java how - @Resource vs @Autowired




example @qualifier (10)

Which annotation, @Resource (jsr250) or @Autowired (Spring-specific) should I use in DI?

I have successfully used both in the past, @Resource(name="blah") and @Autowired @Qualifier("blah")

My instinct is to stick with the @Resource tag since it's been ratified by the jsr people.
Anyone has strong thoughts on this?


Answers

This is what I got from the Spring 3.0.x Reference Manual :-

Tip

If you intend to express annotation-driven injection by name, do not primarily use @Autowired, even if is technically capable of referring to a bean name through @Qualifier values. Instead, use the JSR-250 @Resource annotation, which is semantically defined to identify a specific target component by its unique name, with the declared type being irrelevant for the matching process.

As a specific consequence of this semantic difference, beans that are themselves defined as a collection or map type cannot be injected through @Autowired, because type matching is not properly applicable to them. Use @Resource for such beans, referring to the specific collection or map bean by unique name.

@Autowired applies to fields, constructors, and multi-argument methods, allowing for narrowing through qualifier annotations at the parameter level. By contrast, @Resource is supported only for fields and bean property setter methods with a single argument. As a consequence, stick with qualifiers if your injection target is a constructor or a multi-argument method.


When you analyze critically from the base classes of these two annotations.You will realize the following differences.

@Autowired uses AutowiredAnnotationBeanPostProcessor to inject dependencies.
@Resource uses CommonAnnotationBeanPostProcessor to inject dependencies.

Even though they use different post processor classes they all behave nearly identically. The differences critically lie in their execution paths, which I have highlighted below.

@Autowired / @Inject

1.Matches by Type
2.Restricts by Qualifiers
3.Matches by Name

@Resource

1.Matches by Name
2.Matches by Type
3.Restricts by Qualifiers (ignored if match is found by name)


With @Resource you can do bean self-injection, it might be needed in order to run all extra logic added by bean post processors like transactional or security related stuff.

With Spring 4.3+ @Autowired is also capable of doing this.


@Autowired + @Qualifier will work only with spring DI, if you want to use some other DI in future @Resource is good option.

other difference which I found very significant is @Qualifier does not support dynamic bean wiring, as @Qualifier does not support placeholder, while @Resource does it very well.

For example: if you have an interface with multiple implementations like this

interface parent {

}
@Service("actualService")
class ActualService implements parent{

}
@Service("stubbedService")
class SubbedService implements parent{

}

with @Autowired & @Qualifier you need to set specific child implementation like

@Autowired
@Qualifier("actualService") or 
@Qualifier("stubbedService") 
Parent object;

which does not provide placeholder while with @Resource you can put placeholder and use property file to inject specific child implementation like

@Resource(name="${service.name}")
Parent object;  

where service.name is set in property file as

#service.name=actualService
 service.name=stubbedService

Hope that helps someone :)


@Resource is often used by high-level objects, defined via JNDI. @Autowired or @Inject will be used by more common beans.

As far as I know, it's not a specification, nor even a convention. It's more the logical way standard code will use these annotations.


In spring pre-3.0 it doesn't matter which one.

In spring 3.0 there's support for the standard (JSR-330) annotation @javax.inject.Inject - use it, with a combination of @Qualifier. Note that spring now also supports the @javax.inject.Qualifier meta-annotation:

@Qualifier
@Retention(RUNTIME)
public @interface YourQualifier {}

So you can have

<bean class="com.pkg.SomeBean">
   <qualifier type="YourQualifier"/>
</bean>

or

@YourQualifier
@Component
public class SomeBean implements Foo { .. }

And then:

@Inject @YourQualifier private Foo foo;

This makes less use of String-names, which can be misspelled and are harder to maintain.


As for the original question: both, without specifying any attributes of the annotation, perform injection by type. The difference is:

  • @Resource allows you to specify a name of the injected bean
  • @Autowired allows you to mark it as non-mandatory.

The primary difference is, @Autowired is a spring annotation. Whereas @Resource is specified by the JSR-250, as you pointed out yourself. So the latter is part of Java whereas the former is Spring specific.

Hence, you are right in suggesting that, in a sense. I found folks use @Autowired with @Qualifier because it is more powerful. Moving from some framework to some other is considered very unlikely, if not myth, especially in the case of Spring.


As a note here: SpringBeanAutowiringSupport.processInjectionBasedOnCurrentContext and SpringBeanAutowiringSupport.processInjectionBasedOnServletContext DOES NOT work with @Resource annotation. So, there are difference.


Both @Autowired (or @Inject) and @Resource work equally well. But there is a conceptual difference or a difference in the meaning

  • @Resource means get me a known resource by name. The name is extracted from the name of the annotated setter or field, or it is taken from the name-Parameter.
  • @Inject or @Autowired try to wire in a suitable other component by type.

So, basically these are two quite distinct concepts. Unfortunately the Spring-Implementation of @Resource has a built-in fallback, which kicks in when resolution by-name fails. In this case, it falls back to the @Autowired-kind resolution by-type. While this fallback is convenient, IMHO it causes a lot of confusion, because people are unaware of the conceptual difference and tend to use @Resource for type-based autowiring.


Actually, you should use either JVM managed Objects or Spring-managed Object to invoke methods. from your above code in your controller class, you are creating a new object to call your service class which has an auto-wired object.

MileageFeeCalculator calc = new MileageFeeCalculator();

so it won't work that way.

The solution makes this MileageFeeCalculator as an auto-wired object in the Controller itself.

Change your Controller class like below.

@Controller
public class MileageFeeController {

    @Autowired
    MileageFeeCalculator calc;  

    @RequestMapping("/mileage/{miles}")
    @ResponseBody
    public float mileageFee(@PathVariable int miles) {
        return calc.mileageCharge(miles);
    }
}




java spring dependency-injection annotations autowired