What methods are used to implement dependency injection in application components?

Dependency Injection (DI) is a way of passing dependencies (objects, services) into application components instead of creating them inside the component.

Main methods of dependency injection:

1. Through constructor — dependencies are passed via constructor parameters. This is the preferred method because dependencies become mandatory and immutable.

public class Service {
    private final Repository repository;

    public Service(Repository repository) {
        this.repository = repository;
    }
}

2. Through setters (setter methods) — dependencies are passed through special methods after the object is created.

public class Service {
    private Repository repository;

    public void setRepository(Repository repository) {
        this.repository = repository;
    }
}

3. Through fields (field injection) — dependencies are injected directly into class fields, usually with annotations (e.g., @Autowired in Spring).

public class Service {
    @Autowired
    private Repository repository;
}

Each method has its pros and cons; the choice depends on the context and the framework used.