Self injection with Spring

Question

I tried the following code with Spring 3.x which failed with BeanNotFoundException and it should according to the answers of a question which I asked before - Can I inject same class using Spring?

@Service
public class UserService implements Service{
    @Autowired
    private Service self;
}

Since I was trying this with Java 6, I found the following code works fine:

@Service(value = "someService")
public class UserService implements Service{
    @Resource(name = "someService")
    private Service self;
}

but I don't understand how it resolves the cyclic dependency.

EDIT:
Here's the error message. The OP mentioned it in a comment on one of the answers:

Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No matching bean of type [com.spring.service.Service] found for dependency: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)}

Answer 1

Update: February 2016

Self autowiring will be officially supported in Spring Framework 4.3. The implementation can be seen in this GitHub commit.

---

The definitive reason that you cannot autowire yourself is that the implementation of Spring's DefaultListableBeanFactory.findAutowireCandidates(String, Class, DependencyDescriptor) method explicitly excludes the possibility. This is visible in the following code excerpt from this method:

for (String candidateName : candidateNames) {
    if (!candidateName.equals(beanName) && isAutowireCandidate(candidateName, descriptor)) {
        result.put(candidateName, getBean(candidateName));
    }
}

FYI: the name of the bean (i.e., the bean that's trying to autowire itself) is beanName. That bean is in fact an autowire candidate, but the above if-condition returns false (since candidateName in fact equals the beanName). Thus you simply cannot autowire a bean with itself (at least not as of Spring 3.1 M1).

Now as for whether or not this is intended behavior semantically speaking, that's another question. ;)

I'll ask Juergen and see what he has to say.

Regards,

Sam (Core Spring Committer)

p.s. I've opened a Spring JIRA issue to consider supporting self-autowiring by type using @Autowired. Feel free to watch or vote for this issue here: https://jira.springsource.org/browse/SPR-8450

Answer 2

This code works too:

@Service
public class UserService implements Service {

    @Autowired
    private ApplicationContext applicationContext;

    private Service self;

    @PostConstruct
    private void init() {
        self = applicationContext.getBean(UserService.class);
    }
}

I don't know why, but it seems that Spring can get the bean from ApplicationContext if is created, but not initialized. @Autowired works before initialization and it cannot find the same bean. So, @Resource maybe works after @Autowired and before @PostConstruct.

But I don't know, just speculating. Anyway, good question.