java线程控制、状态同步、volatile、Thread.interupt以及ConcurrentLinkedQueue

在有些严格的系统中，我们需要做到干净的停止线程并清理相关状态。涉及到这个主题会带出很多的相关点，简单的总结如下：

我们知道，在java中，有一个volatile关键字，其官方说明（https://docs.oracle.com/javase/tutorial/essential/concurrency/atomic.html）为：

Using volatile variables reduces the risk of memory consistency errors, because any write to a volatile variable establishes a happens-before relationship with subsequent reads of that same variable. This means that changes to a volatile variable are always visible to other threads. What's more, it also means that when a thread reads a volatilevariable, it sees not just the latest change to the volatile, but also the side effects of the code that led up the change.

基本可以认为，写入volatile变量的值对于其他线程总是可见，一个线程读取到volatile变量时，不仅其值是最新的，其他使用改变了判断的代码也会同时级联发生变化。

不过，volatile的范围仅限于每次从堆读取该值时，进入方法体内后，如果执行的时间较长或者一直处于循环，比如线程的run()方法内，期间对volatile的变更对方法体是不可见的，所以这一点需要注意，如果一定要有效，那就再包装个方法。

如果不想要再加个方法，可以考虑使用ConcurrentLinkedQueue，可以一开始为空，后面轮询判断status.poll() == null的方式实现。

第三，我们知道Thread有stop和interupt这两个终止线程的方法。因为stop已经@Deprecated，所以假设不会使用。再看interupt的副作用，对于很多程序比如log4j来说，其实使用interupt并无副作用，但有些情况下，使用interrupt会导致不必要的系统不一致性，看interrupt的javadoc，如下：

Interrupts this thread.

Unless the current thread is interrupting itself, which is always permitted, the checkAccess method of this thread is invoked, which may cause a SecurityException to be thrown.

If this thread is blocked in an invocation of the wait(), wait(long), or wait(long, int) methods of the Object class, or of the join(), join(long), join(long, int), sleep(long), or sleep(long, int), methods of this class, then its interrupt status will be cleared and it will receive an InterruptedException.

If this thread is blocked in an I/O operation upon an interruptible channel then the channel will be closed, the thread's interrupt status will be set, and the thread will receive a ClosedByInterruptException.

If this thread is blocked in a Selector then the thread's interrupt status will be set and it will return immediately from the selection operation, possibly with a non-zero value, just as if the selector's wakeup method were invoked.

If none of the previous conditions hold then this thread's interrupt status will be set.

Interrupting a thread that is not alive need not have any effect.

所以一般应使用前两种方法。