【名企大厂面试必备】线程池ThreadPoolExecutor的拒绝策略

前言

线程池的拒绝策略，在面试中经常会被问到，本篇文章主要是总结一下对新添加的Task任务有不同的处理策略，有默认的，有我们主动自己添加的，首先说一下有哪几种不同的策略：

AbortPolicy

官方介绍当任务添加到线程池中被拒绝时，它将抛出RejectedExecutionException异常。

CallerRunsPolicy

当我们task添加到线程池中被拒绝时，会使用调用线程池的Thread线程对象处理被拒绝的任务

DiscardOldestPolicy

当Task任务添加到线程池中被拒绝时，线程池会放弃等待队列中最旧的未处理任务，然后将被拒绝的Task添加到等待队列中

DiscardPolicy

当Task添加到线程池中被拒绝时，线程池将丢弃被拒绝的Task

Demo

AbortPolicy-Demo

package com.zcw.demo;

import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

/**
 * @ClassName : AbortPolicyDemo
 * @Description :
 * @Author : Zhaocunwei
 * @Date: 2020-04-22 12:55
 */
@Slf4j
public class AbortPolicyDemo {
    public static void main(String[] args) {
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(4000);
                    log.info(Thread.currentThread().getName()+"end");
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        };
        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(2,3,5,
                TimeUnit.SECONDS, new ArrayBlockingQueue<>(2),
                new ThreadPoolExecutor.AbortPolicy());
        threadPoolExecutor.execute(runnable);
        threadPoolExecutor.execute(runnable);
        threadPoolExecutor.execute(runnable);
        threadPoolExecutor.execute(runnable);
        threadPoolExecutor.execute(runnable);
        threadPoolExecutor.execute(runnable);
        threadPoolExecutor.execute(runnable);
        threadPoolExecutor.execute(runnable);
    }
}

运行结果：

通过运行Demo我们发现报错，这是因为在使用AbortPolicy策略后，当我们在实际项目开发中线程的数量超出了max值时，线程池就会抛出异常

CallerRunsPolicy

package com.zcw.demo;

import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.LinkedBlockingDeque;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

/**
 * @ClassName : MyThreadA
 * @Description :
 * @Author : Zhaocunwei
 * @Date: 2020-04-22 13:05
 */
@Slf4j
public class MyThreadA extends Thread {
    @Override
    public void run(){
        MyThreadB myThreadB = new MyThreadB();
        LinkedBlockingDeque linkedBlockingDeque = new LinkedBlockingDeque(2);
        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(2,3,5,
                TimeUnit.SECONDS,linkedBlockingDeque,
                new ThreadPoolExecutor.CallerRunsPolicy());
        log.info("开始"+Thread.currentThread().getName()+""+System.currentTimeMillis());
        threadPoolExecutor.execute(myThreadB);
        threadPoolExecutor.execute(myThreadB);
        threadPoolExecutor.execute(myThreadB);
        threadPoolExecutor.execute(myThreadB);
        threadPoolExecutor.execute(myThreadB);
        threadPoolExecutor.execute(myThreadB);
        log.info("a 结束"+Thread.currentThread().getName()+" "+System.currentTimeMillis());
        log.info("a 结束"+System.currentTimeMillis());
    }

    public static void main(String[] args) {
        MyThreadA myThreadA = new MyThreadA();
        myThreadA.setName("HH");
        myThreadA.start();
        log.info("程序运行结束。。。。");
    }
}

package com.zcw.demo;

import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.SynchronousQueue;

/**
 * @ClassName : MyThreadB
 * @Description :
 * @Author : Zhaocunwei
 * @Date: 2020-04-22 13:06
 */
@Slf4j
public class MyThreadB extends Thread {
    @Override
    public void run(){
        try {
            Thread.sleep(4000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        log.info("end" + Thread.currentThread().getName()+""+ System.currentTimeMillis());
    }
}

运行结果：

DiscardOldestPolicy

package com.zcw.demo;

import lombok.Data;
import lombok.extern.slf4j.Slf4j;

import java.util.Iterator;
import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

/**
 * @ClassName : DiscardOldestPolicyDemo
 * @Description :
 * @Author : Zhaocunwei
 * @Date: 2020-04-22 13:14
 */
@Data
@Slf4j
public class DiscardOldestPolicyDemo implements Runnable{
    private String name;
    public DiscardOldestPolicyDemo(String name){
        super();
        this.name = name;
    }
    @Override
    public void run(){
        log.info(name+"跑起来....");
        try {
            Thread.sleep(4000);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

    }

    public static void main(String[] args) throws InterruptedException {
        ArrayBlockingQueue arrayBlockingQueue = new ArrayBlockingQueue(2);
        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(2,3,5,
                TimeUnit.SECONDS,arrayBlockingQueue,
                new ThreadPoolExecutor.DiscardOldestPolicy());
        for(int i=0;i<5;i++){
            DiscardOldestPolicyDemo discardOldestPolicyDemo = new DiscardOldestPolicyDemo("Runnable"+(i+1));
            threadPoolExecutor.execute(discardOldestPolicyDemo);
        }
        Thread.sleep(50);
        Iterator iterator = arrayBlockingQueue.iterator();
        while(iterator.hasNext()){
            Object object = iterator.next();
            log.info(((DiscardOldestPolicyDemo)object).getName());
        }
        threadPoolExecutor.execute(new DiscardOldestPolicyDemo("Runnable6"));
        threadPoolExecutor.execute(new DiscardOldestPolicyDemo("Runnable7"));
        iterator =arrayBlockingQueue.iterator();
        while(iterator.hasNext()){
            Object object = iterator.next();
            log.info(((DiscardOldestPolicyDemo)object).getName());
        }

    }
}

运行结果：

DiscardPolicy

package com.zcw.demo;

import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.ArrayBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

/**
 * @ClassName : DiscardPolicyDemo
 * @Description :
 * @Author : Zhaocunwei
 * @Date: 2020-04-22 13:39
 */
@Slf4j
public class DiscardPolicyDemo {
    public static void main(String[] args) throws InterruptedException {
        Runnable runnable = new Runnable() {
            @Override
            public void run() {
                try {
                    Thread.sleep(4000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                log.info(Thread.currentThread().getName()+"end");
            }
        };
        ArrayBlockingQueue arrayBlockingQueue = new ArrayBlockingQueue(2);
        ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(2,3,5,
                TimeUnit.SECONDS,arrayBlockingQueue,
                new ThreadPoolExecutor.DiscardOldestPolicy());
        threadPoolExecutor.execute(runnable);
        threadPoolExecutor.execute(runnable);
        threadPoolExecutor.execute(runnable);
        threadPoolExecutor.execute(runnable);
        threadPoolExecutor.execute(runnable);
        threadPoolExecutor.execute(runnable);
        threadPoolExecutor.execute(runnable);
        threadPoolExecutor.execute(runnable);
        threadPoolExecutor.execute(runnable);
        Thread.sleep(8000);
        log.info(threadPoolExecutor.getPoolSize()+" "+arrayBlockingQueue.size());
    }
}

运行结果：