使用Executors和ThreadPoolExecutor2

线程池负责管理工作线程，包含一个等待执行的任务队列。线程池的任务队列是一个Runnable集合，工作线程负责从任务队列中取出并执行Runnable对象。

java.util.concurrent.executors 提供了 java.util.concurrent.executor 接口的一个Java实现，可以创建线程池。下面是一个简单示例：

首先创建一个Runable 类：

WorkerThread.java

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package com.journaldev.threadpool;   public class WorkerThread implementsRunnable {       privateString command;       publicWorkerThread(String s){         this.command=s;     }       @Override     publicvoid run() {         System.out.println(Thread.currentThread().getName()+" Start. Command = "+command);         processCommand();         System.out.println(Thread.currentThread().getName()+" End.");     }       privatevoid processCommand() {         try{             Thread.sleep(5000);         }catch (InterruptedException e) {             e.printStackTrace();         }     }       @Override     publicString toString(){         returnthis.command;     } }

下面是一个测试程序，从 Executors 框架中创建固定大小的线程池：

SimpleThreadPool.java

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package com.journaldev.threadpool;   import java.util.concurrent.ExecutorService; import java.util.concurrent.Executors;   public class SimpleThreadPool {       publicstatic void main(String[] args) {         ExecutorService executor = Executors.newFixedThreadPool(5);         for(int i = 0; i < 10; i++) {             Runnable worker =new WorkerThread(""+ i);             executor.execute(worker);           }         executor.shutdown();         while(!executor.isTerminated()) {         }         System.out.println("Finished all threads");     }   }

在上面的程序中，我们创建了包含5个工作线程的固定大小线程池。然后，我们向线程池提交10个任务。由于线程池的大小是5，因此首先会启动5个工作线程，其他任务将进行等待。一旦有任务结束，工作线程会从等待队列中挑选下一个任务并开始执行。

以上程序的输出结果如下：

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pool-1-thread-2Start. Command = 1 pool-1-thread-4Start. Command = 3 pool-1-thread-1Start. Command = 0 pool-1-thread-3Start. Command = 2 pool-1-thread-5Start. Command = 4 pool-1-thread-4End. pool-1-thread-5End. pool-1-thread-1End. pool-1-thread-3End. pool-1-thread-3Start. Command = 8 pool-1-thread-2End. pool-1-thread-2Start. Command = 9 pool-1-thread-1Start. Command = 7 pool-1-thread-5Start. Command = 6 pool-1-thread-4Start. Command = 5 pool-1-thread-2End. pool-1-thread-4End. pool-1-thread-3End. pool-1-thread-5End. pool-1-thread-1End. Finished all threads

从输出结果看，线程池中有五个名为“pool-1-thread-1”…“pool-1-thread-5”的工作线程负责执行提交的任务。

Executors 类使用 ExecutorService  提供了一个 ThreadPoolExecutor 的简单实现，但 ThreadPoolExecutor 提供的功能远不止这些。我们可以指定创建 ThreadPoolExecutor 实例时活跃的线程数，并且可以限制线程池的大小，还可以创建自己的 RejectedExecutionHandler 实现来处理不适合放在工作队列里的任务。

下面是一个 RejectedExecutionHandler 接口的自定义实现：

RejectedExecutionHandlerImpl.java

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package com.journaldev.threadpool;   import java.util.concurrent.RejectedExecutionHandler; import java.util.concurrent.ThreadPoolExecutor;   public class RejectedExecutionHandlerImpl implementsRejectedExecutionHandler {       @Override     publicvoid rejectedExecution(Runnable r, ThreadPoolExecutor executor) {         System.out.println(r.toString() +" is rejected");     }   }

ThreadPoolExecutor 提供了一些方法，可以查看执行状态、线程池大小、活动线程数和任务数。所以，我通过一个监视线程在固定间隔输出执行信息。

MyMonitorThread.java

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package com.journaldev.threadpool;   import java.util.concurrent.ThreadPoolExecutor;   public class MyMonitorThread implementsRunnable {     privateThreadPoolExecutor executor;       privateint seconds;       privateboolean run=true;       publicMyMonitorThread(ThreadPoolExecutor executor, intdelay)     {         this.executor = executor;         this.seconds=delay;     }       publicvoid shutdown(){         this.run=false;     }       @Override     publicvoid run()     {         while(run){                 System.out.println(                     String.format("[monitor] [%d/%d] Active: %d, Completed: %d, Task: %d, isShutdown: %s, isTerminated: %s",                         this.executor.getPoolSize(),                         this.executor.getCorePoolSize(),                         this.executor.getActiveCount(),                         this.executor.getCompletedTaskCount(),                         this.executor.getTaskCount(),                         this.executor.isShutdown(),                         this.executor.isTerminated()));                 try{                     Thread.sleep(seconds*1000);                 }catch (InterruptedException e) {                     e.printStackTrace();                 }         }       } }

下面是使用 ThreadPoolExecutor 的线程池实现示例：

WorkerPool.java

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package com.journaldev.threadpool;   import java.util.concurrent.ArrayBlockingQueue; import java.util.concurrent.Executors; import java.util.concurrent.ThreadFactory; import java.util.concurrent.ThreadPoolExecutor; import java.util.concurrent.TimeUnit;   public class WorkerPool {       publicstatic void main(String args[]) throws InterruptedException{         //RejectedExecutionHandler implementation         RejectedExecutionHandlerImpl rejectionHandler =new RejectedExecutionHandlerImpl();         //Get the ThreadFactory implementation to use         ThreadFactory threadFactory = Executors.defaultThreadFactory();         //creating the ThreadPoolExecutor         ThreadPoolExecutor executorPool =new ThreadPoolExecutor(2,4, 10, TimeUnit.SECONDS,new ArrayBlockingQueue<Runnable>(2), threadFactory, rejectionHandler);         //start the monitoring thread         MyMonitorThread monitor =new MyMonitorThread(executorPool,3);         Thread monitorThread =new Thread(monitor);         monitorThread.start();         //submit work to the thread pool         for(inti=0; i<10; i++){             executorPool.execute(newWorkerThread("cmd"+i));         }           Thread.sleep(30000);         //shut down the pool         executorPool.shutdown();         //shut down the monitor thread         Thread.sleep(5000);         monitor.shutdown();       } }

请注意：在初始化 ThreadPoolExecutor 时，初始线程池大小设为2、最大值设为4、工作队列大小设为2。所以，如果当前有4个任务正在运行而此时又有新任务提交，工作队列将只存储2个任务和其他任务将交由RejectedExecutionHandlerImpl 处理。

程序执行的结果如下，确认了上面的结论：

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pool-1-thread-1 Start. Command = cmd0 pool-1-thread-4 Start. Command = cmd5 cmd6 is rejected pool-1-thread-3 Start. Command = cmd4 pool-1-thread-2 Start. Command = cmd1 cmd7 is rejected cmd8 is rejected cmd9 is rejected [monitor] [0/2] Active: 4, Completed: 0, Task: 6, isShutdown:false, isTerminated:false [monitor] [4/2] Active: 4, Completed: 0, Task: 6, isShutdown:false, isTerminated:false pool-1-thread-4 End. pool-1-thread-1 End. pool-1-thread-2 End. pool-1-thread-3 End. pool-1-thread-1 Start. Command = cmd3 pool-1-thread-4 Start. Command = cmd2 [monitor] [4/2] Active: 2, Completed: 4, Task: 6, isShutdown:false, isTerminated:false [monitor] [4/2] Active: 2, Completed: 4, Task: 6, isShutdown:false, isTerminated:false pool-1-thread-1 End. pool-1-thread-4 End. [monitor] [4/2] Active: 0, Completed: 6, Task: 6, isShutdown:false, isTerminated:false [monitor] [2/2] Active: 0, Completed: 6, Task: 6, isShutdown:false, isTerminated:false [monitor] [2/2] Active: 0, Completed: 6, Task: 6, isShutdown:false, isTerminated:false [monitor] [2/2] Active: 0, Completed: 6, Task: 6, isShutdown:false, isTerminated:false [monitor] [2/2] Active: 0, Completed: 6, Task: 6, isShutdown:false, isTerminated:false [monitor] [2/2] Active: 0, Completed: 6, Task: 6, isShutdown:false, isTerminated:false [monitor] [0/2] Active: 0, Completed: 6, Task: 6, isShutdown:true, isTerminated:true [monitor] [0/2] Active: 0, Completed: 6, Task: 6, isShutdown:true, isTerminated:true

请注意活跃线程、已完成线程和任务完成总数的变化。我们可以调用 shutdown() 结束所有已提交任务并终止线程池。

如果希望延迟执行或定期运行任务，那么可以使用 ScheduledThreadPoolExecutor 类。