ExecutorService对象的shutdown()和shutdownNow()的区别

可以关闭 ExecutorService，这将导致其拒绝新任务。提供两个方法来关闭 ExecutorService。shutdown() 方法在终止前允许执行以前提交的任务，而 shutdownNow() 方法阻止等待任务启动并试图停止当前正在执行的任务。在终止时，执行程序没有任务在执行，也没有任务在等待执行，并且无法提交新任务。应该关闭未使用的 ExecutorService 以允许回收其资源。 

下列方法分两个阶段关闭 ExecutorService。第一阶段调用 shutdown 拒绝传入任务，然后调用 shutdownNow（如有必要）取消所有遗留的任务： 

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1. void shutdownAndAwaitTermination(ExecutorService pool) {  
2.   pool.shutdown(); // Disable new tasks from being submitted  
3.   try {  
4.     // Wait a while for existing tasks to terminate  
5.     if (!pool.awaitTermination(60, TimeUnit.SECONDS)) {  
6.       pool.shutdownNow(); // Cancel currently executing tasks  
7.       // Wait a while for tasks to respond to being cancelled  
8.       if (!pool.awaitTermination(60, TimeUnit.SECONDS))  
9.           System.err.println(”Pool did not terminate”);  
10.     }  
11.   } catch (InterruptedException ie) {  
12.     // (Re-)Cancel if current thread also interrupted  
13.     pool.shutdownNow();  
14.     // Preserve interrupt status  
15.     Thread.currentThread().interrupt();  
16.   }  
17. }  

 void shutdownAndAwaitTermination(ExecutorService pool) {
   pool.shutdown(); // Disable new tasks from being submitted
   try {
     // Wait a while for existing tasks to terminate
     if (!pool.awaitTermination(60, TimeUnit.SECONDS)) {
       pool.shutdownNow(); // Cancel currently executing tasks
       // Wait a while for tasks to respond to being cancelled
       if (!pool.awaitTermination(60, TimeUnit.SECONDS))
           System.err.println("Pool did not terminate");
     }
   } catch (InterruptedException ie) {
     // (Re-)Cancel if current thread also interrupted
     pool.shutdownNow();
     // Preserve interrupt status
     Thread.currentThread().interrupt();
   }
 }

shutdown调用后，不可以再submit新的task，已经submit的将继续执行。

shutdownNow试图停止当前正执行的task，并返回尚未执行的task的list