一、线程池：提供了一个线程队列，队列中保存着所有等待状态的线程。避免了创建与销毁额外开销，提高了响应的速度。

     二、线程池的体系结构：       java.util.concurrent.Executor : 负责线程的使用与调度的根接口           |--ExecutorService 子接口: 线程池的主要接口               |--ThreadPoolExecutor 线程池的实现类               |--ScheduledExecutorService 子接口：负责线程的调度                   |--ScheduledThreadPoolExecutor ：继承 ThreadPoolExecutor， 实现 ScheduledExecutorService      三、工具类 : Executors    ExecutorService newFixedThreadPool() : 创建固定大小的线程池   ExecutorService newCachedThreadPool() : 缓存线程池，线程池的数量不固定，可以根据需求自动的更改数量。   ExecutorService newSingleThreadExecutor() : 创建单个线程池。线程池中只有一个线程      ScheduledExecutorService newScheduledThreadPool() : 创建固定大小的线程，可以延迟或定时的执行任务。    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. 创建线程池        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>(){

                                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. 为线程池中的线程分配任务        for (int i = 0; i < 10; i++) {

            pool.submit(tpd);        }                //3. 关闭线程池        pool.shutdown();*/    }     //    new Thread(tpd).start(); //    new Thread(tpd).start(); } class ThreadPoolDemo implements Runnable{     private int i = 0;            public void run() {

        while(i <= 100){

            System.out.println(Thread.currentThread().getName() + " : " + i++);        }    } }