Why is there a need to specify the class in both the xml file and in the getBean() method in Spring

This might be an obvious but I'm having a hard time understanding why we need to define the class of a bean in two places....

From the spring reference manual... ...

<bean id="petStore"
class="org.springframework.samples.jpetstore.services.PetStoreServiceImpl">
<property name="accountDao" ref="accountDao"/>
<property name="itemDao" ref="itemDao"/>
<!-- additional collaborators and conf开发者_如何转开发iguration for this bean go here -->
</bean>

---

// retrieve configured instance
PetStoreServiceImpl service = context.getBean("petStore", PetStoreServiceImpl.class);

Shouldn't the xml fine be enough for the container to know the class of petStore?

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You can use the following method:

context.getBean("petStore")

However, as this returns a java.lang.Object, you'd still need to have a cast:

PetStoreServiceImpl petstore = (PetStoreServiceImpl)context.getBean("petStore");

However, this could lead to problems if your "petStore" bean is not actually a PetStoreServiceImpl, and to avoid casts (which since the advent of Generics are being seen as a bit dirty), you can use the above method to infer the type (and let's spring check whether the bean you're expecting is really of the right class, so hence you've got:

PetStoreServiceImpl service = context.getBean("petStore", PetStoreServiceImpl.class);

Hope that helps.

EDIT:

Personally, I would avoid calling context.getBean() to lookup methods as it goes against the idea of dependency injection. Really, the component that uses the petstore bean should have a property, which can then be injected with the correct component.

private PetStoreService petStoreService;

// setter omitted for brevity

public void someotherMethod() {
    // no need for calling getBean()
    petStoreService.somePetstoreMethod();
}

Then you can hook up the beans in the application context:

You could also do away with the configuration via XML and use annotation to wire up your beans:

@Autowired
private PetStoreService petStoreService;

As long as you've got

in your spring context, the "petStore" bean defined in your application context will automatically be injected. If you've got more than one bean with the type "PetStoreService", then you'd need to add a qualifier:

@Autowired
@Qualifier("petStore")
private PetStoreService petStoreService;

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There's no requirement to specify the class in the getBean() method. It's just a question of safety. Note there's also a getBean() that takes only a class so that you can just look up beans by type instead of needing to know the name.