Java多线程 ThreadPoolExecutor-RejectedExecutionHandler拒绝执行策略

Posted 2022-11-21

(目录)

一、说明

RejectedExecutionHandler

• 当线程池已经被关闭，或者任务数超过maximumPoolSize+workQueue时执行拒绝策略
• ThreadPoolExecutor.AbortPolicy 默认拒绝策略，丢弃任务并抛出RejectedExecutionException异常
• ThreadPoolExecutor.DiscardPolicy 直接丢弃任务，但不抛出异常
• ThreadPoolExecutor.DiscardOldestPolicy 丢弃任务队列最先加入的任务，再执行execute方法把新任务加入队列执行
• ThreadPoolExecutor.CallerRunsPolicy：由创建了线程池的线程来执行被拒绝的任务

二、理解

AbortPolicy

默认拒绝策略，丢弃任务并抛出RejectedExecutionException异常

    private static final RejectedExecutionHandler defaultHandler =
        new AbortPolicy();

    public static class AbortPolicy implements RejectedExecutionHandler r
        public AbortPolicy()  
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) 
            throw new RejectedExecutionException("Task " + r.toString() +
                                                 " rejected from " +
                                                 e.toString());
        
    

DiscardPolicy

直接丢弃任务，但不抛出异常

public static class DiscardPolicy implements RejectedExecutionHandler 
        public DiscardPolicy()  
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) 
        
    

DiscardOldestPolicy

丢弃队列中等待最久的任务，再把新任务添加进去执行，从任务队列弹出最先加入的任务，空出一个位置，然后再次执行execute方法把任务加入队列

    public static class DiscardOldestPolicy implements RejectedExecutionHandler 
        public DiscardOldestPolicy()  
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) 
            if (!e.isShutdown()) 
                e.getQueue().poll();
                e.execute(r);
            
        
    

CallerRunsPolicy

会调用当前线程池的所在的线程去执行被拒绝的任务

    public static class CallerRunsPolicy implements RejectedExecutionHandler 
        public CallerRunsPolicy()  
        public void rejectedExecution(Runnable r, ThreadPoolExecutor e) 
            if (!e.isShutdown()) 
                r.run();
            
        
    

三、实现

1.AbortPolicy

创建 ThreadPoolExecutorTest类，默认使用ThreadPoolExecutor.AbortPolicy拒绝策略，队列是ArrayBlockingQueue，设置核心线程数最大值为1，线程池线程数最大值为2，最大等待时间为5秒，等待队列值为2

public class RejectedExecutionHandlerTest 
    public static void main(String[] args) throws InterruptedException 
        // 1.创建自定义线程池
        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(2, 4, 5,
                TimeUnit.SECONDS,
                new ArrayBlockingQueue<>(2),
                Executors.defaultThreadFactory(),
                new ThreadPoolExecutor.AbortPolicy());


        // 2.创建线程任务
        for (int i = 1; i <= 6; i++) 

            // 3.执行任务
            System.out.println("执行第"+i+"个任务");
            threadPoolExecutor.execute(new runnable("任务"+i));

            System.out.println("当前核心线程数" + threadPoolExecutor.getCorePoolSize());
            System.out.println("当前线程池线程数" + threadPoolExecutor.getPoolSize());
            // 4.迭代器获取等待队列
            Iterator iterator = threadPoolExecutor.getQueue().iterator();
            System.out.print("当前等待队列 ");
            while (iterator.hasNext())
                runnable thread = (runnable) iterator.next();
                System.out.print(thread.name + "\\t");
            
            System.out.print("\\n");
            System.out.println("--------");
        

        Thread.sleep(10000);
        System.out.println("----休眠10秒后----");
        System.out.println("当前核心线程数" + threadPoolExecutor.getCorePoolSize());
        System.out.println("当前线程池线程数" + threadPoolExecutor.getPoolSize());
        System.out.println("当前队列任务数" + threadPoolExecutor.getQueue().size());

        // 5.关闭线程池
        threadPoolExecutor.shutdown();
    
        // 实现Runnable
    static class runnable implements Runnable
        // 设置任务名
        String name;
        public runnable(String setName) 
            this.name = setName;
        
        @Override
        public void run() 
            try 
                System.out.println("线程:"+Thread.currentThread().getName() +" 执行: "+name);
             catch (Exception e) 
                e.printStackTrace();
            
        
    

当线程数达到corePoolSize后，若有新任务加入，则直接进入任务队列等待，超出队列的任务会创建新的线程来执行

一共有1个核心，当线程数超过corePoolSize+workQueue时，将创建非核心线程，核心线程默认情况下不会被回收，不受时间限制，而超时的非核心线程将被回收

但如果再执行1个任务，线程数超过maximumPoolSize+workQueue，再提交任务将被丢弃并抛出RejectedExecutionException异常

2.DiscardPolicy

创建5个任务，让被线程池拒绝的任务直接丢弃，不会抛异常也不会执行

        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(1, 2, 5,
                TimeUnit.SECONDS,
                new ArrayBlockingQueue<>(2),
                Executors.defaultThreadFactory(),
                new ThreadPoolExecutor.DiscardPolicy());

任务5不会执行，恶意不会抛出异常，超时的非核心线程将被回收

3.DiscardOldestPolicy

丢弃任务队列最先加入的任务，再执行execute方法把新任务加入队列执行

        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(1, 2, 5,
                TimeUnit.SECONDS,
                new ArrayBlockingQueue<>(2),
                Executors.defaultThreadFactory(),
                new ThreadPoolExecutor.DiscardOldestPolicy());

添加任务5时，线程数已经超过maximumPoolSize+workQueue，抛弃最先加入队列的任务2并且不执行，再将任务5加进队列中执行

4.CallerRunsPolicy

会调用当前线程池的所在的线程去执行被拒绝的任务

        // 1.创建自定义线程池
        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(1, 2, 5,
                TimeUnit.SECONDS,
                new ArrayBlockingQueue<>(2),
                Executors.defaultThreadFactory(),
                new ThreadPoolExecutor.CallerRunsPolicy());

主线程执行任务1，空闲线程执行任务4，此时队列中有任务2和任务3

添加任务5时，线程数已经超过maximumPoolSize+workQueue，任务5直接调用当前线程池的所在的线程main去执行，这时主线程被阻塞了

当任务5执行完成时，最先的两个任务已经完成了，主线程去执行任务2和任务3，添加任务6也可以直接执行

超时的非核心线程将被回收

5.自定义拒绝执行策略

当线程数已经超过maximumPoolSize+workQueue时，调用新线程去执行任务

    static class MyRejectedExecutionHandler implements RejectedExecutionHandler 
        @Override
        public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) 
            new Thread(r, "新线程"+(new Random().nextInt(4) + 1)).start();
        
    

        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(1, 2, 5,
                TimeUnit.SECONDS,
                new ArrayBlockingQueue<>(2),
                Executors.defaultThreadFactory(),
                new MyRejectedExecutionHandler());