记JAVA线程池的一次扫盲

前言

某日同一同事探讨线程池的几个关键参数：corePoolSize，maximumPoolSize，queueCapacity；进而围绕这几个参数引出几个点：

• a) 应用初始状态下，此时提交任务，将创建线程来处理任务；
• b) 当运行的线程数达到corePoolSize时，新提交的任务将如何操作？
• c) 当queueCapacity满时，新提交的任务将如何处理？
• d) 当运行的线程数达到maximumPoolSize时，新提交的任务该如何处理？
• e) corePoolSize+1个线程运行中，此时有一个工作任务处理完成，该完成任务的线程后续的状态是什么样的？

第一反应

以自己来设计线程池的思路出发，当时个人认为处理方式如下：

在场景b)时，应该会创建新线程来执行任务，直到线程数达到maximumPoolSize后，
再提交的任务将放在等待队列中，直到队列塞满（达到queueCapacity），
此时再提交的任务将根据配置的拒绝模式处理；

然而，被活生生的打脸，该同事好好喝了杯茶说：

JAVA中线程池的实现方式如下（默认maximumPoolSize > corePoolSize,queueCapacity有限大小）：
1. 运行状态的线程数小于corePoolSize时，新增的任务将创建新线程来处理任务；
2. 当运行状态的线程数等于corePoolSize时，新增的任务将被放进任务队列中；
3. 当运行状态的线程数等于corePoolSize并且任务队列的大小达到queueCapacity时，新提交的任务将会创建新线程进行处理；
4. 当运行状态的线程数等于maximumPoolSize 并且任务队列大小达到queueCapacity，此时提交的任务将被拒绝；

故上述的b,c,d场景比较明确处理方式了；此时场景e该如何处理呢？
在细追代码之前，我们直接撸个代码看下运行结果：

• 任务线程代码

package com.test.chan.pools;
import java.util.Date;
public class BlockThread implements Runnable {

    private String tKey = null;
    private long blockTS = 0L;
    public BlockThread(int blockTimes,String threadKey){
        this.blockTS = blockTimes * 1000L;
        this.tKey = threadKey;
    }

    public void run() {
        System.out.printf("[%s]======== START %s - %s ===============\n", new Date(),this.tKey, this.blockTS);
        try {
            Thread.sleep(this.blockTS);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.printf("[%s]======== END %s - %s ===============\n", new Date(),this.tKey, this.blockTS);
    }
}

• 验证代码

package com.test.chan.pools;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.ThreadPoolExecutor;
import java.util.concurrent.TimeUnit;
public class ThreadPoolDemo {
    public static void main(String[] args){
        int corePoolSize = 5;
        int maximumPoolSize = 15;
        long keepAliveTime = 0L;
        int queueCapacity = 10;
        ExecutorService exec = new ThreadPoolExecutor(corePoolSize, maximumPoolSize,
                keepAliveTime, TimeUnit.MILLISECONDS,
                new LinkedBlockingQueue(queueCapacity));
        //前5个正常提交，任务阻塞时间未300s
        //前15个，有10个任务阻塞，任务阻塞时间10s
        //前16个，任务阻塞时间10s，观察队列中的任务是否被执行
        String key_pre = "STEP1-";
        int taskNum = 5;
        int blockTime = 300;
        //STEP1. 提交5个阻塞时间为300s的任务
        for(int i =0;i

• 查看输出结果并分析

/** 初始提交任务  **/
[Thu Aug 20 20:15:30 CST 2020]======== START STEP1-0 - 300000 ===============
[Thu Aug 20 20:15:30 CST 2020]======== START STEP1-4 - 300000 ===============
[Thu Aug 20 20:15:30 CST 2020]======== START STEP1-3 - 300000 ===============
[Thu Aug 20 20:15:30 CST 2020]======== START STEP1-2 - 300000 ===============
[Thu Aug 20 20:15:30 CST 2020]======== START STEP1-1 - 300000 ===============
/** 提交的任务进入任务队列中 **/
/** 任务队列满了，提交任务则将创建新线程处理该任务 **/
[Thu Aug 20 20:15:32 CST 2020]======== START STEP3-0 - 10000 ===============
[Thu Aug 20 20:15:42 CST 2020]======== END STEP3-0 - 10000 ===============
/** 完成任务的线程从任务队列中取任务执行 **/
[Thu Aug 20 20:15:42 CST 2020]======== START STEP2-0 - 10000 ===============
[Thu Aug 20 20:15:52 CST 2020]======== END STEP2-0 - 10000 ===============
[Thu Aug 20 20:15:52 CST 2020]======== START STEP2-1 - 10000 ===============
[Thu Aug 20 20:16:02 CST 2020]======== END STEP2-1 - 10000 ===============
[Thu Aug 20 20:16:02 CST 2020]======== START STEP2-2 - 10000 ===============
[Thu Aug 20 20:16:12 CST 2020]======== END STEP2-2 - 10000 ===============
[Thu Aug 20 20:16:12 CST 2020]======== START STEP2-3 - 10000 ===============
[Thu Aug 20 20:16:22 CST 2020]======== END STEP2-3 - 10000 ===============
[Thu Aug 20 20:16:22 CST 2020]======== START STEP2-4 - 10000 ===============
/**  任务队列满，新增线程处理新提交的任务 **/
[Thu Aug 20 20:16:32 CST 2020]======== START STEP4-5 - 10000 ===============
[Thu Aug 20 20:16:32 CST 2020]======== START STEP4-13 - 10000 ===============
[Thu Aug 20 20:16:32 CST 2020]======== START STEP4-7 - 10000 ===============
[Thu Aug 20 20:16:32 CST 2020]======== START STEP4-6 - 10000 ===============
[Thu Aug 20 20:16:32 CST 2020]======== START STEP4-8 - 10000 ===============
[Thu Aug 20 20:16:32 CST 2020]======== START STEP4-12 - 10000 ===============
[Thu Aug 20 20:16:32 CST 2020]======== START STEP4-11 - 10000 ===============
[Thu Aug 20 20:16:32 CST 2020]======== START STEP4-10 - 10000 ===============
[Thu Aug 20 20:16:32 CST 2020]======== START STEP4-9 - 10000 ===============
[Thu Aug 20 20:16:32 CST 2020]======== END STEP2-4 - 10000 ===============
[Thu Aug 20 20:16:32 CST 2020]======== START STEP2-5 - 10000 ===============
/** 任务队列满了，运行的线程数达到maximumPoolSize，新提交任务被拒绝 **/
Exception in thread "main" java.util.concurrent.RejectedExecutionException: Task com.test.chan.pools.BlockThread@6e0be858 rejected from java.util.concurrent.ThreadPoolExecutor@61bbe9ba[Running, pool size = 15, active threads = 15, queued tasks = 10, completed tasks = 5]
    at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2063)
    at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:830)
    at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1379)
    at com.test.chan.pools.ThreadPoolDemo.main(ThreadPoolDemo.java:67)
[Thu Aug 20 20:16:42 CST 2020]======== END STEP4-13 - 10000 ===============
[Thu Aug 20 20:16:42 CST 2020]======== START STEP2-6 - 10000 ===============
[Thu Aug 20 20:16:42 CST 2020]======== END STEP4-6 - 10000 ===============
[Thu Aug 20 20:16:42 CST 2020]======== END STEP4-7 - 10000 ===============
[Thu Aug 20 20:16:42 CST 2020]======== END STEP4-5 - 10000 ===============
[Thu Aug 20 20:16:42 CST 2020]======== END STEP4-12 - 10000 ===============
[Thu Aug 20 20:16:42 CST 2020]======== START STEP4-0 - 10000 ===============
[Thu Aug 20 20:16:42 CST 2020]======== END STEP4-11 - 10000 ===============
[Thu Aug 20 20:16:42 CST 2020]======== END STEP4-8 - 10000 ===============
[Thu Aug 20 20:16:42 CST 2020]======== START STEP4-2 - 10000 ===============
[Thu Aug 20 20:16:42 CST 2020]======== END STEP4-9 - 10000 ===============
[Thu Aug 20 20:16:42 CST 2020]======== END STEP4-10 - 10000 ===============
[Thu Aug 20 20:16:42 CST 2020]======== START STEP4-4 - 10000 ===============
[Thu Aug 20 20:16:42 CST 2020]======== START STEP2-8 - 10000 ===============
[Thu Aug 20 20:16:42 CST 2020]======== START STEP2-7 - 10000 ===============
[Thu Aug 20 20:16:42 CST 2020]======== START STEP4-3 - 10000 ===============
[Thu Aug 20 20:16:42 CST 2020]======== START STEP4-1 - 10000 ===============
[Thu Aug 20 20:16:42 CST 2020]======== START STEP2-9 - 10000 ===============
[Thu Aug 20 20:16:42 CST 2020]======== END STEP2-5 - 10000 ===============
[Thu Aug 20 20:16:52 CST 2020]======== END STEP2-6 - 10000 ===============
[Thu Aug 20 20:16:52 CST 2020]======== END STEP4-0 - 10000 ===============
[Thu Aug 20 20:16:52 CST 2020]======== END STEP2-7 - 10000 ===============
[Thu Aug 20 20:16:52 CST 2020]======== END STEP4-2 - 10000 ===============
[Thu Aug 20 20:16:52 CST 2020]======== END STEP2-8 - 10000 ===============
[Thu Aug 20 20:16:52 CST 2020]======== END STEP4-4 - 10000 ===============
[Thu Aug 20 20:16:52 CST 2020]======== END STEP4-1 - 10000 ===============
[Thu Aug 20 20:16:52 CST 2020]======== END STEP2-9 - 10000 ===============
[Thu Aug 20 20:16:52 CST 2020]======== END STEP4-3 - 10000 ===============
[Thu Aug 20 20:20:30 CST 2020]======== END STEP1-0 - 300000 ===============
[Thu Aug 20 20:20:30 CST 2020]======== END STEP1-4 - 300000 ===============
[Thu Aug 20 20:20:30 CST 2020]======== END STEP1-3 - 300000 ===============
[Thu Aug 20 20:20:30 CST 2020]======== END STEP1-1 - 300000 ===============
[Thu Aug 20 20:20:30 CST 2020]======== END STEP1-2 - 300000 ===============

从结果回推： 场景e下，完成工作的线程将会从任务队列中取任务并执行，同时此时若新增任务，则先放入任务队列中（有空位）；
即当运行状态的线程数等于corePoolSize并且任务队列的大小达到queueCapacity时，此时提交的任务将会被优先执行（新建线程来执行该任务）；当任务队列中的堆积任务被处理完后，则线程池的可用线程数将逐步降低至corePoolSize（allowCoreThreadTimeOut=false）

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