JAVA并发任务中止的isinterrupted,interupted()

将一个任务的不同步骤划分成子任务放到不同的线程里，这样实现的并行化并不是一个很好的办法。这些异质的子任务之间有协作关系，需要有效的机制 同步——而额外的同步会对性能有一定影响，更重要的是会影响scalability。一个更好的办法是将任务划分成多个独立的同质子任务，这样子任务可以在不需要 同步的情况下并行执行，性能提升的效果更明显，scalability也更强。

2.4. 任务的取消与终止

• Java中没有安全的终止一个线程的方法，因此必须采用协作的方式来实现。

There is no safe way to preemptively stop a thread in Java, and therefore no safe way to preemptively stop a task. There are only cooperative mechanisms, by which the task and the code requesting cancellation follow an agreed-upon protocol.

• 再抓InterruptedException的时候要三思，要想好该做些什么，因为：

Thread interruption is a cooperative mechanism for a thread to signal another thread that it should, at its convenience and if it feels like it, stop what it is doing and do something else.

• isInterrupted方法返回线程当前的interrupted状态；interrupted方法清除并返回interrupted状态。
• 如果打断发生时线程阻塞在sleep，wait之类的方法中，那它们会重置interrupted状态并抛出InterruptedException。这意味着在抓InterruptedException的时候调用isInterrupted会返回false。
• 如果打断发生在线程正常运行时，那打断只会将interrupted标志改成true，线程需要自己检查并清除这个标志。
• 如果在进入阻塞方法之前线程就已经被打断并且interrupted标志未被重置，那这些阻塞方法会立刻抛出InterruptedException。见如下例子：

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public class InterruptBeforeSleep {     public static void main(String[] args) {         Runnable task = new Runnable() {                           public void run() {                 System.out.println("Running task");                 double dummy = 0.11111;                 for (double i = 2.0; i < 100000000.0; i++) {                     dummy *= (i / (i-1));                 }                 System.out.println("Result is: " + dummy);                 System.out.println("About to sleep");                 try {                     Thread.sleep(SECONDS.toMillis(20));                 } catch (InterruptedException e) {                     System.out.println("I'm interrupted");                     // The interrupt status is cleared when Thread.sleep throws InterruptedException                     System.out.println("I'm interrupted? " + Thread.currentThread().isInterrupted());                 }             }         };                   Thread t = new Thread(task);         t.start();         t.interrupt();         System.out.println("Main interrupted the task thread");     } }

这个程序的输出：

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Main interrupted the task thread Running task Result is: 1.1110999894874712E7 About to sleep I'm interrupted I'm interrupted? false

• Because each thread has its own interruption policy, you should not interrupt a thread unless you know what interruption means to that thread.