gh-giuseppe-trisciuoglio-developer-kit-developer-kit/skills/spring-boot-dependency-injection/references/spring-official-dependency-injection.md

Spring Framework Official Guidance: Dependency Injection (Clean Excerpt)

Source: https://docs.spring.io/spring-framework/reference/core/beans/dependencies/factory-collaborators.html (retrieved via u2m -v on current date).

Key Highlights

• Emphasize constructor-based dependency injection to make collaborators explicit and enable immutable design.
• Use setter injection only for optional dependencies or when a dependency can change after initialization.
• Field injection is supported but discouraged because it hides dependencies and complicates testing.
• The IoC container resolves constructor arguments by type, name, and order; prefer unique types or qualify arguments with @Qualifier or XML attributes when ambiguity exists.
• Static factory methods behave like constructors for dependency injection and can receive collaborators through arguments.

Constructor-Based DI

public class SimpleMovieLister {
    private final MovieFinder movieFinder;

    public SimpleMovieLister(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }
}

• The container selects the matching constructor and provides dependencies by type.
• When argument types are ambiguous, specify indexes (@ConstructorProperties, XML index attribute) or qualifiers.

Setter-Based DI

public class SimpleMovieLister {
    private MovieFinder movieFinder;

    @Autowired
    public void setMovieFinder(MovieFinder movieFinder) {
        this.movieFinder = movieFinder;
    }
}

• Invoke only when a collaborator is optional or changeable.
• Use @Autowired(required = false) or ObjectProvider<T> to guard optional collaborators.

Reference Snippets

<bean id="exampleBean" class="examples.ExampleBean">
    <constructor-arg ref="anotherExampleBean"/>
    <constructor-arg ref="yetAnotherBean"/>
    <constructor-arg value="1"/>
</bean>

<bean id="exampleBean" class="examples.ExampleBean">
    <property name="beanOne" ref="anotherExampleBean"/>
    <property name="beanTwo" ref="yetAnotherBean"/>
</bean>

• Spring treats constructor-arg entries as positional parameters unless index or type is provided.
• Setter injection uses <property> elements mapped by name.

Additional Notes

• Combine configuration classes with @Import to wire dependencies declared in different modules.
• Lazy initialization (@Lazy) delays bean creation but defers error detection; prefer eager initialization unless startup time is critical.
• Profiles (@Profile) activate different wiring scenarios per environment (for example, @Profile("test")).
• Testing support allows constructor injection in production code while wiring mocks manually (no container required) or relying on the TestContext framework for integration tests.