线程池用来管理工作线程的数量,它持有一个等待被执行的线程的队列。
java.util.concurrent.Executors 提供了 java.util.concurrent.Executor 接口实现来创建 Java 里的线程池。我们写一个简单的程序来解释一下它的工作机制。
首先我们需要有一个 Runnable 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;
- }
- }
这里是我们使用 Executors 框架创建了一个固定的线程池的测试程序。
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");
- }
-
- }
程序中我们创建了固定大小为五个工作线程的线程池。然后分配给线程池十个工作,因为线程池大小为五,它将启动五个工作线程先处理五个工作,其他的工作则处于等待状态,一旦有工作完成,空闲下来工作线程就会捡取等待队列里的其他工作进行执行。
这里是以上程序的输出。
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
输出表明线程池中至始至终只有五个名为 "pool-1-thread-1" 到 "pool-1-thread-5" 的五个线程,这五个线程不随着工作的完成而消亡,会一直存在,并负责执行分配给线程池的任务,直到线程池消亡。
Executors 类提供了使用了 ThreadPoolExecutor 的简单的 ExecutorService 实现,但是 ThreadPoolExecutor 提供的功能远不止于此。我们可以在创建 ThreadPoolExecutor 实例时指定活动线程的数量,我们也可以限制线程池的大小并且创建我们自己的 RejectedExecutionHandler 实现来处理不能适应工作队列的工作。
这里是我们自定义的 RejectedExecutionHandler 接口的实现。
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 提供了一些方法,我们可以使用这些方法来查询 executor 的当前状态,线程池大小,活动线程数量以及任务数量。因此我是用来一个监控线程在特定的时间间隔内打印 executor 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();
- }
- }
-
- }
- }
这里是使用 ThreadPoolExecutor 的线程池实现例子。
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{
-
- RejectedExecutionHandlerImpl rejectionHandler = new RejectedExecutionHandlerImpl();
-
- ThreadFactory threadFactory = Executors.defaultThreadFactory();
-
- ThreadPoolExecutor executorPool = new ThreadPoolExecutor(2, 4, 10, TimeUnit.SECONDS, new ArrayBlockingQueue<Runnable>(2), threadFactory, rejectionHandler);
-
- MyMonitorThread monitor = new MyMonitorThread(executorPool, 3);
- Thread monitorThread = new Thread(monitor);
- monitorThread.start();
-
- for(int i=0; i<10; i++){
- executorPool.execute(new WorkerThread("cmd"+i));
- }
-
- Thread.sleep(30000);
-
- executorPool.shutdown();
-
- Thread.sleep(5000);
- monitor.shutdown();
-
- }
- }
注意在初始化 ThreadPoolExecutor 时,我们保持初始池大小为 2,最大池大小为 4 而工作队列大小为 2。因此如果已经有四个正在执行的任务而此时分配来更多任务的话,工作队列将仅仅保留他们(新任务)中的两个,其他的将会被 RejectedExecutionHandlerImpl 处理。
上面程序的输出可以证实以上观点。
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 executor 的活动任务、完成任务以及所有完成任务,这些数量上的变化。我们可以调用 shutdown() 方法来结束所有提交的任务并终止线程池。
原文链接: http://www.journaldev.com/1069/java-thread-pool-example-using-executors-and-threadpoolexecutor