Service layer

Understand the idea

It sits between HTTP handling and storage. It can trim inputs, enforce ownership, coordinate operations, and decide how missing data should be handled. Constructor injection makes its dependencies visible and permits isolated unit tests.

A concrete example

public Todo update(String id, ...) {
    Todo todo = get(id);
    // Change editable fields and save.
}

In the application

TodoService handles shared CRUD. PersonalTodoService requires an authenticated owner for every operation.

@Service identifies a component; it does not automatically apply every validation rule or authorization rule.

See it in a working stage

• Stage 9: Move a product rule into a service

Related concepts

• Controller
• Repository
• Dependency injection
• Authorization / ownership