How Does Spring's @Autowired Annotation Handle Dependency Injection?

Understanding Spring @Autowired Usage

The @Autowired annotation eliminates the need for manual wiring in XML configurations. It automates the injection of dependencies based on type matching.

Autowired Examples

Let's revisit the example classes:

public class SimpleMovieLister {

    private MovieFinder movieFinder;

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

public class MovieRecommender {

    private MovieCatalog movieCatalog;

    private CustomerPreferenceDao customerPreferenceDao;

    @Autowired
    public void prepare(MovieCatalog movieCatalog,
                    CustomerPreferenceDao customerPreferenceDao) {
        this.movieCatalog = movieCatalog;
        this.customerPreferenceDao = customerPreferenceDao;
    }
}

How Spring Autowires Multiple Implementing Beans

In the @Autowired declaration, Spring uses the parameter type (MovieCatalog and CustomerPreferenceDao) to search for matching beans. If multiple matching beans are found, it raises an exception.

Handling Beans of the Same Type

If your application has multiple beans implementing the same interface, you can use the @Qualifier annotation to specify which bean should be injected.

@Qualifier("redBean")
class Red implements Color {
    // Class code
}

@Qualifier("blueBean")
class Blue implements Color {
    // Class code
}

// Main Class
@Autowired
@Qualifier("redBean")
private Color color;

Alternatively, you can use @Resource to combine @Autowired and @Qualifier into a single annotation:

@Resource(name="redBean")
private Color color;

Best Practices

Consider using @Inject (part of JSR-330) instead of @Autowired for portability. Additionally, it's recommended to annotate constructors rather than methods for better exception handling.

Conclusion

@Autowired streamlines dependency injection by automating the bean wiring process, ensuring that the required dependencies are injected where they are needed. By employing best practices, you can ensure efficient and reliable autowiring in your Spring applications.

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