服务端没接受一次任务就创建一个线程，如果成千上万次请求，那么将创建万计的线程，这样会导入服务器频繁的进行线程的切换，增加系统负担。
线程池技术能很好的解决这个问题。

• 消除了频繁创建和消亡线程的系统资源开销。
• 面对过量任务的提交能够平缓的劣化。

客户端线程将任务添加到任务队列jobs后就返回。工作者线程Worker不断从任务队列取出任务进行处理，当任务为空的时候，工作者线程等待，一旦有任务添加到队列，工作者线程立马被激活。
ThreadPool.java

package com.threadpooltest;

public interface ThreadPool<Job extends Runnable>{
    //执行一个Job，这个job需要实现runnable
    void execute(Job job);
    //关闭线程池
    void shutdown();
    //增加工作者线程
    void addWorkers(int num);
    //减少工作者线程
    void removeWorker(int num);
    //得到正在等待执行的任务数量
    int getJobSize();
}

DefaultThreadPool.java

package com.threadpooltest;

import java.util.ArrayList;
import java.util.Collections;
import java.util.LinkedList;
import java.util.List;
import java.util.concurrent.atomic.AtomicLong;

public class DefaultThreadPool<Job extends Runnable> implements ThreadPool<Job> {
    //线程池最大限制数
    private static final int MAX_WORKER_NUMBERS = 10;
    //线程池默认数量
    private static final int DEFAULT_WORKER_NUMBER = 5;
    //线程池最小数量
    private static final int MIN_WORKER_NUMBER = 1;
    //这是一个工作列表，将会向里面插入工作
    private final LinkedList<Job> jobs = new LinkedList<Job>();
    //工作者列表
    //Collections.synchronizedList使非同步的集合变成同步的（我理解这样在多线程对该集合进行操作的时候就不用手动加锁了）。
    private final List<Worker> workers = Collections.synchronizedList(new ArrayList<Worker>());
    //工作者线程的数量
    private int workerNum = DEFAULT_WORKER_NUMBER;
    //线程编号生成
    private AtomicLong threadNum = new AtomicLong();
    
    public DefaultThreadPool(){
        initializeWorkers(DEFAULT_WORKER_NUMBER);
    }
    public DefaultThreadPool(int num){
        workerNum = num > MAX_WORKER_NUMBERS? MAX_WORKER_NUMBERS : num<MIN_WORKER_NUMBER?MIN_WORKER_NUMBER:num;
        initializeWorkers(workerNum);
    }
    //初始化线程工作者
    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() );//incrementAndGet原子的方式加一
            thread.start();
        }
    }
    
    @Override
    public void execute(Job job) {
        if(job != null){
            //添加一个工作，然后进行通知
            synchronized(jobs){
                jobs.add(job);
                jobs.notifyAll();
            }
        }
    }

    @Override
    public void shutdown() {
        for(Worker worker : workers){
            worker.shutdown();
        }
    }

    @Override
    public void addWorkers(int num) {
        synchronized(jobs){//这个加锁有必要用吗？我觉得这里不需要。
            //限制新增的worker数量不能超过最大值
            if(num + this.workerNum > MAX_WORKER_NUMBERS){
                num = MAX_WORKER_NUMBERS - this.workerNum;
            }
            initializeWorkers(num);
            this.workerNum += num;
        }
    }

    @Override
    public void removeWorker(int num) {
        synchronized(jobs){
            if(num >= this.workerNum){
                throw new IllegalArgumentException("beyond worknum.");
            }
            //按照给定的数量停止worker
            int count = 0;
            while(count<num){
                Worker worker = workers.get(count);
                worker.shutdown();
                count++;
            }
            this.workerNum -= num;
        }
    }

    @Override
    public int getJobSize() {
        return jobs.size();
    }
    
    class Worker implements Runnable{
        //是否工作
        private volatile boolean running = true;
        
        public void run() {
            while(running){
                Job job = null;
                synchronized(jobs){
                    //如果工作者列表是空的，那么久wait
                    while(jobs.isEmpty()){
                        try {
                            jobs.wait();
                        } catch (InterruptedException e) {
                            //感知到外部对workerThread的中断操作，返回
                            Thread.currentThread().interrupt();
                            e.printStackTrace();
                        }
                    }
                    //取出一个job
                    job = jobs.removeFirst();
                }
                if(job != null){
                    System.out.println(Thread.currentThread().getName() + "处理一个job。");
                    job.run();
                }
            }
        }
        public void shutdown(){
            running = false;
        }
    }
}

ThreadPoolMain.java

package com.threadpooltest;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class ThreadPoolMain {

    public static void main(String[] args) {
        
        ThreadPool tp = new DefaultThreadPool<Task>();
        int i = 0;
        while(i++ < 7){
            tp.execute(new Task());
        }
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String command = "";
        while(true){
            try {
                System.out.println("请输入指令：");
                command = br.readLine();
            } catch (IOException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
            if(command.equals("quit")){
                break;
            }else if(command.equals("add")){
                i = 0;
                while(i++ < 7){
                    tp.execute(new Task());
                }
            }else if(command.equals("minus")){
            }else if(command.equals("shutdown")){
                tp.shutdown();
            }
        }
    }
}
class Task implements Runnable{
    @Override
    public void run() {
        System.out.println("Task 处理完毕！");
    }
}