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
package com.bijian.study;
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;
}
}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
package com.bijian.study;
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：
pool-1-thread-1 Start. Command = 0 pool-1-thread-4 Start. Command = 3 pool-1-thread-5 Start. Command = 4 pool-1-thread-2 Start. Command = 1 pool-1-thread-3 Start. Command = 2 pool-1-thread-2 End. pool-1-thread-4 End. pool-1-thread-1 End. pool-1-thread-5 End. pool-1-thread-3 End. pool-1-thread-5 Start. Command = 8 pool-1-thread-1 Start. Command = 7 pool-1-thread-2 Start. Command = 5 pool-1-thread-4 Start. Command = 6 pool-1-thread-3 Start. Command = 9 pool-1-thread-3 End. pool-1-thread-2 End. pool-1-thread-1 End. pool-1-thread-4 End. pool-1-thread-5 End. 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
package com.bijian.study;
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
package com.bijian.study;
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();
}
}
}
}下面是使用 ThreadPoolExecutor 的线程池实现示例：
WorkerPool.java
package com.bijian.study;
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(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，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，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：
pool-1-thread-2 Start. Command = cmd1 cmd6 is rejected cmd7 is rejected cmd8 is rejected cmd9 is rejected pool-1-thread-4 Start. Command = cmd5 pool-1-thread-1 Start. Command = cmd0 pool-1-thread-3 Start. Command = cmd4 [monitor] [0/2] Active: 0, Completed: 0, Task: 6, isShutdown: false, isTerminated: false [monitor] [4/2] Active: 4, Completed: 0, Task: 6, isShutdown: false, isTerminated: false pool-1-thread-2 End. pool-1-thread-4 End. pool-1-thread-1 End. pool-1-thread-3 End. pool-1-thread-4 Start. Command = cmd3 pool-1-thread-2 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-2 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。
再修改WorkerPool.java，将工作队列大小设为3，如下所示：
//creating the ThreadPoolExecutor
ThreadPoolExecutor executorPool = new ThreadPoolExecutor(2, 4, 10, TimeUnit.SECONDS, new ArrayBlockingQueue(3), threadFactory, rejectionHandler); 运行结果如下：
pool-1-thread-1 Start. Command = cmd0 pool-1-thread-2 Start. Command = cmd1 pool-1-thread-4 Start. Command = cmd6 cmd7 is rejected pool-1-thread-3 Start. Command = cmd5 cmd8 is rejected cmd9 is rejected [monitor] [0/2] Active: 0, Completed: 0, Task: 6, isShutdown: false, isTerminated: false [monitor] [4/2] Active: 4, Completed: 0, Task: 7, isShutdown: false, isTerminated: false pool-1-thread-3 End. pool-1-thread-2 End. pool-1-thread-4 End. pool-1-thread-1 End. pool-1-thread-4 Start. Command = cmd4 pool-1-thread-2 Start. Command = cmd3 pool-1-thread-3 Start. Command = cmd2 [monitor] [4/2] Active: 3, Completed: 4, Task: 7, isShutdown: false, isTerminated: false [monitor] [4/2] Active: 3, Completed: 4, Task: 7, isShutdown: false, isTerminated: false pool-1-thread-2 End. pool-1-thread-3 End. pool-1-thread-4 End. [monitor] [4/2] Active: 0, Completed: 7, Task: 7, isShutdown: false, isTerminated: false [monitor] [3/2] Active: 0, Completed: 7, Task: 7, isShutdown: false, isTerminated: false [monitor] [3/2] Active: 0, Completed: 7, Task: 7, isShutdown: false, isTerminated: false [monitor] [2/2] Active: 0, Completed: 7, Task: 7, isShutdown: false, isTerminated: false [monitor] [2/2] Active: 0, Completed: 7, Task: 7, isShutdown: false, isTerminated: false [monitor] [2/2] Active: 0, Completed: 7, Task: 7, isShutdown: false, isTerminated: false [monitor] [0/2] Active: 0, Completed: 7, Task: 7, isShutdown: true, isTerminated: true [monitor] [0/2] Active: 0, Completed: 7, Task: 7, isShutdown: true, isTerminated: true
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3。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3个任务，其他任务将交由RejectedExecutionHandlerImpl 处理。
进一步查看ThreadPoolExecutor的此构造方法源代码如下：
/**
* Creates a new {@code ThreadPoolExecutor} with the given initial
* parameters.
*
* @param corePoolSize the number of threads to keep in the pool, even
* if they are idle, unless {@code allowCoreThreadTimeOut} is set
* @param maximumPoolSize the maximum number of threads to allow in the
* pool
* @param keepAliveTime when the number of threads is greater than
* the core, this is the maximum time that excess idle threads
* will wait for new tasks before terminating.
* @param unit the time unit for the {@code keepAliveTime} argument
* @param workQueue the queue to use for holding tasks before they are
* executed. This queue will hold only the {@code Runnable}
* tasks submitted by the {@code execute} method.
* @param threadFactory the factory to use when the executor
* creates a new thread
* @param handler the handler to use when execution is blocked
* because the thread bounds and queue capacities are reached
* @throws IllegalArgumentException if one of the following holds:
* {@code corePoolSize < 0}
* {@code keepAliveTime < 0}
* {@code maximumPoolSize <= 0}
* {@code maximumPoolSize < corePoolSize}
* @throws NullPointerException if {@code workQueue}
* or {@code threadFactory} or {@code handler} is null
*/
public ThreadPoolExecutor(int corePoolSize,
int maximumPoolSize,
long keepAliveTime,
TimeUnit unit,
BlockingQueue workQueue,
ThreadFactory threadFactory,
RejectedExecutionHandler handler) {
if (corePoolSize < 0 ||
maximumPoolSize <= 0 ||
maximumPoolSize < corePoolSize ||
keepAliveTime < 0)
throw new IllegalArgumentException();
if (workQueue == null || threadFactory == null || handler == null)
throw new NullPointerException();
this.corePoolSize = corePoolSize;
this.maximumPoolSize = maximumPoolSize;
this.workQueue = workQueue;
this.keepAliveTime = unit.toNanos(keepAliveTime);
this.threadFactory = threadFactory;
this.handler = handler;
} 如果希望延迟执行或定期运行任务，那么可以使用 ScheduledThreadPoolExecutor 类。要了解更多，请参见 Java Schedule Thread Pool Executor。
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