为什么要使用拒绝策略？

等待队列已经排满了，再也塞不下新任务了
同时，线程池中的max线程也达到了，无法继续为新任务服务。

这个是时候我们就需要拒绝策略机制合理的处理这个问题。

四种拒绝策略介绍

jdk中自带的拒绝策略均实现了java.util.concurrent.RejectedExecutionHandler接口

AbortPolicy

默认的拒绝策略。直接抛出 java.util.concurrent.RejectedExecutionException异常

 new ThreadPoolExecutor.AbortPolicy()

import java.util.concurrent.*;

public class Test {

    public static void main(String[] args) {
        ExecutorService executorService = new ThreadPoolExecutor(2, 5, 2, TimeUnit.SECONDS, new LinkedBlockingQueue<>(3), Executors.defaultThreadFactory(), new ThreadPoolExecutor.AbortPolicy());
        try {
            for (int i = 1; i <= 9; i++) {
                executorService.execute(new Runnable() {
                    @Override
                    public void run() {
                        System.out.println(Thread.currentThread().getName() + " 办理业务");
                    }
                });
            }
        } finally {
            executorService.shutdown();
        }

    }
}

CallerRunsPolicy

将任务返还给调用者线程执行

new ThreadPoolExecutor.CallerRunsPolicy()

import java.util.concurrent.*;

public class Test {

    public static void main(String[] args) {
        ExecutorService executorService = new ThreadPoolExecutor(2, 5, 2, TimeUnit.SECONDS, new LinkedBlockingQueue<>(3), Executors.defaultThreadFactory(), new ThreadPoolExecutor.CallerRunsPolicy());
        try {
            for (int i = 1; i <= 9; i++) {
                executorService.execute(new Runnable() {
                    @Override
                    public void run() {
                        System.out.println(Thread.currentThread().getName() + " 办理业务");
                    }
                });
            }
        } finally {
            executorService.shutdown();
        }

    }
}

执行结果： 任务果然返还给main线程了

image.png

DiscardPolicy

直接抛弃无法处理的任务，不予处理不抛异常。如果业务汇总允许任务丢失，这是最好的策略

new ThreadPoolExecutor.DiscardPolicy()

查看执行结果，只有8个。第9个被抛弃

image.png

DiscardOldestPolicy

抛弃队列中等待最久的任务，然后把当前任务加入队列中尝试再次提交当前任务

new ThreadPoolExecutor.DiscardOldestPolicy()

image.png