Java thread pool manages the pool of worker threads. It contains a queue that keeps tasks waiting to get executed. We can use ThreadPoolExecutor to create thread pool in Java.

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 thread pool manages the collection of Runnable threads. The worker threads execute Runnable threads from the queue. java.util.concurrent.Executors provide factory and support methods for java.util.concurrent.Executor interface to create the thread pool in java.

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 is a utility class that also provides useful methods to work with ExecutorService, ScheduledExecutorService, ThreadFactory, and Callable classes through various factory methods.

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。

Let’s write a simple program to explain it’s working.

让我们编写一个简单的程序来说明它的工作原理。

First, we need to have a Runnable class, named WorkerThread.java

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

package com.journaldev.threadpool;public class WorkerThread implements Runnable {private String command;public WorkerThread(String s){this.command=s;}@Overridepublic 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();}}@Overridepublic String toString(){return this.command;}
}

ExecutorService示例 (ExecutorService Example)

Here is the test program class SimpleThreadPool.java, where we are creating fixed thread pool from Executors framework.

这是测试程序类SimpleThreadPool.java ，在这里我们从Executors框架创建固定线程池。

package com.journaldev.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");}
}

In the above program, we are creating a fixed-size thread pool of 5 worker threads. Then we are submitting 10 jobs to this pool, since the pool size is 5, it will start working on 5 jobs and other jobs will be in wait state, as soon as one of the job is finished, another job from the wait queue will be picked up by worker thread and get’s executed.

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。

Here is the output of the above program.

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

The output confirms that there are five threads in the pool named from “pool-1-thread-1” to “pool-1-thread-5” and they are responsible to execute the submitted tasks to the pool.

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 (ThreadPoolExecutor Example)

Executors class provide simple implementation of ExecutorService using ThreadPoolExecutor but ThreadPoolExecutor provides much more feature than that. We can specify the number of threads that will be alive when we create ThreadPoolExecutor instance and we can limit the size of thread pool and create our own RejectedExecutionHandler implementation to handle the jobs that can’t fit in the worker queue.

Executors类提供简单实现的ExecutorService的使用的ThreadPoolExecutor但ThreadPoolExecutor的提供了更多的功能不止于此。 我们可以指定创建ThreadPoolExecutor实例时仍处于活动状态的线程数，并且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。

Here is our custom implementation of RejectedExecutionHandler interface.

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

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

ThreadPoolExecutor provides several methods using which we can find out the current state of the executor, pool size, active thread count and task count. So I have a monitor thread that will print the executor information at a certain time interval.

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.journaldev.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;}@Overridepublic 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();}}}
}

Here is the thread pool implementation example using ThreadPoolExecutor.

这是使用ThreadPoolExecutor的线程池实现示例。

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 {public static void main(String args[]) throws InterruptedException{//RejectedExecutionHandler implementationRejectedExecutionHandlerImpl rejectionHandler = new RejectedExecutionHandlerImpl();//Get the ThreadFactory implementation to useThreadFactory threadFactory = Executors.defaultThreadFactory();//creating the ThreadPoolExecutorThreadPoolExecutor executorPool = new ThreadPoolExecutor(2, 4, 10, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(2), threadFactory, rejectionHandler);//start the monitoring threadMyMonitorThread monitor = new MyMonitorThread(executorPool, 3);Thread monitorThread = new Thread(monitor);monitorThread.start();//submit work to the thread poolfor(int i=0; i<10; i++){executorPool.execute(new WorkerThread("cmd"+i));}Thread.sleep(30000);//shut down the poolexecutorPool.shutdown();//shut down the monitor threadThread.sleep(5000);monitor.shutdown();}
}

Notice that while initializing the ThreadPoolExecutor, we are keeping initial pool size as 2, maximum pool size to 4 and work queue size as 2. So if there are 4 running tasks and more tasks are submitted, the work queue will hold only 2 of them and the rest of them will be handled by RejectedExecutionHandlerImpl.

请注意，在初始化ThreadPoolExecutor时，我们将初始池大小保持为2，最大池大小为4，工作队列大小为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。

Here is the output of the above program that confirms the above statement.

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

Notice the change in active, completed and total completed task count of the executor. We can invoke shutdown() method to finish execution of all the submitted tasks and terminate the thread pool.

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。

If you want to schedule a task to run with delay or periodically then you can use ScheduledThreadPoolExecutor class. Read more about them at Java Schedule Thread Pool Executor.

如果要安排任务延迟或定期运行，则可以使用ScheduledThreadPoolExecutor类。 在Java Schedule Thread Pool Executor中了解有关它们的更多信息。

翻译自: https://www.journaldev.com/1069/threadpoolexecutor-java-thread-pool-example-executorservice