线程安全

线程安全问题

线程安全产生的原因是多个线程在访问全局变量，共享资源，或者在执行一段代码因为存在对共享资源的访问所导致的竞争。这种情况下将导致错误甚至无法预料的结果。

 

定义

很多人在定义“线程安全”的时候总是感觉很困恼。

 

Brian Göetz在Java Concurrency In Practice中是这样的：

Java Concurrency In Practice 写道

Defining thread safety is surprisingly tricky. The more formal attempts are so complicated as to offer little practical guidance or intuitive understanding, and the rest are informal descriptions that can seem downright circular. A quick Google search turns up numerous "definitions" like these:
...can be called from multiple program threads without unwanted interactions between the threads.
...may be called by more than one thread at a time without requiring any other action on the caller's part.
Given definitions like these,it's no wonder we find thread safety confusing! They sound suspiciously like "a class is thread-safe if it can be used safely from multiple threads." You can't really argue with such a statement, but it doesn't offer much practical help either. How do we tell a thread-safe class from an unsafe one? What do we even mean by "safe"?

 

 

Java Concurrency In Practice 写道

A class is thread safe if it behaves correctly when accessed from multiple threads, regardless of the scheduling or interleaving of the execution of those threads by the runtime environment, and with no additional synchronization or other coordination on the part of the calling code.

 

线程安全：

 

多个线程在访问一个对象或执行一个过程时能无状态的，或有状态时进行同步的、或有序的、或隔离的执行，返回正确结果。

 

或者更简单的，多个线程可以同步有序的访问并返回正确结果。