java编写属于自己的线程池

public interface ThreadPool<Job extends Runnable>{
  //执行一个任务(Job),这个Job必须实现Runnable
  void execute(Job job);
 //关闭线程池
 void shutdown();
 //增加工作者线程，即用来执行任务的线程
 void addWorkers(int num);
 //减少工作者线程
 void removeWorker(int num);
 //获取正在等待执行的任务数量
 void getJobSize();
}

public class DefaultThreadPool<Job extends Runnable> implements ThreadPool<Job>{

  // 线程池维护工作者线程的最大数量
  private static final int MAX_WORKER_NUMBERS = 10;
  // 线程池维护工作者线程的默认值
  private static final int DEFAULT_WORKER_NUMBERS = 5;
  // 线程池维护工作者线程的最小数量
  private static final int MIN_WORKER_NUMBERS = 1;
  // 维护一个工作列表,里面加入客户端发起的工作
  private final LinkedList<Job> jobs = new LinkedList<Job>();
  // 工作者线程的列表
  private final List<Worker> workers = Collections.synchronizedList(new ArrayList<Worker>());
  // 工作者线程的数量
  private int workerNum;
  // 每个工作者线程编号生成
  private AtomicLong threadNum = new AtomicLong();

 //生成线程池
public DefaultThreadPool() {
    this.workerNum = DEFAULT_WORKER_NUMBERS;
    initializeWorkers(this.workerNum);
  }

  public DefaultThreadPool(int num) {
    if (num > MAX_WORKER_NUMBERS) {
      this.workerNum =DEFAULT_WORKER_NUMBERS;
    } else {
      this.workerNum = num;
    }
    initializeWorkers(this.workerNum);
  }
//初始化每个工作者线程
private void initializeWorkers(int num) {
    for (int i = 0; i < num; i++) {
      Worker worker = new Worker();
      //添加到工作者线程的列表
      workers.add(worker);
      //启动工作者线程
      Thread thread = new Thread(worker);
      thread.start();
    }
  }

public void execute(Job job) {
    if (job != null) {
    //根据线程的"等待/通知机制"这里必须对jobs加锁
      synchronized (jobs) {
        jobs.addLast(job);
        jobs.notify();
      }
    }
  }
  //关闭线程池即关闭每个工作者线程
   public void shutdown() {
    for (Worker w : workers) {
      w.shutdown();
    }
  }
   //增加工作者线程
    public void addWorkers(int num) {
    //加锁，防止该线程还么增加完成而下个线程继续增加导致工作者线程超过最大值
    synchronized (jobs) {
      if (num + this.workerNum > MAX_WORKER_NUMBERS) {
        num = MAX_WORKER_NUMBERS - this.workerNum;
      }
      initializeWorkers(num);
      this.workerNum += num;
    }
  }
  //减少工作者线程
public void removeWorker(int num) {
    synchronized (jobs) {
    if(num>=this.workerNum){
         throw new IllegalArgumentException("超过了已有的线程数量");
         }
      for (int i = 0; i < num; i++) {
        Worker worker = workers.get(i);
        if (worker != null) {
        //关闭该线程并从列表中移除
          worker.shutdown();
          workers.remove(i);
        }
      }
      this.workerNum -= num;
    }
  }

public int getJobSize() {
    // TODO Auto-generated method stub
    return workers.size();
  }
//定义工作者线程
class Worker implements Runnable {
    // 表示是否运行该worker
    private volatile boolean running = true;

    public void run() {
      while (running) {
        Job job = null;
        //线程的等待/通知机制
        synchronized (jobs) {
          if (jobs.isEmpty()) {
            try {
              jobs.wait();//线程等待唤醒
            } catch (InterruptedException e) {
              //感知到外部对该线程的中断操作，返回
              Thread.currentThread().interrupt();
              return;
            }
          }
          // 取出一个job
          job = jobs.removeFirst();
        }
        //执行job
        if (job != null) {
          job.run();
        }
      }
    }

    // 终止该线程
    public void shutdown() {
      running = false;
    }
  }
}