We use three class-level annotations to describe a class's intended thread-safety promises: @Immutable, @ThreadSafe, and @NotThreadSafe. @Immutable means, of course, that the class is immutable, and implies @ThreadSafe. @NotThreadSafe is optionalif a class is not annotated as thread-safe, it should be presumed not to be thread-safe, but if you want to make it extra clear, use @NotThreadSafe.
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 codeanalysis 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.