@Resource vs @Component

[Rakesh Basani]

Hi guys,

I had a horrible day by searching for the difference betweeen @Resource and @Component.
I know how it works if we use @Resource at field-level and method-level. But, how to use @Resource at class-level. Is it similar to @Component.

Please reply guys.
Thanks in advance.

[Bill Gorder]

@Component is the root stereo type annotation. Such classes are considered as candidates for auto-detection when using annotation-based configuration and classpath scanning. The JSR 330 annotation that is more of a drop in replacement for this is the @Named annotation.

Since most projects I am on are unlikely to ever operate without Spring I prefer to use the subclasses of @Component as they more specifically define the behavior of the class and in some cases (such as @Repository) add additional behavior. There are lots of annotations that extend Component some examples are @Service, @Repository, @MessageEndpoint etc.


For more information you can see the reference documentation
http://static.springsource.org/spring/docs/3.1.x/spring-framework-reference/html/beans.html#beans-standard-annotations

Here is a blog discussing the differences between @Resource, @Inject and @Autowired
http://blogs.sourceallies.com/2011/08/spring-injection-with-resource-and-autowired/

[Rakesh Basani]

Thanks for your quick reply Bill,

The blog you mentioned for @Resource explained only about field level and method level injections. But I desperately need an example on how to use @Resource at class level. That is what my major concern.

[Bill Gorder]

I have never used nor seen @Resource used in place of @Component. Will it work? I don't know you would have to try but my guess is it won't. Read the Spring documentation here:

http://static.springsource.org/spring/docs/3.1.x/spring-framework-reference/html/beans.html#beans-resource-annotation

Its says the following:

Spring also supports injection using the JSR-250 @Resource annotation on fields or bean property setter methods.

This implies that Spring only supports this annotation on fields or setters. Like I said I prefer to use the Spring annotations since the applications I work on will always be using Spring. If you have a really odd scenario where that is not the case or you just really want to use an JSR annotation instead than use @Named instead of @Component as was described in a link in my previous post.

Note the @Resource annotation is also useful for wiring in collections that are defined as @Bean's

Thanks,
Bill

[Rakesh Basani]

Have a look at these links Bill

http://www.java.net/node/664016
http://docs.oracle.com/javaee/6/tutorial/doc/bnaeo.html (topic--> Declaring reference to a resource)

finally look at this example http://webspherepersistence.blogspot.in/

They are discussing about @Resources tag at class level. But I did not get that.. Could you please verify that once.

[Bill Gorder]

Yup and those links have nothing at all to do with Spring. My impression from your original question was you were asking about using @Resource inside a Spring application and you were trying to draw comparisons between @Resource and @Component. As far that question is concerned I think my previous posts addressed it.

Now you are moving out of the realm of Spring and into EJB 3.1. @Resource of course is not a Spring annotation but Spring does support its usage in its own way. Without Spring this annotation would be used in a full JEE container and the class that is annotated would be managed by the container. In this case if the @Resource is applied at the class level a name must be specified and that would result in a JNDI (environment) look-up for the resource. See the java doc here:
http://docs.oracle.com/javase/6/docs/api/javax/annotation/Resource.html