一个线程池管理工作线程池，它包含一个队列，使任务等待被执行。一个线程池管理的可运行线程和辅助线程集合执行运行队列中的.
java.util.concurrent.executors 创建为实现 java.util.concurrent.executor 接口的Java线程池中让我们写程序变得更简单
首先创建一个Runable 类：
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;
}
}
我们创建一个测试线程程序，创建一个固定大小的线程池从 Executors 框架中。
SimpleThreadPool.java
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");
}
}
在上面的程序，我们创建了5个固定大小的线程池线程。然后，我们提交10个任务到线程池中，但是由于线程池的大小是5，它会开始5个线程工作和其他工作将在等待状态，当你的工作完成后，另一个任务从等待队列的线程将拿起并得到执行。
下面为输出结果：
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
从输出结果看，池中有线程名从“pool-1-thread-1”到“pool-1-thread-5”来进行执行以上提交的任务。
Executors 提供了使用的 ThreadPoolExecutor 的ExecutorService简单的实现 ，但的 ThreadPoolExecutor 提供远不止这些功能。我们可以指定将线程活着的时候，我们创建的ThreadPoolExecutor实例的线程数，我们可以限制线程池的大小，并创建自 己的 RejectedExecutionHandler 实现来处理，可以不适合在工作队列中的作业。
下面是一个自定义实现RejectedExecutionHandler 接口的例子：
RejectedExecutionHandlerImpl.java
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提供了使用它我们可以找出执行者，池大小，活动线程数和任务数的当前状态的几种方法。所以，我有一个监视线程，这将在一定的时间间隔打印的执行人的信息。
MyMonitorThread.java
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();
}
}
}
}
这里是实现线程池的例子使用线程池：
WorkerPool.java
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 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，当初始化的ThreadPoolExecutor，我们保持最初的池大小为2，最大池大小为4，工作队列大小为2。所以，如果有4个正在运 行的任务和更多的任务提交，工作队列将其中只有2保持和他们的其余部分将通过RejectedExecutionHandlerImpl处理。
下面是上面的程序，确认上述语句的输出。
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
注意在主动的改变，完成和执行者完成的总任务数。我们可以调用 shutdown() 方法执行完成所有提交的任务并终止线程池。
如果要计划任务与延迟或定期运行，那么你可以使用 ScheduledThreadPoolExecutor 类。了解更多关于他们在Java Schedule Thread Pool Executor.
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