Java线程池简单实现

（已迁移）

--　　线程池优势：

　　不会一个任务一个线程，进而创建以万记的线程量，导致频繁的线程上下文切换，消耗系统资源。且线程的创建和销毁同样浪费着系统资源。

public interface JobRun {
    //Job的具体工作内容
    void concreteJob();
}

import java.util.concurrent.TimeUnit;

public class Job implements JobRun {
    @Override
    public void concreteJob(){
        System.out.println("执行某一个工作，耗时1s");
        try{
            TimeUnit.SECONDS.sleep(1);
        }catch(InterruptedException e){
            e.printStackTrace();
        }
    }
}

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

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 JobRun> implements ThreadPool<Job> {
    //线程池最大限制数
    private static final int MAX_WORKER_NUMBERS = 10;
    //线程池默认线程数
    private static final int DEFAULT_WORKER_NUMBERS = 5;
    //线程池最小线程数
    private static final int MIN_WORKER_NUMBERS = 2;
    //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
    private final List<Worker> workers = Collections.synchronizedList(new ArrayList<Worker>());
    //工作者线程数
    private int workerNum = DEFAULT_WORKER_NUMBERS;
    //线程编号生成
    private AtomicLong threadNum = new AtomicLong();

    public DefaultThreadPool(){
        initializeWorkers(DEFAULT_WORKER_NUMBERS);
    }

    public DefaultThreadPool(int num){
        if(num > MAX_WORKER_NUMBERS){
            workerNum = MAX_WORKER_NUMBERS;
        }
        if(num<MIN_WORKER_NUMBERS){
            workerNum = MIN_WORKER_NUMBERS;
        }
        initializeWorkers(workerNum);
    }

    public void execute(Job job){
        if(job != null){
            synchronized (jobs){
                //增加一个工作，然后进行通知
                jobs.addLast(job);
                jobs.notify();
            }
        }
    }

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

    public void addWorkers(int num){
        synchronized (jobs){
            //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;
        }
    }

    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(0);
                if(workers.remove(worker)){
                    worker.shutdown();
                    ++count;
                }
            }
            this.workerNum -= count;
        }
    }

    public int getJobSize(){
        return jobs.size();
    }

    //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());
            thread.start();
        }
    }

    //工作者，负责消费任务
    class Worker implements Runnable{
        //是否工作
        private volatile boolean running = true;
        @Override
        public void run(){
            while (running){
                Job job = null;
                synchronized (jobs){
                    //如果工作者列表为空，那么wait
                    while(jobs.isEmpty()){
                        try{
                            jobs.wait();
                        }catch(InterruptedException ex){
                            //感知到外部对WorkerThread的中断操作，返回
                            Thread.currentThread().interrupt();
                            return;
                        }
                    }
                    //取出一个job
                    job = jobs.removeFirst();
                }
                if(job != null){
                    try{
                        job.concreteJob();
                    }catch(Exception ex){
                        System.out.println(ex.getMessage());
                    }
                }
            }
        }

        public void shutdown(){
            running = false;
        }
    }
}

public class ApplicationDemo {
    public static void main(String[] args){
        DefaultThreadPool<Job> defaultThreadPool = new DefaultThreadPool<Job>();
        defaultThreadPool.removeWorker(3);
        for(int i = 0;i<20;++i){
            Job job = new Job();
            defaultThreadPool.execute(job);
        }
    }
}

--　　关于Collects.synchronizedList

　　ArrayList是非线程安全的，多线程下向list插入数据可能会造成数据丢失。并且一个线程变量list，另一个线程修改list，会报ConcurrentModificationException（并发修改异常）错误。

　　Vector是线程安全的list，但是它的线程安全实现方式是对所有操作都加上synchronized关键字，这种方式严重影响效率，不推荐使用。

　　推荐使用：

List<String> list = Collections.synchronizedList(new ArrayList<String>());
list.add("1");
list.add("2");
list.add("3");

synchronized(list){
    Iterator iter = list.iterator();
    while(iter.hasNext()){
        System.out.println(iter.next());
    }
}

Collections.synchronizedList会对add方法加锁。
遍历的时候，应避免数据被其他线程串改，应加锁保护。
（但是上文线程池实现的时候，Collections.synchronizedList貌似并非是必要的）

（参考：Java并发编程的艺术）
（参考原文：https://www.cnblogs.com/luojiabao/p/11308803.html）