Blog of Silvio Wangler Thoughts about software and stuff…

Spring Framework – Understand @Autowired

Yesterday I have learned that I did not correctly understand how Spring 3.0.5 (and below) works with @Autowired annotations. Imagine we define a Spring bean using the following annotations and additionally have two lists as Spring beans defined.

@Component
public class MightyValidator {
   @Autowired
   private List<String> validationRegexes;

   @Autowired
   private List<String> otherMightyStuff;

   // ... some more mighty code ....
}

<beans>
  <util:list id="validationRegexes">
    <value>1</value>
  </util:list>

  <util:list id="otherMightyStuff">
    <value>1</value>
  </util:list>
</beans>

My previous understanding of @Autowired was that it injects the depending Spring bean by analyzing its type and the field name. And then I run into this given example that my @Component class defined several fields with the same datatype (according to this example java.util.List<String>). I ended up debugging the whole stuff and found out that the Spring bean validationRegexes was injected into both List<String> fields of MightyValidator. So what is going on?

Obviously @Autowired is handled in a different way than I understud. Its more like

Hey I found a List<String> field that has to get auto wired. I will consult my application context and try to find a bean of type List<String>. If I find one I will inject it to the target bean. Done and delivered!

So how to solve that issue? There are the following possibilities to solve this problem.

• If you dependency is not optional use @Resource
• If you like to stay flexible or if you have to mark a dependency as optional stay with @Autowired but additionally use @Qualifier

Solution A - Introduce @Resource
@Component
public class MightyValidator {
   @Resource("validationRegexes")
   private List<String> validationRegexes;

   @Resource("otherMightyStuff")
   private List<String> otherMightyStuff;

   // ... some more mighty code ....
}

Solution B - Add @Qualifier
@Component
public class MightyValidator {
   @Autowired
   @Qualifier("validationRegexes")
   private List<String> validationRegexes;

   @Autowired
   @Qualifier("otherMightyStuff")
   private List<String> otherMightyStuff;

   // ... some more mighty code ....
}

<beans>
<bean id="validationRegexes" class="org.springframework.beans.factory.config.ListFactoryBean">
 <qualifier name="validationRegexes"/>
<property name="sourceList">
      <list>
        <value>1</value>
      </list>
  </property>
</bean>
<bean id="otherMightyStuff" class="org.springframework.beans.factory.config.ListFactoryBean">
 <qualifier name="otherMightyStuff"/>
<property name="sourceList">
      <list>
        <value>1</value>
      </list>
  </property>
</bean>
</beans>


      

        

        
        

      
    
      

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Tagged as: Java, Spring
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