The Command Pattern in Java: Eliminating Fat Service Classes with Commands and Handlers

Fat Service classes that accumulate too much logic are a common anti-pattern in Java applications. The Command pattern offers a clean solution by separating business intent into discrete Command objects and dedicated Handler classes. Each Handler handles exactly one Command, enforcing single responsibility. The pattern involves four layers: a DTO capturing external input, a Command encapsulating domain intent, a Repository interface abstracting persistence, and a Handler containing the actual business logic. The Controller simply maps requests to Commands and delegates to Handlers. Benefits include improved scannability (package structure reflects use cases), easier testability (single-purpose Handlers have minimal dependencies to mock), and safer maintenance (changes are isolated to specific Handlers).