spring Creating and using beans Autowiring specific bean instances with @Qualifier
Example
If you've got multiple implementations of the same interface, Spring needs to know which one it should autowire into a class. I'm going to use a Validator pattern in this example.1
Foo Class:
public class Foo {
private String name;
private String emailAddress;
private String errorMessage;
/** Getters & Setters omitted **/
}
Interface:
public interface FooValidator {
public Foo validate(Foo foo);
}
Name Validator Class:
@Component(value="FooNameValidator")
public class FooNameValidator implements FooValidator {
@Override
public Foo validate(Foo foo) {
//Validation logic goes here.
}
}
Email Validator Class:
@Component(value="FooEmailValidator")
public class FooEmailValidator implements FooValidator {
@Override
public Foo validate(Foo foo) {
//Different validation logic goes here.
}
}
You can now autowire these validators individually into a class.
Interface:
public interface FooService {
public void handleFoo(Foo foo);
}
Class:
@Service
public class FooServiceImpl implements FooService {
/** Autowire validators individually **/
@Autowired
/*
* Notice how the String value here matches the value
* on the @Component annotation? That's how Spring knows which
* instance to autowire.
*/
@Qualifier("FooNameValidator")
private FooValidator nameValidator;
@Autowired
@Qualifier("FooEmailValidator")
private FooValidator emailValidator;
@Override
public void handleFoo(Foo foo) {
/**You can use just one instance if you need**/
foo = nameValidator.validate(foo);
}
}
It's worth noting that if you have more than one implementation of an interface in the Spring IoC container and don't specify which one you want to use with the @Qualifier annotation, Spring will throw an exception when trying to start, because it won't know which instance to use.
1: This is not the right way to do such simple validations. This is a simple example about autowiring. If you want an idea of a much easier validation method look up how Spring does validation with annotations.
