为什么Spring可以“自己注入自己”
引言
在相同Bean中使用@Transactional时一个很常见的场景，程序员基本也都知道使用自我注入（Self-Injection）的方式去解决，这个本身没什么难度。
本文只是想考据一下Spring Framework，具体是哪里的代码支持了自我注入。
同类调用中@Transactional的失效问题
我们偶尔会在想调用同一个类的方法时，对调用的方法添加事务，如下
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@Service public class MyService { public void doSomething() { // ... doSomethingElse(); } @Transactional public void doSomethingElse() { // ... } }
但是此时@Transactional会失效，起不到事务的作用。主要原因在于@Transactional采用的是Spring中的AOP机制。AOP机制的介绍可以看以下链接
Chapter 6. Aspect Oriented Programming with Spring
其中有一句需要注意
In addition, AspectJ itself has type-based semantics and at an execution join point both 'this' and 'target' refer to the same object - the object executing the method. Spring AOP is a proxy based system and differentiates between the proxy object itself (bound to 'this') and the target object behind the proxy (bound to 'target').
此处告诉我们AOP是基于代理系统的，作用于代理的目标对象，所以直接调用自身class的方法时是失效的。除非注入自身class作为代理对象 (self injection)。
如何自我注入(Self-Injection)
关于如何注入自身，可以看以下文章
Self-Injection With Spring | Baeldung
从这里我们可以看出，有两种方式，一种是使用***@Autowired注解，一种是使用ApplicationContextAware***，下面我们列举一下这两种方式
使用***@Autowired***
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@Component public class MyBean { @Autowired private MyBean self; public void doSomething() { // use self reference here } }
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@Component public class MyBean { @Autowired private MyBean self; public void doSomething() { // use self reference here } }
使用ApplicationContextAware
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@Component public class MyBean implements ApplicationContextAware { private ApplicationContext context; @Override public void setApplicationContext(ApplicationContext context) throws BeansException { this.context = context; } public void doSomething() { MyBean self = context.getBean(MyBean.class); // ... } }
为什么自我注入(Self-Injection)不会引发循环引用**(circular dependency)**
Spring Framework官方是在4.3版本之后支持的，具体可以看这条PR
可以看到判断Self-Injection最关键的方法是isSelfReference，入参beanName是需要注入的self bean，candidateName是当前正在初始化的bean
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/** * Determine whether the given beanName/candidateName pair indicates a self reference, * i.e. whether the candidate points back to the original bean or to a factory method * on the original bean. */ private boolean isSelfReference(@Nullable String beanName, @Nullable String candidateName) { return (beanName != null && candidateName != null && (beanName.equals(candidateName) || (containsBeanDefinition(candidateName) && beanName.equals(getMergedLocalBeanDefinition(candidateName).getFactoryBeanName())))); }
其次我们看一下isSelfReference的调用方法findAutowireCandidates，beanName指的是当前需要注册的bean，requiredType是需要注入的class类型，descriptor主要是对需要注入的bean的具体描述，如name、required等等。考虑到requiredType有可能是接口类，故通过BeanFactoryUtils.beanNamesForTypeIncludingAncestors方法可以拿到具体的需要注入的beanNames，并放在candidateNames。
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/** * Find bean instances that match the required type. * Called during autowiring for the specified bean. * @param beanName the name of the bean that is about to be wired * @param requiredType the actual type of bean to look for * (may be an array component type or collection element type) * @param descriptor the descriptor of the dependency to resolve * @return a Map of candidate names and candidate instances that match * the required type (never {@code null}) * @throws BeansException in case of errors * @see #autowireByType * @see #autowireConstructor */ protected Map<String, Object> findAutowireCandidates( @Nullable String beanName, Class<?> requiredType, DependencyDescriptor descriptor) { String[] candidateNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors( this, requiredType, true, descriptor.isEager()); Map<String, Object> result = CollectionUtils.newLinkedHashMap(candidateNames.length); for (Map.Entry<Class<?>, Object> classObjectEntry : this.resolvableDependencies.entrySet()) { Class<?> autowiringType = classObjectEntry.getKey(); if (autowiringType.isAssignableFrom(requiredType)) { Object autowiringValue = classObjectEntry.getValue(); autowiringValue = AutowireUtils.resolveAutowiringValue(autowiringValue, requiredType); if (requiredType.isInstance(autowiringValue)) { result.put(ObjectUtils.identityToString(autowiringValue), autowiringValue); break; } } } for (String candidate : candidateNames) { if (!***isSelfReference***(beanName, candidate) && isAutowireCandidate(candidate, descriptor)) { addCandidateEntry(result, candidate, descriptor, requiredType); } } if (result.isEmpty()) { boolean multiple = indicatesMultipleBeans(requiredType); // Consider fallback matches if the first pass failed to find anything... DependencyDescriptor fallbackDescriptor = descriptor.forFallbackMatch(); for (String candidate : candidateNames) { if (!***isSelfReference***(beanName, candidate) && isAutowireCandidate(candidate, fallbackDescriptor) && (!multiple || getAutowireCandidateResolver().hasQualifier(descriptor))) { addCandidateEntry(result, candidate, descriptor, requiredType); } } if (result.isEmpty() && !multiple) { // Consider self references as a final pass... // but in the case of a dependency collection, not the very same bean itself. for (String candidate : candidateNames) { if (***isSelfReference***(beanName, candidate) && (!(descriptor instanceof MultiElementDescriptor) || !beanName.equals(candidate)) && isAutowireCandidate(candidate, fallbackDescriptor)) { addCandidateEntry(result, candidate, descriptor, requiredType); } } } } return result; }
Spring 4.2版本测试
现在我们使用Spring 4.2的版本做一下测试。我们使用1.3.8.RELEAS版本的spring-boot-starter-parent，其对应的spring.version是4.2.8.RELEASE。测试类如下：
- BeanA使用@Autowired自我注入
- BeanC使用ApplicationContextAware自我注入
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@Component public class BeanA { @Autowired private BeanB b; @Autowired private BeanA a; } @Component public class BeanB { } @Component public class BeanC implements ApplicationContextAware { private ApplicationContext context; @Override public void setApplicationContext(ApplicationContext applicationContext) throws BeansException { this.context = context; } public void doSomething() { BeanC self = context.getBean(BeanC.class); } }
启动之后报错如下：
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Caused by: org.springframework.beans.factory.BeanCreationException: Could not autowire field: private org.example.BeanA org.example.BeanA.a; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type [org.example.BeanA] 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)} at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:573) ~[spring-beans-4.2.8.RELEASE.jar:4.2.8.RELEASE] at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:88) ~[spring-beans-4.2.8.RELEASE.jar:4.2.8.RELEASE] at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor.postProcessPropertyValues(AutowiredAnnotationBeanPostProcessor.java:331) ~[spring-beans-4.2.8.RELEASE.jar:4.2.8.RELEASE] ... 16 common frames omitted Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type [org.example.BeanA] found for dependency: expected at least 1 bean which qualifies as autowire candidate for this dependency. Dependency annotations: {@org.springframework.beans.factory.annotation.Autowired(required=true)} at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoSuchBeanDefinitionException(DefaultListableBeanFactory.java:1380) ~[spring-beans-4.2.8.RELEASE.jar:4.2.8.RELEASE] at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1126) ~[spring-beans-4.2.8.RELEASE.jar:4.2.8.RELEASE] at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1021) ~[spring-beans-4.2.8.RELEASE.jar:4.2.8.RELEASE] at org.springframework.beans.factory.annotation.AutowiredAnnotationBeanPostProcessor$AutowiredFieldElement.inject(AutowiredAnnotationBeanPostProcessor.java:545) ~[spring-beans-4.2.8.RELEASE.jar:4.2.8.RELEASE] ... 18 common frames omitted
由此可以看到，BeanA的写法造成了循环引用导致NoSuchBeanDefinitionException异常；BeanC的写法没有报错。
Spring 4.3版本测试
使用1.4.0.RELEASE版本的spring-boot-starter-parent，其对应的spring.version是4.3.2.RELEASE。类似构建如上的BeanA、BeanB和BeanC，启动后发现并无异常。
打断点到DefaultListableBeanFactory可以看到：
总结
Spring是在4.3版本支持了”自己注入自己”，对应到springboot的版本是1.4.0