Java线程池管理工作线程的池。它包含一个队列，用于存放等待执行的任务。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提供了用于在Java中创建线程池的java.util.concurrent.Executor接口的工厂和支持方法。Executors是一个实用类，通过各种工厂方法还提供了与ExecutorService、ScheduledExecutorService、ThreadFactory和Callable类一起使用的有用方法。让我们编写一个简单的程序来解释它的工作原理。首先，我们需要有一个Runnable类，命名为WorkerThread.java
package com.journaldev.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，我们在其中使用Executors框架创建了一个固定大小的线程池。
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");
}
}
在上述程序中，我们创建了一个由5个工作线程组成的固定大小线程池。然后，我们向此池提交了10个作业，由于池大小为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，工作线程将从等待队列中选择另一个作业并执行。以下是上述程序的输出。
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，分别命名为“pool-1-thread-1”到“pool-1-thread-5”，它们负责执行提交给池的任务。
ThreadPoolExecutor 示例
Executors 类提供了ExecutorService的简单实现，使用了ThreadPoolExecutor，但ThreadPoolExecutor提供了更多的功能。我们可以指定在创建ThreadPoolExecutor实例时将存活的线程数量，还可以限制线程池的大小，并创建我们自己的RejectedExecutionHandler实现，以处理无法放入工作队列的作业。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 {
@Override
public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
System.out.println(r.toString() + " is rejected");
}
}
ThreadPoolExecutor提供了几种方法，我们可以通过这些方法了解执行器的当前状态、池大小、活动线程数和任务数量。因此，我有一个监视线程，会在一定时间间隔内打印执行器信息。
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;
}
@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();
}
}
}
}
以下是使用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 实现
RejectedExecutionHandlerImpl rejectionHandler = new RejectedExecutionHandlerImpl();
//获取 ThreadFactory 实现以使用
ThreadFactory threadFactory = Executors.defaultThreadFactory();
//创建 ThreadPoolExecutor
ThreadPoolExecutor executorPool = new ThreadPoolExecutor(2, 4, 10, TimeUnit.SECONDS, new ArrayBlockingQueue(2), threadFactory, rejectionHandler);
//启动监控线程
MyMonitorThread monitor = new MyMonitorThread(executorPool, 3);
Thread monitorThread = new Thread(monitor);
monitorThread.start();
//提交任务至线程池
for(int i=0; i<10; i++){
executorPool.execute(new WorkerThread("cmd"+i));
}
Thread.sleep(30000);
//关闭线程池
executorPool.shutdown();
//关闭监控线程
Thread.sleep(5000);
monitor.shutdown();
}
}
请注意，在初始化 ThreadPoolExecutor 时，我们将初始池大小设置为 2，最大池大小设置为 4，并将工作队列大小设置为 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的 2 个，其余的将由 RejectedExecutionHandlerImpl 处理。以下是确认上述说明的程序输出。
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
注意执行器中活动、已完成和总完成任务计数的变化。我们可以调用 shutdown() 方法来完成所有已提交任务的执行并终止线程池。如果您想要延迟运行任务或定期运行任务，则可以使用 ScheduledThreadPoolExecutor 类。更多信息，请阅读Java 定时器线程池执行者。