JAVA线程池核心数满了后什么时候开始创建新的线程

首先，我们来创建了一个线程池，设置核心数5个，最大线程数10个，任务队列10个，alive时间是2s，每个任务就是随机的sleep一会，主要用来占用时间。
创建完线程后，每隔一秒钟，获取一下线程池的状态。
根据输出发现，当任务数大于5个后，active数依然没变，只要任务数为16个的时候，active才变为6，也就是说当5个核心数满了，任务队列也满了后，才开始创建新的线程。
当线程执行完任务的时候，等待设置的alive时间后，开始销毁，知道active=0。
也就是核心线程也是会被销毁的。

环境
jdk8，ThreadPoolExecutor
测试代码

package com.koolearn;

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

/**
 * @author 
 * @version 1.0
 * @date 
 */
public class ThreadPoolTest {

    /**
     * 测试JAVA线程池 核心池满了后什么情况下创建新的线程
     * 结论：当任务来的时候，判断是否有线程，没有就创建，当大于核心线程池后，放到任务队列，队列满了后创建新的线程执行，直到线程数等于最大线程数
     * 当任务执行完后，线程会等待设置的时长后销毁，包括核心线程，都会销毁，直到active的线程数是0
     * @param args
     */
    public static void main(String[] args) {
        BlockingQueue<Runnable> workQueue = new ArrayBlockingQueue<>(10);
        ThreadPoolExecutor es = new ThreadPoolExecutor(5,10,2, TimeUnit.SECONDS,workQueue);

        for (int i = 0; i < 20; i++) {
            int a = i;
            es.submit(new Runnable() {
                @Override
                public void run() {
                    try {
                        Thread.sleep(a*500);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            });
            String name = Thread.currentThread().getName()+"-"+i;
            System.out.println(name+":es.getActiveCount():"+es.getActiveCount());
            System.out.println(name+":es.getCorePoolSize():"+es.getCorePoolSize());
            System.out.println(name+":es.getMaximumPoolSize():"+es.getMaximumPoolSize());
            System.out.println(name+":es.getTaskCount():"+es.getTaskCount());
            System.out.println("--------------------------------");
        }

        for (int i = 0; i < 20; i++) {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            String name = Thread.currentThread().getName()+"-"+"last"+i;
            System.out.println(name+":es.getActiveCount():"+es.getActiveCount());
            System.out.println(name+":es.getCorePoolSize():"+es.getCorePoolSize());
            System.out.println(name+":es.getMaximumPoolSize():"+es.getMaximumPoolSize());
            System.out.println(name+":es.getTaskCount():"+es.getTaskCount());
            System.out.println("--------------------------------");
        }
        
        es.shutdown();
    }

}

输出结果

main-0:es.getActiveCount():1
main-0:es.getCorePoolSize():5
main-0:es.getMaximumPoolSize():10
main-0:es.getTaskCount():1
--------------------------------
main-1:es.getActiveCount():2
main-1:es.getCorePoolSize():5
main-1:es.getMaximumPoolSize():10
main-1:es.getTaskCount():2
--------------------------------
main-2:es.getActiveCount():3
main-2:es.getCorePoolSize():5
main-2:es.getMaximumPoolSize():10
main-2:es.getTaskCount():3
--------------------------------
main-3:es.getActiveCount():4
main-3:es.getCorePoolSize():5
main-3:es.getMaximumPoolSize():10
main-3:es.getTaskCount():4
--------------------------------
main-4:es.getActiveCount():5
main-4:es.getCorePoolSize():5
main-4:es.getMaximumPoolSize():10
main-4:es.getTaskCount():5
--------------------------------
main-5:es.getActiveCount():5
main-5:es.getCorePoolSize():5
main-5:es.getMaximumPoolSize():10
main-5:es.getTaskCount():6
--------------------------------
main-6:es.getActiveCount():5
main-6:es.getCorePoolSize():5
main-6:es.getMaximumPoolSize():10
main-6:es.getTaskCount():7
--------------------------------
main-7:es.getActiveCount():5
main-7:es.getCorePoolSize():5
main-7:es.getMaximumPoolSize():10
main-7:es.getTaskCount():8
--------------------------------
main-8:es.getActiveCount():5
main-8:es.getCorePoolSize():5
main-8:es.getMaximumPoolSize():10
main-8:es.getTaskCount():9
--------------------------------
main-9:es.getActiveCount():5
main-9:es.getCorePoolSize():5
main-9:es.getMaximumPoolSize():10
main-9:es.getTaskCount():10
--------------------------------
main-10:es.getActiveCount():5
main-10:es.getCorePoolSize():5
main-10:es.getMaximumPoolSize():10
main-10:es.getTaskCount():11
--------------------------------
main-11:es.getActiveCount():5
main-11:es.getCorePoolSize():5
main-11:es.getMaximumPoolSize():10
main-11:es.getTaskCount():12
--------------------------------
main-12:es.getActiveCount():4
main-12:es.getCorePoolSize():5
main-12:es.getMaximumPoolSize():10
main-12:es.getTaskCount():13
--------------------------------
main-13:es.getActiveCount():5
main-13:es.getCorePoolSize():5
main-13:es.getMaximumPoolSize():10
main-13:es.getTaskCount():14
--------------------------------
main-14:es.getActiveCount():5
main-14:es.getCorePoolSize():5
main-14:es.getMaximumPoolSize():10
main-14:es.getTaskCount():15
--------------------------------
main-15:es.getActiveCount():5
main-15:es.getCorePoolSize():5
main-15:es.getMaximumPoolSize():10
main-15:es.getTaskCount():16
--------------------------------
main-16:es.getActiveCount():6
main-16:es.getCorePoolSize():5
main-16:es.getMaximumPoolSize():10
main-16:es.getTaskCount():17
--------------------------------
main-17:es.getActiveCount():7
main-17:es.getCorePoolSize():5
main-17:es.getMaximumPoolSize():10
main-17:es.getTaskCount():18
--------------------------------
main-18:es.getActiveCount():8
main-18:es.getCorePoolSize():5
main-18:es.getMaximumPoolSize():10
main-18:es.getTaskCount():19
--------------------------------
main-19:es.getActiveCount():9
main-19:es.getCorePoolSize():5
main-19:es.getMaximumPoolSize():10
main-19:es.getTaskCount():20
--------------------------------
main-last0:es.getActiveCount():9
main-last0:es.getCorePoolSize():5
main-last0:es.getMaximumPoolSize():10
main-last0:es.getTaskCount():20
--------------------------------
main-last1:es.getActiveCount():9
main-last1:es.getCorePoolSize():5
main-last1:es.getMaximumPoolSize():10
main-last1:es.getTaskCount():20
--------------------------------
main-last2:es.getActiveCount():9
main-last2:es.getCorePoolSize():5
main-last2:es.getMaximumPoolSize():10
main-last2:es.getTaskCount():20
--------------------------------
main-last3:es.getActiveCount():9
main-last3:es.getCorePoolSize():5
main-last3:es.getMaximumPoolSize():10
main-last3:es.getTaskCount():20
--------------------------------
main-last4:es.getActiveCount():9
main-last4:es.getCorePoolSize():5
main-last4:es.getMaximumPoolSize():10
main-last4:es.getTaskCount():20
--------------------------------
main-last5:es.getActiveCount():9
main-last5:es.getCorePoolSize():5
main-last5:es.getMaximumPoolSize():10
main-last5:es.getTaskCount():20
--------------------------------
main-last6:es.getActiveCount():9
main-last6:es.getCorePoolSize():5
main-last6:es.getMaximumPoolSize():10
main-last6:es.getTaskCount():20
--------------------------------
main-last7:es.getActiveCount():8
main-last7:es.getCorePoolSize():5
main-last7:es.getMaximumPoolSize():10
main-last7:es.getTaskCount():20
--------------------------------
main-last8:es.getActiveCount():5
main-last8:es.getCorePoolSize():5
main-last8:es.getMaximumPoolSize():10
main-last8:es.getTaskCount():20
--------------------------------
main-last9:es.getActiveCount():4
main-last9:es.getCorePoolSize():5
main-last9:es.getMaximumPoolSize():10
main-last9:es.getTaskCount():20
--------------------------------
main-last10:es.getActiveCount():3
main-last10:es.getCorePoolSize():5
main-last10:es.getMaximumPoolSize():10
main-last10:es.getTaskCount():20
--------------------------------
main-last11:es.getActiveCount():2
main-last11:es.getCorePoolSize():5
main-last11:es.getMaximumPoolSize():10
main-last11:es.getTaskCount():20
--------------------------------
main-last12:es.getActiveCount():2
main-last12:es.getCorePoolSize():5
main-last12:es.getMaximumPoolSize():10
main-last12:es.getTaskCount():20
--------------------------------
main-last13:es.getActiveCount():1
main-last13:es.getCorePoolSize():5
main-last13:es.getMaximumPoolSize():10
main-last13:es.getTaskCount():20
--------------------------------
main-last14:es.getActiveCount():0
main-last14:es.getCorePoolSize():5
main-last14:es.getMaximumPoolSize():10
main-last14:es.getTaskCount():20
--------------------------------
main-last15:es.getActiveCount():0
main-last15:es.getCorePoolSize():5
main-last15:es.getMaximumPoolSize():10
main-last15:es.getTaskCount():20
--------------------------------
main-last16:es.getActiveCount():0
main-last16:es.getCorePoolSize():5
main-last16:es.getMaximumPoolSize():10
main-last16:es.getTaskCount():20
--------------------------------
main-last17:es.getActiveCount():0
main-last17:es.getCorePoolSize():5
main-last17:es.getMaximumPoolSize():10
main-last17:es.getTaskCount():20
--------------------------------
main-last18:es.getActiveCount():0
main-last18:es.getCorePoolSize():5
main-last18:es.getMaximumPoolSize():10
main-last18:es.getTaskCount():20
--------------------------------
main-last19:es.getActiveCount():0
main-last19:es.getCorePoolSize():5
main-last19:es.getMaximumPoolSize():10
main-last19:es.getTaskCount():20
--------------------------------