一些注解用来标示一个类是否线程安全

http://www.javaconcurrencyinpractice.com/annotations/doc/

这些注解只有在生成java doc时使用，作用是

These annotations are relatively unintrusive and are beneficial to both users and maintainers. Users can see immediately whether a class is thread-safe, and maintainers can see immediately whether thread-safety guarantees must be preserved. Annotations are also useful to a third constituency: tools. Static code-analysis tools may be able to verify that the code complies with the contract indicated by the annotation, such as verifying that a class annotated with @Immutable actually is immutable.