Annotations should always be immutable.

Static state is dangerous to begin with, but much worse for annotations. Annotation instances are usually constants, and it is very surprising if their state ever changes, or if they are not thread-safe.

TIP: prefer [@AutoAnnotation] to writing annotation implementations by hand.

To make annotation implementations immutable, ensure:

• All fields are final.
• The types of all fields are deeply immutable. For example, use ImmutableList and ImmutableSet instead of List and Set.
• To signal to Error Prone that the type of a field is immutable, add com.google.errorprone.annotations.Immutable to its declaration.
• If the implementation is a member class, ensure it is static to avoid capturing state from a mutable enclosing instance. If it is an anonymous class, convert it to a named member class so it can be made static.

TIP: annotating the declaration of an annotation with @Immutable is unnecessary -- Error Prone assumes annotations are immutable by default.