Auto-wiring a List using util schema gives NoSuchBeanDefinitionException

Question

I have a bean that i want to inject with a named list using Spring util namespace <util:list id="myList"> but Spring is looking for a collection of beans of type String instead. My broken test is:

@RunWith(SpringJUnit4ClassRunner.class) @ContextConfiguration public class ListInjectionTest {      @Autowired @Qualifier("myList") private List<String> stringList;      @Test public void testNotNull() {         TestCase.assertNotNull("stringList not null", stringList);     } } 

My context is:

<beans xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"    xmlns:util="http://www.springframework.org/schema/util"    xmlns="http://www.springframework.org/schema/beans"    xmlns:context="http://www.springframework.org/schema/context"    xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-2.5.xsd    http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util-2.5.xsd     http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-2.5.xsd">     <util:list id="myList">        <value>foo</value>        <value>bar</value>    </util:list>  </beans> 

But I get

Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: 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.Autowired(required=true), @org.springframework.beans.factory.annotation.Qualifier(value=myList)}     at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoSuchBeanDefinitionException(DefaultListableBeanFactory.java:726)     at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:571)     at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:412) 

Which puzzles me rather as I figured this would be the way it was expected to work.

Answer 1

This is due to a rather obscure part of @Autowired's behaviour, specified in 3.11.2. @Autowired:

It is also possible to provide all beans of a particular type from the ApplicationContext by adding the annotation to a field or method that expects an array of that type...

The same applies for typed collections...

In other words, by saying @Autowired @Qualifier("myList") List<String>, you're actually asking for "give me the list of all beans of type java.lang.String that have the qualifier "myList".

The solution is mentioned in 3.11.3. Fine-tuning annotation-based autowiring with qualifiers:

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, prefer 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 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.

So use this in your test, and it works fine:

@Resource(name="myList") private List<String> stringList;