Java 多线程 ：入门- 简单试验ExecuteorService

Posted 2020-08-01 ajjiangxin

首先，根据其他文章提到的，我也实验了的Runnable跟Callable的差别：

1）Callable接口的call()方法有返回值，这样方便处理“需要将一个对象加工并返回”的需求（Runnable的run()没有返回值，但可以通过回调的方式在run()内部解决）；

2）都使用ExecutorService来submit()时，均返回Future<T>，但前者的future.get()为空，后者可以有内容；

 

 

试验一下Executors.newFixedThreadPool(), 代码如下：

public class testExecutorShutdown{
    public static void main(String[] args) throws InterruptedException, ExecutionException{
        ExecutorService executor = Executors.newFixedThreadPool(10);
        executor.submit(new CallableA(executor));
    }
}

class CallableA implements Callable<Object> {
    
    class newRunnable implements Runnable{
        private int id;
        public newRunnable(int id){
            this.id = id;
        }
        @Override
        public void run() {
            System.out.println("newRunnable:"+id+" executed; Thread hashCode: "+Thread.currentThread().hashCode());
        }
    }
    ExecutorService executor = null;

    public CallableA(ExecutorService executor){
        this.executor = executor;
    }
    
    @Override
    public Object call() {
        for(int i=0; i<100; i++){
            executor.submit(new newRunnable(i));
        }
        return null;
    }
}

结果：

newRunnable:9 executed; Thread hashCode: 777173972
newRunnable:10 executed; Thread hashCode: 777173972
newRunnable:3 executed; Thread hashCode: 622087230
newRunnable:12 executed; Thread hashCode: 622087230
newRunnable:7 executed; Thread hashCode: 348776228
newRunnable:13 executed; Thread hashCode: 622087230
newRunnable:14 executed; Thread hashCode: 622087230
newRunnable:6 executed; Thread hashCode: 948531615
newRunnable:17 executed; Thread hashCode: 948531615
newRunnable:18 executed; Thread hashCode: 948531615
newRunnable:19 executed; Thread hashCode: 948531615
newRunnable:20 executed; Thread hashCode: 948531615 ......

这里只贴了结果的一部分，可以看到newFixedThreadPool会把一个Runnable或Callable任务拆分给最多n个（new的时候定义的）线程来做，且各做一部分，不会重复，但顺序不一定。

 

另外，我尝试了在run()方法中，插入if(i==50){executor.shutdownNow()； System.out.pringln(...); //或shutdown()}, 结果还是有其他的线程的内容打印出来，这里说明了两个问题：

1）任务的拆分是submit()方法(或execute()方法)一开始调用时就做好的，然后没有特定顺序地同时开始，这印证了上一部分的结论，更是由于此，shutdown()和shutdownNow()的执行点if(i==50)其实是没有意义的，因为什么时候i==50的那个newRunnable线程是不一定的，且就算执行了，System.out.println（）方法打印出结果的时刻并不准确代表了所有线程的执行进度，可能shutdownNow()之前很多线程已经开启，只是打印到控制台比较晚，才出现了上述结果；

2）另外，文档给出shutdownNow()方法的解释是“对于已经开始执行的线程，不一定能彻底shutdown”，原文如下，说明这种方法来完全shutdown线程池中的线程是不靠谱的：

shutdownNow
List<Runnable> shutdownNow()

Attempts to stop all actively executing tasks, halts the processing of waiting tasks, and returns a list of the tasks that were awaiting execution. 
This method does not wait for actively executing tasks to terminate. Use awaitTermination to do that. 

There are no guarantees beyond best-effort attempts to stop processing actively executing tasks. For example, typical implementations will cancel via Thread.interrupt(), so any task that fails to respond to interrupts may never terminate.

 

 

写的很差哈，一步步简单理解Executor线程池的作用，未完待续。。。