服務端沒接受一次任務就創建一個線程，如果成千上萬次請求，那么將創建萬計的線程，這樣會導入服務器頻繁的進行線程的切換，增加系統負擔。
線程池技術能很好的解決這個問題。

• 消除了頻繁創建和消亡線程的系統資源開銷。
• 面對過量任務的提交能夠平緩的劣化。

客戶端線程將任務添加到任務隊列jobs后就返回。工作者線程Worker不斷從任務隊列取出任務進行處理，當任務為空的時候，工作者線程等待，一旦有任務添加到隊列，工作者線程立馬被激活。
ThreadPool.java

package com.threadpooltest;

public interface ThreadPool<Job extends Runnable>{
    //執行一個Job，這個job需要實現runnable
    void execute(Job job);
    //關閉線程池
    void shutdown();
    //增加工作者線程
    void addWorkers(int num);
    //減少工作者線程
    void removeWorker(int num);
    //得到正在等待執行的任務數量
    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> {
    //線程池最大限制數
    private static final int MAX_WORKER_NUMBERS = 10;
    //線程池默認數量
    private static final int DEFAULT_WORKER_NUMBER = 5;
    //線程池最小數量
    private static final int MIN_WORKER_NUMBER = 1;
    //這是一個工作列表，將會向里面插入工作
    private final LinkedList<Job> jobs = new LinkedList<Job>();
    //工作者列表
    //Collections.synchronizedList使非同步的集合變成同步的（我理解這樣在多線程對該集合進行操作的時候就不用手動加鎖了）。
    private final List<Worker> workers = Collections.synchronizedList(new ArrayList<Worker>());
    //工作者線程的數量
    private int workerNum = DEFAULT_WORKER_NUMBER;
    //線程編號生成
    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);
    }
    //初始化線程工作者
    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){
            //添加一個工作，然后進行通知
            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){//這個加鎖有必要用嗎？我覺得這里不需要。
            //限制新增的worker數量不能超過最大值
            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.");
            }
            //按照給定的數量停止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) {
                            //感知到外部對workerThread的中斷操作，返回
                            Thread.currentThread().interrupt();
                            e.printStackTrace();
                        }
                    }
                    //取出一個job
                    job = jobs.removeFirst();
                }
                if(job != null){
                    System.out.println(Thread.currentThread().getName() + "處理一個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("請輸入指令：");
                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 處理完畢！");
    }
}