Spring : context:component-scan vs context:annotation-config

Whats the difference between component-scan and annotation-config

• <context:annotation-config/>  -   Looks for @Autowired and JSR annotation, It does not identify the @component, @configuration annotations.  It just autowires existing beans
  
• whereas <context:component-scan  base-package="org.first" />  Does recoginize @Autowired, @component and other annotations also.
  
• <context:component-scan> also does  <context:annotation-config/>  and also scans packages to find registeredbeans

---

Example  1 :  Let's define the classes in beans.xml and also annotate with @Component and @Autowired

package org.first;

@Component
public class FirstServiceImpl implements FirstService{

    public Name userName;

    public FirstServiceImpl(){
        System.out.println("new FirstServiceImpl()");
    }


    @Autowired
    public void setUserName(final Name userName){
        System.out.println("setUserName(final Name userName)");
        this.userName = userName;
    }



   //getters, setters, other constructors

}

package org.first;

@Component

public class Name{

    String name = "new";

    public Name(){
        super();

        System.out.println("new Name()");
    }

   //getters, setters, other constructors
}

beans.xml

<beans><!-- Add Namesapece here -->

    <bean name="userName" class="org.first.Name">
        <property name="name" value="Vidhya"></property>
    </bean>

    <bean name="firstService" class="org.first.FirstServiceImpl">
        <property name="userName" ref="userName"></property>
    </bean>
</beans>

OUTPUT : This will instantiate the userName and firstService classes, because both classes are defined in the xml

new Name()
new FirstServiceImpl()
setUserName(final Name userName)

Example  2 :  Lets try to use <context-annotation-config> :  

package org.first;
@Component
public class FirstServiceImpl implements FirstService{

    public Name userName;

    public FirstServiceImpl(){
        System.out.println("new FirstServiceImpl()");
    }


    @Autowired
    public void setUserName(final Name userName){
        System.out.println("setUserName(final Name userName)");
        this.userName = userName;
    }



   //getters, setters, other constructors

}


package org.first;

public class Name{
    String name = "new";

    public Name(){
        super();
        System.out.println("new Name()");
    }
   //getters, setters, other constructors
}




beans.xml

 <beans><!-- Add Namesapece here -->
    <context:annotation-config/>
</beans>










OUTPUT : This will NOT do anything. NO OUTPUT. Why? The annotation-config cannot know about the beans unless defined. 

How to fix this? We need to add bean definitions. Lets add the bean definitions in the beans.xml





Example  3 :  Lets use <context-annotation-config> and bring back the bean definitions.



beans.xml

 <beans><!-- Add Namesapece here -->
    <context:annotation-config/>


    <bean name="userName" class="org.first.Name">
        <property name="name" value="Vidhya"></property>
    </bean>

    <bean name="firstService" class="org.first.FirstServiceImpl">
        <property name="userName" ref="userName"></property>
    </bean>
</beans>













OUTPUT : This will instantiate the userName and firstService classes, because both classes are defined in the xml

new Name()
new FirstServiceImpl()
setUserName(final Name userName)








Example  4 :  So, what's the use of annotation-config? We saw that annotation-config cannot load @component, Now lets see if it can autowire, with same class as in Example 2.



beans.xml


 <beans><!-- Add Namesapece here -->
    <context:annotation-config/>


    <bean name="userName" class="org.first.Name">
        <property name="name" value="Vidhya"></property>
    </bean>

    <bean name="firstService" class="org.first.FirstServiceImpl">
        <property name="userName" ref="userName"></property>
    </bean>
</beans>













OUTPUT : 


new Name()
new FirstServiceImpl()
setUserName(final Name userName)





Looks like annotation-config can autowire, but cannot instantiate @Component

Example  5 :  Lets use <context:component-scan> now. As per definition, it can read all@component classes and also autowire. We will check that

package org.first;

@Component
public class FirstServiceImpl implements FirstService{
    public Name userName;


    public FirstServiceImpl(){
        System.out.println("new FirstServiceImpl()");
    }


    @Autowired
    public void setUserName(final Name userName){
        System.out.println("setUserName(final Name userName)");
        this.userName = userName;
    }



   //getters, setters, other constructors

}


package org.first;
@Component
public class Name{

    String name = "new";

    public Name(){
        super();
        System.out.println("new Name()");
    }
   //getters, setters, other constructors
}




beans.xml

 <beans><!-- Add Namespace here -->
     <context:component-scan base-package="org.first" />
</beans>

OUTPUT : There you go.. Now we achieve with single line in beans.xml, which all @Component and @Autowired annotations.













new FirstServiceImpl()

new Name()

setUserName(final Name userName)







SUMMARY:
1. <context:component-scan> is one stop solution for @Autowired and @Component 
2.<context:annotation-config> can only do @Autowired.
3. I am readying the annotation-config is very useful in the web context with DispatcherServlet. But need to try that. Probably will write about it later when I try it.



References
http://docs.spring.io/spring/docs/3.0.0.M3/reference/html/ch04s11.html



http://www.tutorialspoint.com/spring/spring_annotation_based_configuration.htm
http://stackoverflow.com/questions/7414794/difference-between-contextannotation-config-vs-contextcomponent-scan