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011-Executors创建线程池.md 9.67 KB
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ConstXiong authored 2019-09-08 00:01 . sync github concurrency

Executors如何创建线程池?

Executors 类是从 JDK 1.5 开始就新增的线程池创建的静态工厂类,它就是创建线程池的,但是很多的大厂已经不建议使用该类去创建线程池。原因在于,该类创建的很多线程池的内部使用了无界任务队列,在并发量很大的情况下会导致 JVM 抛出 OutOfMemoryError,直接让 JVM 崩溃,影响严重。

但是 Executors 类究竟是如何使用的?

  1. newFixedThreadPool,创建定长线程池,每当提交一个任务就创建一个线程,直到达到线程池的最大数量,这时线程数量不再变化,当线程发生错误结束时,线程池会补充一个新的线程。
package constxiong.concurrency.a011;

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

/**
 * 测试创建定长线程池
 * @author ConstXiong
 */
public class TestNewFixedThreadPool {

	public static void main(String[] args) {
		//创建工作线程数为 3 的线程池,每当提交一个任务就创建一个线程,直到达到线程池的最大数量,这时线程数量不再变化,当线程发生错误结束时,线程池会补充一个新的线程
		ExecutorService fixedThreadPool = Executors.newFixedThreadPool(3);
		//提交 6 个任务
		for (int i = 0; i < 6; i++) {
			final int index = i;
			fixedThreadPool.execute(() -> {
				try {
					//休眠 3 秒
					Thread.sleep(3000);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
				System.out.println(Thread.currentThread().getName() + " index:" + index);
			});
		}
		
		try {
			Thread.sleep(4000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		System.out.println("4秒后...");
		
		//关闭线程池后,已提交的任务仍然会执行完
		fixedThreadPool.shutdown();
	}
	
}

打印结果:

pool-1-thread-2 index:1
pool-1-thread-3 index:2
pool-1-thread-1 index:0
4秒后...
pool-1-thread-1 index:4
pool-1-thread-3 index:5
pool-1-thread-2 index:3
  1. newCachedThreadPool,创建可缓存的线程池,如果线程池的容量超过了任务数,自动回收空闲线程,任务增加时可以自动添加新线程,线程池的容量不限制。
package constxiong.concurrency.a011;

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

/**
 * 测试创建可缓存的线程池
 * @author ConstXiong
 */
public class TestNewCachedThreadPool {
	
	public static void main(String[] args) {
		//创建可缓存的线程池,如果线程池的容量超过了任务数,自动回收空闲线程,任务增加时可以自动添加新线程,线程池的容量不限制
		ExecutorService cachedThreadPool = Executors.newCachedThreadPool();

		for (int i = 0; i < 6; i++) {
			final int index = i;
			cachedThreadPool.execute(() -> {
				try {
					Thread.sleep(3000);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
				System.out.println(Thread.currentThread().getName() + " index:" + index);
			});
		}
		
		try {
			Thread.sleep(4000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		System.out.println("4秒后...");
		
		cachedThreadPool.shutdown();
		
	}
	
}

打印结果可以,创建的线程数于任务数相等

pool-1-thread-1 index:0
pool-1-thread-3 index:2
pool-1-thread-6 index:5
pool-1-thread-4 index:3
pool-1-thread-5 index:4
pool-1-thread-2 index:1
4秒后...
  1. newScheduledThreadPool,创建定长线程池,可执行周期性的任务。
package constxiong.concurrency.a011;

import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

/**
 * 测试创建定长线程池,可执行周期性的任务
 * @author ConstXiong
 */
public class TestNewScheduledThreadPool {

	public static void main(String[] args) {
		//创建定长线程池,可执行周期性的任务
		ScheduledExecutorService scheduledThreadPool = Executors.newScheduledThreadPool(3);
		
		for (int i = 0; i < 3; i++) {
			final int index = i;
			//scheduleWithFixedDelay 固定的延迟时间执行任务; scheduleAtFixedRate 固定的频率执行任务
			scheduledThreadPool.scheduleWithFixedDelay(() -> {
					System.out.println(Thread.currentThread().getName() + " index:" + index);
			}, 0, 3, TimeUnit.SECONDS);
		}
		
		try {
			Thread.sleep(4000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		System.out.println("4秒后...");
		
		scheduledThreadPool.shutdown();

	}
}

打印结果:

pool-1-thread-1 index:0
pool-1-thread-3 index:2
pool-1-thread-2 index:1
pool-1-thread-1 index:0
pool-1-thread-2 index:1
pool-1-thread-3 index:2
4秒后...
  1. newSingleThreadExecutor,创建单线程的线程池,线程异常结束,会创建一个新的线程,能确保任务按提交顺序执行。
package constxiong.concurrency.a011;

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

/**
 * 测试单线程的线程池
 * @author ConstXiong
 */
public class TestNewSingleThreadExecutor {
	
	public static void main(String[] args) {
		//单线程的线程池,线程异常结束,会创建一个新的线程,能确保任务按提交顺序执行
		ExecutorService singleThreadPool = Executors.newSingleThreadExecutor();
		
		//提交 3 个任务
		for (int i = 0; i < 3; i++) {
			final int index = i;
			singleThreadPool.execute(() -> {
				
				//执行第二个任务时,报错,测试线程池会创建新的线程执行任务三
				if (index == 1) {
					throw new RuntimeException("线程执行出现异常");
				}
				
				try {
					Thread.sleep(3000);
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
				System.out.println(Thread.currentThread().getName() + " index:" + index);
			});
		}
		
		try {
			Thread.sleep(4000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		System.out.println("4秒后...");
		
		singleThreadPool.shutdown();
	}

}

打印结果可以看出,即使任务出现了异常,线程池还是会自动补充一个线程继续执行下面的任务

pool-1-thread-1 index:0Exception in thread "pool-1-thread-1" 
java.lang.RuntimeException: 线程执行出现异常
	at constxiong.concurrency.a011.TestNewSingleThreadExecutor.lambda$0(TestNewSingleThreadExecutor.java:21)
	at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
	at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
	at java.lang.Thread.run(Thread.java:748)
4秒后...
pool-1-thread-2 index:2
  1. newSingleThreadScheduledExecutor,创建单线程可执行周期性任务的线程池。
package constxiong.concurrency.a011;

import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

/**
 * 测试单线程可执行周期性任务的线程池
 * @author ConstXiong
 */
public class TestNewSingleThreadScheduledExecutor {

	public static void main(String[] args) {
		//创建单线程可执行周期性任务的线程池
		ScheduledExecutorService singleScheduledThreadPool = Executors.newSingleThreadScheduledExecutor();
		
		//提交 3 个固定频率执行的任务
		for (int i = 0; i < 3; i++) {
			final int index = i;
			//scheduleWithFixedDelay 固定的延迟时间执行任务; scheduleAtFixedRate 固定的频率执行任务
			singleScheduledThreadPool.scheduleAtFixedRate(() -> {
				System.out.println(Thread.currentThread().getName() + " index:" + index);
			}, 0, 3, TimeUnit.SECONDS);
		}
		
		try {
			Thread.sleep(4000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		System.out.println("4秒后...");
		
		singleScheduledThreadPool.shutdown();
	}
	
}

打印机结果可以看出 0-2 任务都被执行了 2 个周期

pool-1-thread-1 index:0
pool-1-thread-1 index:1
pool-1-thread-1 index:2
pool-1-thread-1 index:0
pool-1-thread-1 index:1
pool-1-thread-1 index:2
4秒后...
  1. newWorkStealingPool,创建任务可窃取线程池,空闲线程可以窃取其他任务队列的任务,不保证执行顺序,适合任务耗时差异较大。
package constxiong.concurrency.a011;

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

/**
 * 测试可任务窃取线程池
 * @author ConstXiong
 */
public class TestNewWorkStealingPool {

	public static void main(String[] args) {
		//创建 4个工作线程的 任务可窃取线程池,如果不设置并行数,默认取 CPU 总核数
		ExecutorService workStealingThreadPool = Executors.newWorkStealingPool(4);
		
		for (int i = 0; i < 10; i++) {
			final int index = i;
			workStealingThreadPool.execute(() -> {
				try {
					//模拟任务执行时间为 任务编号为0 1 2 的执行时间需要 3秒;其余任务200 毫秒,导致任务时间差异较大
					if (index <= 2) {
						Thread.sleep(3000);
					} else {
						Thread.sleep(200);
					}
				} catch (InterruptedException e) {
					e.printStackTrace();
				}
				System.out.println(Thread.currentThread().getName() + " index:" + index);
			});
		}
		
		try {
			Thread.sleep(10000);//休眠 10 秒
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		System.out.println("10秒后...");
	}
	
}

打印机结果可以看出,线程 ForkJoinPool-1-worker-0 把3-9的任务都执行完

ForkJoinPool-1-worker-0 index:3
ForkJoinPool-1-worker-0 index:4
ForkJoinPool-1-worker-0 index:5
ForkJoinPool-1-worker-0 index:6
ForkJoinPool-1-worker-0 index:7
ForkJoinPool-1-worker-0 index:8
ForkJoinPool-1-worker-0 index:9
ForkJoinPool-1-worker-1 index:0
ForkJoinPool-1-worker-3 index:2
ForkJoinPool-1-worker-2 index:1
10秒后...



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