Java ThreadPoolExecutor机制

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

核心构造方法讲解

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

参数名字 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。
一个任务(即调用的一个方法)通过 execute(Runnable)方法被添加到线程池，任务就是一个 Runnable类型的对象，任务的执行方法就是Runnable类型对象的run()方法。

当一个任务通过execute(Runnable)方法欲添加到线程池时：

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（此时workQueue已满），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：

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>());      }  

1. 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>());      }  

1. 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>()));      }  

1. 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);      }  

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10个,最大线程池大小，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(out of memory)，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

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(out of memory)，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);      }  

实战

private final ExecutorService es = new ThreadPoolExecutor(10, 30, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue<Runnable>(50));@Overridepublic Response<List<ShopOwnerDTO>> getFoodOwnersIdByPoiShopIds(List<Integer> poiShopIds, final Integer platform){        List<ShopOwnerDTO> shopOwnerDTOs = Lists.newArrayList();        try {            for (Integer poiShopId : poiShopIds) {                final int shopId = poiShopId;                Future<Object> shopOwnerCallable = es.submit(new Callable<Object>() {@Overridepublic Object call() throws Exception {     return getFoodOwnerIdByPoiShopId(shopId, platform);       }   });  Response<ShopOwnerDTO> response = (Response<ShopOwnerDTO>) shopOwnerCallable.get();   if(response.isSuccess()) {   shopOwnerDTOs.add(response.getObj());                }            }        } catch (ExecutionException ex) {            logger.error(ex.getMessage(),ex);        } catch (InterruptedException e) {            logger.error(e.getMessage(),e);        }        return Response.success(shopOwnerDTOs);    }

总结：
1、用ThreadPoolExecutor自定义线程池，看线程的用途，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(out of memory)
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 （Intel 赛扬G460是单核心，双线程的CPU，Intel 酷睿i3 3220是双核心 四线程，Intel 酷睿i7 4770K是四核心 八线程 ，Intel 酷睿i5 4570是四核心 四线程等等）
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(out of memory)，所以最好异步开个线程获取结果.

参考：
http://825635381.iteye.com/blog/2184680
http://blog.csdn.net/wangwenhui11/article/details/6760474
http://blog.csdn.net/mccand1234/article/details/51972820
http://blog.csdn.net/yu132563/article/details/45222935