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。