What methods can be used to implement dependency injection in Spring?

Dependency injection in Spring can be implemented in the following ways:

1. Through constructor — dependencies are passed via the class constructor parameters. This is the preferred method as it makes dependencies mandatory and simplifies testing.

@Component
public class Service {
    private final Repository repository;

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

2. Through setters (setter injection) — dependencies are set via setter methods annotated with @Autowired.

@Component
public class Service {
    private Repository repository;

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

3. Through fields (field injection) — dependencies are injected directly into class fields using @Autowired.

@Component
public class Service {
    @Autowired
    private Repository repository;
}

Field injection is simpler but less flexible and more difficult to test.

You can also use XML configuration, but in modern projects, annotations and Java configuration are preferred.