ThreadPoolExecutor

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、定制属于自己的非阻塞线程池

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(3000);                    } 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3秒，保证提交10任务时，线程数目被占用完，再提交30任务时，阻塞队列被占用完，，这样提交第41个任务是，会交给CustomRejectedExecutionHandler 异常处理类来处理。
提交任务的代码如下：

public void execute(Runnable command) {        if (command == null)            throw new NullPointerException();        /*         * Proceed in 3 steps:         *         * 1. If fewer than corePoolSize threads are running, try to         * start a new thread with the given command as its first         * task.  The call to addWorker atomically checks runState and         * workerCount, and so prevents false alarms that would add         * threads when it shouldn't, by returning false.         *         * 2. If a task can be successfully queued, then we still need         * to double-check whether we should have added a thread         * (because existing ones died since last checking) or that         * the pool shut down since entry into this method. So we         * recheck state and if necessary roll back the enqueuing if         * stopped, or start a new thread if there are none.         *         * 3. If we cannot queue task, then we try to add a new         * thread.  If it fails, we know we are shut down or saturated         * and so reject the task.         */        int c = ctl.get();        if (workerCountOf(c) < corePoolSize) {            if (addWorker(command, true))                return;            c = ctl.get();        }        if (isRunning(c) && workQueue.offer(command)) {            int recheck = ctl.get();            if (! isRunning(recheck) && remove(command))                reject(command);            else if (workerCountOf(recheck) == 0)                addWorker(null, false);        }        else if (!addWorker(command, false))            reject(command);    }

注意：41以后提交的任务就不能正常处理了，因为，execute中提交到任务队列是用的offer方法，如上面代码，这个方法是非阻塞的，所以就会交给CustomRejectedExecutionHandler 来处理，所以对于大数据量的任务来说，这种线程池，如果不设置队列长度会OOM，设置队列长度，会有任务得不到处理，接下来我们构建一个阻塞的自定义线程池；
五、定制属于自己的阻塞线程池

package com.tongbanjie.trade.test.commons;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----1      * maximumPoolSize 最大线程池大小----3      * 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>(5)====5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(                  1,                  3,                  30,                  TimeUnit.MINUTES,                  new ArrayBlockingQueue<Runnable>(5),                  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) {              try {                                // 核心改造点，由blockingqueue的offer改成put阻塞方法                executor.getQueue().put(r);            } catch (InterruptedException e) {                e.printStackTrace();            }        }      }      // 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 {                          System.out.println(">>>task is running=====");                         TimeUnit.SECONDS.sleep(10);                    } catch (InterruptedException e) {                          e.printStackTrace();                      }                  }              });          }          // 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();          }      }  }  

解释：当提交任务被拒绝时，进入拒绝机制，我们实现拒绝方法，把任务重新用阻塞提交方法put提交，实现阻塞提交任务功能，防止队列过大，OOM，提交被拒绝方法在下面

public void execute(Runnable command) {        if (command == null)            throw new NullPointerException();        int c = ctl.get();        if (workerCountOf(c) < corePoolSize) {            if (addWorker(command, true))                return;            c = ctl.get();        }        if (isRunning(c) && workQueue.offer(command)) {            int recheck = ctl.get();            if (! isRunning(recheck) && remove(command))                reject(command);            else if (workerCountOf(recheck) == 0)                addWorker(null, false);        }        else if (!addWorker(command, false))            // 进入拒绝机制， 我们把runnable任务拿出来，重新用阻塞操作put，来实现提交阻塞功能            reject(command);    }

总结：
1、用ThreadPoolExecutor自定义线程池，看线程是的用途，如果任务量不大，可以用无界队列，如果任务量非常大，要用有界队列，防止OOM
2、如果任务量很大，还要求每个任务都处理成功，要对提交的任务进行阻塞提交，重写拒绝机制，改为阻塞提交。保证不抛弃一个任务
3、最大线程数一般设为2N+1最好，N是CPU核数
4、核心线程数，看应用，如果是任务，一天跑一次，设置为0，合适，因为跑完就停掉了，如果是常用线程池，看任务量，是保留一个核心还是几个核心线程数
5、如果要获取任务执行结果，用CompletionService，但是注意，获取任务的结果的要重新开一个线程获取，如果在主线程获取，就要等任务都提交后才获取，就会阻塞大量任务结果，队列过大OOM，所以最好异步开个线程获取结果