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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 volatile
variable, 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:
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。
java线程控制、状态同步、volatile、Thread.interupt以及ConcurrentLinkedQueue