ThreadPoolExecutor机制 

二、核心构造方法讲解 
下面是ThreadPoolExecutor最核心的构造方法

构造方法参数讲解 

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);@Overridepublic 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 {@Overridepublic 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() {@Overridepublic 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