使用 Spring 进行自注入 [英] Self injection with Spring

问题描述

我在 Spring 3.x 中尝试了以下代码，但失败了 BeanNotFoundException 并且它应该根据我之前问过的一个问题的答案 - 我可以使用 Spring 注入同一个类吗?

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;
}

自从我用 Java 6 尝试这个之后，我发现以下代码工作正常:

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;
}

但我不明白它是如何解决循环依赖的.

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

这是错误消息.OP 在对其中一个答案的评论中提到了这一点:

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

引起:org.springframework.beans.factory.NoSuchBeanDefinitionException:找不到类型为 [com.spring.service.Service] 的匹配 bean 依赖项:预期至少有 1 个 bean 有资格作为此依赖项的自动装配候选者.依赖注解:{@org.springframework.beans.factory.annotation.Autowired(required=true)}

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)}

推荐答案

更新:2016 年 2 月

Self autowiring 将在 Spring Framework 4.3 中得到正式支持.实现可以在这个 GitHub 提交中看到.

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

您不能自己自动装配的明确原因是 Spring 的 DefaultListableBeanFactory.findAutowireCandidates(String, Class, DependencyDescriptor) 方法的实现明确排除了这种可能性.这在此方法的以下代码摘录中可见:

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));
    }
}

仅供参考:bean 的名称(即尝试自动装配自己的 bean)是 beanName.该 bean 实际上是自动装配候选者，但上述 if 条件返回 false(因为 candidateName 实际上等于 beanName).因此，您根本无法将 bean 与自身自动装配在一起(至少从 Spring 3.1 M1 开始不是这样).

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).

至于这是否是语义上的预期行为，这是另一个问题.;)

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

我会问 Juergen，看看他有什么要说的.

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

问候，

Sam(核心 Spring 提交者)

Sam (Core Spring Committer)

附言我已经打开了一个 Spring JIRA 问题，以考虑使用 @Autowired 按类型支持自动装配.请随时在此处观看或投票支持此问题:https://jira.springsource.org/browse/SPR-8450

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

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