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什么是线程池，这个池字是什么

线程池，主要利用池化思想，线程池，字符串常量池等

为什么要有一个线程池？

正常线程的创建：1，手动创建一个线程

2.给该线程分配任务，线程执行任务

3.执行完成，手动释放线程

有了线程池后，默认线程数为3，最大线程数为5，等待队列为6，在正常状态下，我们只有三个默认线程去处理任务，流程是这样的：

1.等待队列按照顺序分出任务给空闲线程，直到任务为空或者无空闲线程

2.线程执行完任务后不会销毁，而是继续执行任务，如果此时没有任务，那么线程进入空闲状态，不会被销毁

如果任务变多，等待队列满呢

此时只有三个线程，但是任务队列已经满了，证明在这个状态下只有三个线程工作是不够的，所以我们开启最大线程数，全部开始处理任务，如果此时仍然想有任务进入队列，拒绝。

在将任务处理完毕后，队列处于空闲或者任务很少的状态，销毁线程，使当前的线程数为默认线程数

优点：

提高线程利用率

提高响应速度（省去创建线程的时间了）

控制并发数（控制最大线程数量)

便于统一管理(都位于线程池中)

代码实现：

package org.my;import java.util.ArrayList;
import java.util.Collections;import java.util.LinkedList;import java.util.List;import java.util.concurrent.atomic.AtomicInteger;import java.util.concurrent.atomic.AtomicLong;import java.util.stream.Collectors;/*** 创建了num个线程* 每个线程做woker这个任务* 而woker这个任务是不断取jobs中的一个任务进行执行* 直到jobs为空，然后进入等待状态* 如果jobs又重新exe添加了新job* jobs唤醒worker可以开始运行* 线程就可以继续开始工作。*/
public class ThreadPoolLearn313<Job extends Runnable> implements ThreadPool<Job> {//最大线程数量private static final int MAX_WORKER_NUMBERS = 10;//默认线程数private static final int DEFAULT_WORKER_NUMBERS = 5;//最小线程数private static final int MIN_WORKER_NUMBERS = 1;//任务队列private final LinkedList<Job> jobs =new LinkedList<>();private final List<Worker> workers = Collections.synchronizedList(new ArrayList<Worker>());//AtomicLong可理解为加了synchronized的longprivate AtomicLong threadNum = new AtomicLong();private  int workerNum = DEFAULT_WORKER_NUMBERS;public ThreadPoolLearn313() {initializeWorkers(DEFAULT_WORKER_NUMBERS);}private void initializeWorkers(int num) {for (int i = 0; i < num; i++) {Worker worker = new Worker();workers.add(worker);Thread thread = new Thread(worker,"ThreadPool-Worker-"+threadNum.incrementAndGet());thread.start();}}public ThreadPoolLearn313(int num) {this.workerNum = num>MAX_WORKER_NUMBERS?MAX_WORKER_NUMBERS:num<MIN_WORKER_NUMBERS?MIN_WORKER_NUMBERS:num;initializeWorkers(workerNum);}@Overridepublic void execute(Job job) {if (job!=null){synchronized (jobs){jobs.addLast(job);jobs.notify();}}}//关闭线程池@Overridepublic void shutdown() {for (Worker worker : workers) {worker.shutdown();}}@Overridepublic void addWorks(int num) {synchronized (jobs){if(num+workerNum>MAX_WORKER_NUMBERS){num =MAX_WORKER_NUMBERS-workerNum;}initializeWorkers(num);workerNum+=num;}}@Overridepublic void removeWorks(int num) {synchronized (jobs){if(num>=workerNum){throw new IllegalArgumentException("beyond workNum");}int count=0;while (count<num){Worker worker = workers.get(count);if(workers.remove(worker)){worker.shutdown();count++;}}workerNum-=count;}}@Overridepublic int getJobSize() {return jobs.size();}class Worker implements Runnable{private volatile  boolean running = true;@Overridepublic void run() {while (running){Job job =null;synchronized (jobs){while (jobs.isEmpty()){try {jobs.wait();} catch (InterruptedException e) {Thread.currentThread().interrupt();return;}}job = jobs.removeFirst();}if(job!=null){job.run();}}}public  void  shutdown(){running = false;}}
}