JAVA进阶----ThreadPoolExecutor机制

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 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());
     }

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());
     }

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()));
     }

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(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(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(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(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