线程池-ExecutorService

先创建固定的线程放到一个池子里，当有任务需要处理的时候就从池子里面拿，处理完就把线程放回池子里面，不进行销毁，减少资源的浪费

一：例子

创建一个只有三个线程的线程池，往池子里面添加5个任务，每个任务循环10次

public class ThreadPool {

    public static void main(String[] args) {
        ExecutorService threadPool = Executors.newFixedThreadPool(3);
        for (int i = 1; i < 6; i++) {
            final int task=i;
            threadPool.execute(
                    new Runnable() {
                        @Override
                        public void run() {
                            for (int j = 0; j < 10; j++) {
                                System.out.println(Thread.currentThread().getName() + " look for " + j+" ;task is "+task);
                            }
                        }
                    }
            );
        }
    }
}

执行结果

pool-1-thread-1 look for 0 ;task is 1                                             
pool-1-thread-1 look for 1 ;task is 1                   
pool-1-thread-2 look for 0 ;task is 2
pool-1-thread-1 look for 2 ;task is 1                   
pool-1-thread-3 look for 0 ;task is 3
pool-1-thread-1 look for 3 ;task is 1
pool-1-thread-2 look for 1 ;task is 2
pool-1-thread-1 look for 4 ;task is 1
pool-1-thread-3 look for 1 ;task is 3
pool-1-thread-1 look for 5 ;task is 1
pool-1-thread-2 look for 2 ;task is 2
pool-1-thread-1 look for 6 ;task is 1
pool-1-thread-3 look for 2 ;task is 3
pool-1-thread-1 look for 7 ;task is 1
pool-1-thread-2 look for 3 ;task is 2
pool-1-thread-1 look for 8 ;task is 1
pool-1-thread-3 look for 3 ;task is 3
pool-1-thread-1 look for 9 ;task is 1
pool-1-thread-2 look for 4 ;task is 2
pool-1-thread-2 look for 5 ;task is 2
pool-1-thread-3 look for 4 ;task is 3
pool-1-thread-2 look for 6 ;task is 2
pool-1-thread-1 look for 0 ;task is 4
pool-1-thread-2 look for 7 ;task is 2
pool-1-thread-3 look for 5 ;task is 3
pool-1-thread-2 look for 8 ;task is 2
pool-1-thread-1 look for 1 ;task is 4
pool-1-thread-2 look for 9 ;task is 2
pool-1-thread-3 look for 6 ;task is 3
pool-1-thread-2 look for 0 ;task is 5
pool-1-thread-1 look for 2 ;task is 4
pool-1-thread-2 look for 1 ;task is 5
pool-1-thread-3 look for 7 ;task is 3
pool-1-thread-2 look for 2 ;task is 5
pool-1-thread-1 look for 3 ;task is 4
pool-1-thread-2 look for 3 ;task is 5
pool-1-thread-3 look for 8 ;task is 3
pool-1-thread-2 look for 4 ;task is 5
pool-1-thread-1 look for 4 ;task is 4
pool-1-thread-2 look for 5 ;task is 5
pool-1-thread-3 look for 9 ;task is 3
pool-1-thread-2 look for 6 ;task is 5
pool-1-thread-2 look for 7 ;task is 5
pool-1-thread-2 look for 8 ;task is 5
pool-1-thread-2 look for 9 ;task is 5
pool-1-thread-1 look for 5 ;task is 4
pool-1-thread-1 look for 6 ;task is 4
pool-1-thread-1 look for 7 ;task is 4
pool-1-thread-1 look for 8 ;task is 4
pool-1-thread-1 look for 9 ;task is 4

可以task1,taks2,task,会先循环10次，只有当task1.task2,task3中有一个工作完成之后，task4才会开始，也就是说始终只有三个线程（看线程名称，只有thread-1，thread-2，thread-3）在工作，有先任务来的时候就会先进行等待

Executors.newFixedThreadPool：   创建固定大小线程数的池子

Executors.newCachedThreadPool： 来几个任务就创建几个线程，如果任务变少了，过段时间就把超时的任务收回

Executors.newSingleThreadExecutor：创建唯一线程，就是单线程，好处就是时刻保存有一个现场，假设创建的现场死了，会立刻创建出一个

Executors.newScheduledThreadPool(3).schedule()：创建定时任务的线程池，定时执行

CompletionService：执行一组Callable对象，那个先执行完成就先获取到结果，可通过take方法获取；

二：execute 与submit的区别

execute 不返回结果，submit返回执行结果