JAVA的ThreadPoolExecutor机制

一、概述 

1、ThreadPoolExecutor作为java.util.concurrent包对外提供基础实现,以内部线程池的形式对外提供管理任务执行,线程调度,线程池管理等等服务; 

2、Executors方法提供的线程服务,都是通过参数设置来实现不同的线程池机制。

 3、先来了解其线程池管理的机制,有助于正确使用,避免错误使用导致严重故障。同时可以根据自己的需求实现自己的线程池 二、核心构造方法讲解 下面是ThreadPoolExecutor最核心的构造方法

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;
    }
构造方法参数讲解
参数名 作用
corePoolSize 核心线程池大小
maximumPoolSize 最大线程池大小
keepAliveTime 线程池中超过corePoolSize数目的空闲线程最大存活时间;可以allowCoreThreadTimeOut(true)使得核心线程有效时间
TimeUnit keepAliveTime时间单位
workQueue 阻塞任务队列
threadFactory 新建线程工厂
RejectedExecutionHandler 当提交任务数超过maxmumPoolSize+workQueue之和时,任务会交给RejectedExecutionHandler来处理

重点讲解: 其中比较容易让人误解的是:corePoolSize,maximumPoolSize,workQueue之间关系。 

1.当线程池小于corePoolSize时,新提交任务将创建一个新线程执行任务,即使此时线程池中存在空闲线程。 

2.当线程池达到corePoolSize时,新提交任务将被放入workQueue中,等待线程池中任务调度执行 

3.当workQueue已满,且maximumPoolSize>corePoolSize时,新提交任务会创建新线程执行任务 

4.当提交任务数超过maximumPoolSize时,新提交任务由RejectedExecutionHandler处理

 5.当线程池中超过corePoolSize线程,空闲时间达到keepAliveTime时,关闭空闲线程

 6.当设置allowCoreThreadTimeOut(true)时,线程池中corePoolSize线程空闲时间达到keepAliveTime也将关闭 线程管理机制图示: 三、Executors提供的线程池配置方案

1、构造一个固定线程数目的线程池,配置的corePoolSize与maximumPoolSize大小相同,同时使用了一个无界LinkedBlockingQueue存放阻塞任务,因此多余的任务将存在再阻塞队列,不会由RejectedExecutionHandler处理

public static ExecutorService newFixedThreadPool(int nThreads) {
        return new ThreadPoolExecutor(nThreads, nThreads,
                                      0L, TimeUnit.MILLISECONDS,
                                      new LinkedBlockingQueue<Runnable>());
    }
2、构造一个缓冲功能的线程池,配置corePoolSize=0,maximumPoolSize=Integer.MAX_VALUE,keepAliveTime=60s,以及一个无容量的阻塞队列 SynchronousQueue,因此任务提交之后,将会创建新的线程执行;线程空闲超过60s将会销毁
public static ExecutorService newCachedThreadPool() {
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                      60L, TimeUnit.SECONDS,
                                      new SynchronousQueue<Runnable>());
    }
3、构造一个只支持一个线程的线程池,配置corePoolSize=maximumPoolSize=1,无界阻塞队列LinkedBlockingQueue;保证任务由一个线程串行执行
public static ExecutorService newSingleThreadExecutor() {
        return new FinalizableDelegatedExecutorService
            (new ThreadPoolExecutor(1, 1,
                                    0L, TimeUnit.MILLISECONDS,
                                    new LinkedBlockingQueue<Runnable>()));
    }
4、构造有定时功能的线程池,配置corePoolSize,无界延迟阻塞队列DelayedWorkQueue;有意思的是:maximumPoolSize=Integer.MAX_VALUE,由于DelayedWorkQueue是无界队列,所以这个值是没有意义的
public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
        return new ScheduledThreadPoolExecutor(corePoolSize);
    }
public static ScheduledExecutorService newScheduledThreadPool(
            int corePoolSize, ThreadFactory threadFactory) {
        return new ScheduledThreadPoolExecutor(corePoolSize, threadFactory);
    }
public ScheduledThreadPoolExecutor(int corePoolSize,
                             ThreadFactory threadFactory) {
        super(corePoolSize, Integer.MAX_VALUE, 0, TimeUnit.NANOSECONDS,
              new DelayedWorkQueue(), threadFactory);
    }
四、定制属于自己的线程池
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.RejectedExecutionHandler;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
import java.util.concurrent.atomic.AtomicInteger;
public class CustomThreadPoolExecutor {
	private ThreadPoolExecutor pool = null;
	/**
	 * 线程池初始化方法
	 * 
	 * corePoolSize 核心线程池大小----10
	 * maximumPoolSize 最大线程池大小----30
	 * keepAliveTime 线程池中超过corePoolSize数目的空闲线程最大存活时间----30+单位TimeUnit
	 * TimeUnit keepAliveTime时间单位----TimeUnit.MINUTES
	 * workQueue 阻塞队列----new ArrayBlockingQueue<Runnable>(10)====10容量的阻塞队列
	 * threadFactory 新建线程工厂----new CustomThreadFactory()====定制的线程工厂
	 * rejectedExecutionHandler 当提交任务数超过maxmumPoolSize+workQueue之和时,
	 * 							即当提交第41个任务时(前面线程都没有执行完,此测试方法中用sleep(100)),
	 * 						          任务会交给RejectedExecutionHandler来处理
	 */
	public void init() {
		pool = new ThreadPoolExecutor(
				10,
				30,
				30,
				TimeUnit.MINUTES,
				new ArrayBlockingQueue<Runnable>(10),
				new CustomThreadFactory(),
				new CustomRejectedExecutionHandler());
	}
	public void destory() {
		if(pool != null) {
			pool.shutdownNow();
		}
	}
	public ExecutorService getCustomThreadPoolExecutor() {
		return this.pool;
	}
	private class CustomThreadFactory implements ThreadFactory {
		private AtomicInteger count = new AtomicInteger(0);
		@Override
		public Thread newThread(Runnable r) {
			Thread t = new Thread(r);
			String threadName = CustomThreadPoolExecutor.class.getSimpleName() + count.addAndGet(1);
			System.out.println(threadName);
			t.setName(threadName);
			return t;
		}
	}
	private class CustomRejectedExecutionHandler implements RejectedExecutionHandler {
		@Override
		public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
			// 记录异常
			// 报警处理等
			System.out.println("error.............");
		}
	}
	// 测试构造的线程池
	public static void main(String[] args) {
		CustomThreadPoolExecutor exec = new CustomThreadPoolExecutor();
		// 1.初始化
		exec.init();
		ExecutorService pool = exec.getCustomThreadPoolExecutor();
		for(int i=1; i<100; i++) {
			System.out.println("提交第" + i + "个任务!");
			pool.execute(new Runnable() {
				@Override
				public void run() {
					try {
						Thread.sleep(300);
					} catch (InterruptedException e) {
						e.printStackTrace();
					}
					System.out.println("running=====");
				}
			});
		}
		// 2.销毁----此处不能销毁,因为任务没有提交执行完,如果销毁线程池,任务也就无法执行了
		// exec.destory();
		try {
			Thread.sleep(10000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
	}
}
方法中建立一个核心线程数为30个,缓冲队列有10个的线程池。每个线程任务,执行时会先睡眠0.1秒,保证提交40个任务时没有任务被执行完,这样提交第41个任务是,会交给CustomRejectedExecutionHandler 类来处理。
本文系转载文章,感谢原作者的辛勤付出!
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