来运行的一样。

4.关于shutdown（）（以下的这段话摘自http://my.oschina.net/bairrfhoinn/blog/177639，笔者认为他解释的已经比較清楚。事实上关键是笔者比較懒，不喜欢打字）

为了关闭在 ExecutorService 中的线程，你须要调用 shutdown() 方法。ExecutorService 并不会立即关闭，而是不再接收新的任务，一旦全部的线程结束运行当前任务，ExecutorServie 才会真的关闭。

全部在调用 shutdown() 方法之前提交到 ExecutorService 的任务都会运行。

假设你希望立即关闭 ExecutorService，你能够调用 shutdownNow() 方法。这种方法会尝试立即关闭全部正在运行的任务。而且跳过全部已经提交可是还没有运行的任务。可是对于正在运行的任务。能否够成功关闭它是无法保证的，有可能他们真的被关闭掉了，也有可能它会一直运行到任务结束。

总结：这一章节主要介绍Executors的使用。

这一章节就到这里，谢谢。

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