Understanding Spring @Autowired usage
autowired, java, spring, spring-3
Solution
TL;DR
The `@Autowired` annotation spares you the need to do the wiring by yourself in the XML file (or any other way) and just finds for you what needs to be injected where and does that for you.
Full explanation
The `@Autowired` annotation allows you to skip configurations elsewhere of what to inject and just does it for you. Assuming your package is `com.mycompany.movies` you have to put this tag in your XML (application context file):
<context:component-scan base-package="com.mycompany.movies" />
This tag will do an auto-scanning. Assuming each class that has to become a bean is annotated with a correct annotation like `@Component` (for simple bean) or `@Controller` (for a servlet control) or `@Repository` (for `DAO` classes) and these classes are somewhere under the package `com.mycompany.movies`, Spring will find all of these and create a bean for each one. This is done in 2 scans of the classes - the first time it just searches for classes that need to become a bean and maps the injections it needs to be doing, and on the second scan it injects the beans. Of course, you can define your beans in the more traditional XML file or with an `@Configuration` class (or any combination of the three).
The `@Autowired` annotation tells Spring where an injection needs to occur. If you put it on a method `setMovieFinder` it understands (by the prefix `set` + the `@Autowired` annotation) that a bean needs to be injected. In the second scan, Spring searches for a bean of type `MovieFinder`, and if it finds such bean, it injects it to this method. If it finds two such beans you will get an `Exception`. To avoid the `Exception`, you can use the `@Qualifier` annotation and tell it which of the two beans to inject in the following manner:
@Qualifier("redBean")
class Red implements Color {
// Class code here
}
@Qualifier("blueBean")
class Blue implements Color {
// Class code here
}
Or if you prefer to declare the beans in your XML, it would look something like this:
<bean id="redBean" class="com.mycompany.movies.Red"/>
<bean id="blueBean" class="com.mycompany.movies.Blue"/>
In the `@Autowired` declaration, you need to also add the `@Qualifier` to tell which of the two color beans to inject:
@Autowired
@Qualifier("redBean")
public void setColor(Color color) {
this.color = color;
}
If you don't want to use two annotations (the `@Autowired` and `@Qualifier`) you can use `@Resource` to combine these two:
@Resource(name="redBean")
public void setColor(Color color) {
this.color = color;
}
The `@Resource` (you can read some extra data about it in the first comment on this answer) spares you the use of two annotations and instead, you only use one.
I'll just add two more comments:
- Good practice would be to use `@Inject` instead of `@Autowired` because it is not Spring-specific and is part of the `JSR-330` standard.
- Another good practice would be to put the `@Inject` / `@Autowired` on a constructor instead of a method. If you put it on a constructor, you can validate that the injected beans are not null and fail fast when you try to start the application and avoid a `NullPointerException` when you need to actually use the bean.
Update: To complete the picture, I created a new question about the `@Configuration` class.
Problem
I am reading the spring 3.0.x reference documentation to understand Spring Autowired annotation: 3.9.2 @Autowired and @Inject I am not able to understand the below examples. Do we need to do something in the XML for it to work? EXAMPLE 1 ``` public class SimpleMovieLister { private MovieFinder movieFinder; @Autowired public void setMovieFinder(MovieFinder movieFinder) { this.movieFinder = movieFinder; } // ... } ``` EXAMPLE 2 ``` public class MovieRecommender { private MovieCatalog movieCatalog; private CustomerPreferenceDao customerPreferenceDao; @Autowired public void prepare(MovieCatalog movieCatalog, CustomerPreferenceDao customerPreferenceDao) { this.movieCatalog = movieCatalog; this.customerPreferenceDao = customerPreferenceDao; } // ... } ``` How can the two classes be autowired implementing the same interface and using the same class? Example: ``` class Red implements Color class Blue implements Color class myMainClass{ @Autowired private Color color; draw(){ color.design(); } } ``` Which design method will be called? How do I make sure the design method of Red class will be called and not Blue?