線程池

1、線程池：提供了一個線程隊列，隊列中保存着全部等待狀態的線程。避免了建立與銷燬額外開銷，提升了響應的速度。
  
  2、線程池的體系結構：
      java.util.concurrent.Executor : 負責線程的使用與調度的根接口

          |--ExecutorService 子接口: 線程池的主要接口
              |--ThreadPoolExecutor 線程池的實現類
              |--ScheduledExecutorService 子接口：負責線程的調度
                  |--ScheduledThreadPoolExecutor ：繼承 ThreadPoolExecutor， 實現 ScheduledExecutorService
  
  3、工具類 : Executors 
  ExecutorService newFixedThreadPool() : 建立固定大小的線程池
  ExecutorService newCachedThreadPool() : 緩存線程池，線程池的數量不固定，能夠根據需求自動的更改數量。
  ExecutorService newSingleThreadExecutor() : 建立單個線程池。線程池中只有一個線程
  
  ScheduledExecutorService newScheduledThreadPool() : 建立固定大小的線程，能夠延遲或定時的執行任務。

    import java.util.ArrayList;
    import java.util.List;
    import java.util.concurrent.Callable;
    import java.util.concurrent.ExecutorService;
    import java.util.concurrent.Executors;
    import java.util.concurrent.Future;
   
public class TestThreadPool {
    
    public static void main(String[] args) throws Exception {
        //1. 建立線程池
        ExecutorService pool = Executors.newFixedThreadPool(5);
        
        List<Future<Integer>> list = new ArrayList<>();
        
        for (int i = 0; i < 10; i++) {
            Future<Integer> future = pool.submit(new Callable<Integer>(){

                @Override
                public Integer call() throws Exception {
                    int sum = 0;
                    
                    for (int i = 0; i <= 100; i++) {
                        sum += i;
                    }
                    
                    return sum;
                }
                
            });

            list.add(future);
        }
        
        pool.shutdown();
        
        for (Future<Integer> future : list) {
            System.out.println(future.get());
        }
        
        
        
        /*ThreadPoolDemo tpd = new ThreadPoolDemo();
        
        //2. 爲線程池中的線程分配任務
        for (int i = 0; i < 10; i++) {
            pool.submit(tpd);
        }
        
        //3. 關閉線程池
        pool.shutdown();*/
    }
    
//    new Thread(tpd).start();
//    new Thread(tpd).start();

}

class ThreadPoolDemo implements Runnable{

    private int i = 0;
    
    @Override
    public void run() {
        while(i <= 100){
            System.out.println(Thread.currentThread().getName() + " : " + i++);
        }
    }
}