Class, field, and method level annotations for describing thread-safety policies.

Three class-level annotations describe the intended thread-safety promises of a class: `@Immutable`, `@ThreadSafe`, and `@NotThreadSafe`. `@Immutable` means that the class is immutable, and implies `@ThreadSafe`. `@NotThreadSafe` is optional; if 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 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.

## Field and method annotations

The class-level annotations above are part of the public documentation for the class. Other aspects of a class's thread-safety strategy are entirely for maintainers and are not part of its public documentation.

Classes that use locking should document which state variables are guarded with which locks, and which locks are used to guard those variables. A common source of inadvertent non-thread-safety is when a thread-safe class consistently uses locking to guard its state, but is later modified to add either new state variables that are not adequately guarded by locking, or new methods that do not use locking properly to guard the existing state variables. Documenting which variables are guarded by which locks can help prevent both types of omissions.

The `@GuardedBy(lock)` annotation documents that a field or method should be accessed only with a specific lock held.

## Copyright and license

This software is copyright (c) 2005 Brian Goetz and Tim Peierls and is released under the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.5). The official home for this software is http://www.jcip.net. Any republication or derived work distributed in source code form must include the copyright and license notice.