Annotation used to indicate that the annotated method is inherently blocking and so should not be executed on a non-blockable thread (I/O thread, event loops...).

Frameworks can add support for this annotation and offload the work to another thread if the current thread cannot be blocked. It's particularly useful for frameworks using a reactive execution model. Framework relying on this annotation must specify the exact behavior:

• what thread is considered non-blockable;
• on which thread is the execution offloaded;
• whether, when the current thread can block, the execution of the annotated method is still offloaded to another thread, or stays on the same thread;
• if the execution of the method is offloaded, whether the initial thread is restored after the method execution.

When this annotation is used on a class, all the methods declared by the annotated class are considered blocking.

This annotation is not inheritable, so the user must repeat the annotation when sub-classing the class or overriding the method.