Observer in Java: choosing and reviewing in-process listener contracts for ordering, errors, threads, registration lifetime, reentrancy and notification outside locks. Use when adding listeners, investigating retained listeners or missed callbacks, or assessing a move from local notifications to a broker. Covers migration contract changes; detailed transaction/outbox design belongs to event-driven-architecture, broker guarantees to delivery-semantics, demand protocols to reactive-backpressure, and hub coordination to gof-mediator.
