Java线程池管理工作线程池。
它包含一个使任务等待执行的队列。
我们可以使用ThreadPoolExecutor在Java中创建线程池。

Java线程池管理可运行线程的集合。
工作线程从队列中执行可运行线程。
java.util.concurrent.Executors为java.util.concurrent.Executor接口提供工厂和支持方法，以在Java中创建线程池。

Executors是一个实用程序类，它还提供了有用的方法，可以通过各种工厂方法与ExecutorService，ScheduledExecutorService，ThreadFactory和Callable类一起使用。

让我们编写一个简单的程序来说明它的工作原理。

首先，我们需要有一个名为WorkerThread.java的Runnable类。

package com.theitroad.threadpool;

public class WorkerThread implements Runnable {

  private String command;
  
  public WorkerThread(String s){
      this.command=s;
  }

  @Override
  public void run() {
      System.out.println(Thread.currentThread().getName()+" Start. Command = "+command);
      processCommand();
      System.out.println(Thread.currentThread().getName()+" End.");
  }

  private void processCommand() {
      try {
          Thread.sleep(5000);
      } catch (InterruptedException e) {
          e.printStackTrace();
      }
  }

  @Override
  public String toString(){
      return this.command;
  }
}

ExecutorService示例

这是测试程序类SimpleThreadPool.java，我们其中从Executors框架创建固定线程池。

package com.theitroad.threadpool;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class SimpleThreadPool {

  public static 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，我们正在创建一个由5个工作线程组成的固定大小的线程池。
然后，我们将10个作业提交到该池中，因为该池的大小为5，它将开始处理5个作业，其他作业将处于等待状态，一旦其中一个作业完成，来自等待队列的另一个作业将被工作线程拾取并执行。

这是上面程序的输出。

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

输出确认池中有五个线程从" pool-1-thread-1"到" pool-1-thread-5"命名，它们负责执行向池中提交的任务。

ThreadPoolExecutor示例

Executors类使用ThreadPoolExecutor提供了ExecutorService的简单实现，但是ThreadPoolExecutor提供了更多的功能。
我们可以指定创建ThreadPoolExecutor实例时仍处于活动状态的线程数，并可以限制线程池的大小并创建自己的RejectedExecutionHandler实现以处理无法容纳在工作队列中的作业。

这是我们对RejectedExecutionHandler接口的自定义实现。

package com.theitroad.threadpool;

import java.util.concurrent.RejectedExecutionHandler;
import java.util.concurrent.ThreadPoolExecutor;

public class RejectedExecutionHandlerImpl implements RejectedExecutionHandler {

  @Override
  public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
      System.out.println(r.toString() + " is rejected");
  }

}

ThreadPoolExecutor提供了几种方法，通过这些方法我们可以找到执行器的当前状态，池大小，活动线程数和任务数。
因此，我有一个监视线程，该线程将在特定时间间隔打印执行程序信息。

package com.theitroad.threadpool;

import java.util.concurrent.ThreadPoolExecutor;

public class MyMonitorThread implements Runnable
{
  private ThreadPoolExecutor executor;
  private int seconds;
  private boolean run=true;

  public MyMonitorThread(ThreadPoolExecutor executor, int delay)
  {
      this.executor = executor;
      this.seconds=delay;
  }
  public void shutdown(){
      this.run=false;
  }
  @Override
  public void 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();
              }
      }
          
  }
}

这是使用ThreadPoolExecutor的线程池实现示例。

package com.theitroad.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 {

  public static 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(int i=0; i<10; i++){
          executorPool.execute(new WorkerThread("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，我们将初始池大小保持为2，最大池大小保持为4，工作队列大小保持为2。
因此，如果有4个正在运行的任务并且提交了更多任务，则工作队列将仅容纳其中2个其余的将由RejectedExecutionHandlerImpl处理。

这是上述程序的输出，确认了上述声明。

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

注意执行程序的活动，已完成和已完成任务总数的变化。
我们可以调用shutdown()方法来完成所有提交的任务的执行并终止线程池。

如果要安排任务延迟或者定期运行，则可以使用ScheduledThreadPoolExecutor类。