Autowiring a collection via the constructor with Spring

Question

I have what seems to be a simple problem, as stated in the title. Here is the kind of class I have :

public class Foo {
    @Autowired
    public Foo(@Qualifier("bar") Set<String> bar) {
        // ...
    }
}

Which I try to run with the following spring context :

<context:annotation-config />
<util:set id="bar">
    <value>tata</value>
    <value>titi</value>
    <value>toto</value>
</util:set>
<bean id="foo" class="Foo" />

This fails to run with :

No matching bean of type [java.lang.String] found for dependency [collection of java.lang.String]: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {@org.springframework.beans.factory.annotation.Qualifier(value=bar)}

Note that if I add other parameters to my constructor, it works fine. If I use setter injection, it works fine. I'm sure I miss something obvious ... do you know what ?

Answer 1

I think this is because Spring interprets the autowiring of a collection as "give me all beans of type String", rather than "give me the bean which is a collection of String". The error message supports that idea.

I don't think you can use autowiring for this. Short of manually wiring it up in the XML, the best I can suggest is:

public class Foo {  
   private @Resource Set<String> bar;
}

Answer 2

Autowiring collections is not possible using the @Autowired annotation. An autowired collection means "to provide all beans of a particular type". Using the JSR-250 @Resource annotation, you can declare that you want a resource injected by its name, not its type. Or you inject the dependency explicitly.

[...] beans which are themselves defined as a collection or map type cannot be injected via @Autowired since type matching is not properly applicable to them. Use @Resource for such beans, referring to the specific collection/map bean by unique name.

See the Spring documentation for more details.

Answer 3

As others stated it is impossible to use @Autowired for Strings and collections of String. You can use @Value with spring EL here assuming you have spring in version 3:

public class Foo {
    public Foo(@Value("#{bar}") Set<String> bar) {
        // ...
    }
}