Spring Boot何时注入@Autowired标注的属性？

• 是在Bean实例化后，填充Bean的时候注入@Autowired标注的属性

如果注入类型的Bean存在多个，Spring Boot是如何处理的？

• 如果存在多个类型的Bean，会根据primary—>javax.annotation.Priority—>名称依次过滤，得到最终匹配的bean名称

一、定义接口

public interface PersonService{String hello(String name);
}

二、定义接口的两个实现

@Service(value = "studentService")
public class StudentServiceImpl implements PersonService{@Overridepublic String hello(String name){return "[student service] hello " + name;}
}

@Service(value = "teacerService")
public class TeacherServiceImpl implements PersonService{@Overridepublic String hello(String name){return "[teacher service] hello " + name;}
}

三、定义控制器

@RestController
public class TestController{@Autowiredprivate PersonService studentService;@Autowiredprivate PersonService teacherService;@GetMapping("/hello")public String hello(@RequestParam(name = "name") String name){return studentService.hello(name) + "=======>" + teacherService.hello(name);}
}

分析

在TestController类上打一个断点，然后查看调用链。
基于调用链路，我们看到有一个doCreateBean方法，该方法就是用来创建bean的

实例化Bean

protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)throws BeanCreationException {// Instantiate the bean.BeanWrapper instanceWrapper = null;if (mbd.isSingleton()) {instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);}if (instanceWrapper == null) {// 创建beaninstanceWrapper = createBeanInstance(beanName, mbd, args);}final Object bean = instanceWrapper.getWrappedInstance();Class<?> beanType = instanceWrapper.getWrappedClass();// ...省略部分代码// Initialize the bean instance.Object exposedObject = bean;try {// 填充bean，也就是我们上面提到的调用对象的set方法设置对象属性populateBean(beanName, mbd, instanceWrapper);exposedObject = initializeBean(beanName, exposedObject, mbd);}// ...省略部分代码return exposedObject;
}

填充bean

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {// ...省略代码PropertyDescriptor[] filteredPds = null;if (hasInstAwareBpps) {if (pvs == null) {pvs = mbd.getPropertyValues();}// 遍历所有的后置处理器for (BeanPostProcessor bp : getBeanPostProcessors()) {if (bp instanceof InstantiationAwareBeanPostProcessor) {InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;// 通过断点分析我们可以得知此处调用的是AutowiredAnnotationBeanPostProcessor#postProcessPropertiesPropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);if (pvsToUse == null) {if (filteredPds == null) {filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);}pvsToUse = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);if (pvsToUse == null) {return;}}pvs = pvsToUse;}}}if (needsDepCheck) {if (filteredPds == null) {filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);}checkDependencies(beanName, mbd, filteredPds, pvs);}if (pvs != null) {applyPropertyValues(beanName, mbd, bw, pvs);}
}

处理属性

public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {// 查找当前bean需要注入的元数据信息，以TestController为例，那么需要注入的就是studentService和teacherService两个属性InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);try {// 注入属性metadata.inject(bean, beanName, pvs);}catch (BeanCreationException ex) {throw ex;}catch (Throwable ex) {throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);}return pvs;
}

注入属性 AutowiredAnnotationBeanPostProcessor.AutowiredFieldElement#inject

protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {// 获取属性，此处的属性就是studentServiceField field = (Field) this.member;// 属性对应的valueObject value;if (this.cached) {value = resolvedCachedArgument(beanName, this.cachedFieldValue);}else {DependencyDescriptor desc = new DependencyDescriptor(field, this.required);desc.setContainingClass(bean.getClass());Set<String> autowiredBeanNames = new LinkedHashSet<>(1);Assert.state(beanFactory != null, "No BeanFactory available");TypeConverter typeConverter = beanFactory.getTypeConverter();try {// 解析属性依赖value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);}catch (BeansException ex) {throw new UnsatisfiedDependencyException(null, beanName, new InjectionPoint(field), ex);}synchronized (this) {if (!this.cached) {if (value != null || this.required) {this.cachedFieldValue = desc;registerDependentBeans(beanName, autowiredBeanNames);if (autowiredBeanNames.size() == 1) {String autowiredBeanName = autowiredBeanNames.iterator().next();if (beanFactory.containsBean(autowiredBeanName) &&beanFactory.isTypeMatch(autowiredBeanName, field.getType())) {this.cachedFieldValue = new ShortcutDependencyDescriptor(desc, autowiredBeanName, field.getType());}}}else {this.cachedFieldValue = null;}this.cached = true;}}}if (value != null) {ReflectionUtils.makeAccessible(field);// 给属性设置值，完成注入功能field.set(bean, value);}
}

解析属性依赖 DefaultListableBeanFactory#resolveDependency

public Object resolveDependency(DependencyDescriptor descriptor, @Nullable String requestingBeanName,@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {descriptor.initParameterNameDiscovery(getParameterNameDiscoverer());if (Optional.class == descriptor.getDependencyType()) {return createOptionalDependency(descriptor, requestingBeanName);}else if (ObjectFactory.class == descriptor.getDependencyType() ||ObjectProvider.class == descriptor.getDependencyType()) {return new DependencyObjectProvider(descriptor, requestingBeanName);}else if (javaxInjectProviderClass == descriptor.getDependencyType()) {return new Jsr330Factory().createDependencyProvider(descriptor, requestingBeanName);}else {Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(descriptor, requestingBeanName);if (result == null) {// 解析依赖result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);}return result;}
}

解析属性依赖 DefaultListableBeanFactory#doResolveDependency

public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName,@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);try {// ...省略代码// 解析多个Bean，比如Array、List、Map类型，有兴趣可以自己查看分析Object multipleBeans = resolveMultipleBeans(descriptor, beanName, autowiredBeanNames, typeConverter);if (multipleBeans != null) {return multipleBeans;}// 根据类型获取候选对象，针对studentService而言，该属性的类型为PersonService// PersonService有2个实现类，StudentServiceImpl和TeacherServiceImpl// 所以此处获取结果为StudentServiceImpl对象和TeacherServiceImpl对象Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);if (matchingBeans.isEmpty()) {if (isRequired(descriptor)) {raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);}return null;}String autowiredBeanName;Object instanceCandidate;// 重点处理，如果存在多个匹配的beanif (matchingBeans.size() > 1) {// 从已经匹配的bean中选择一个符合的beanautowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor);if (autowiredBeanName == null) {// 如果bean必须注入或者存在多个匹配的bean，则抛出异常if (isRequired(descriptor) || !indicatesMultipleBeans(type)) {return descriptor.resolveNotUnique(descriptor.getResolvableType(), matchingBeans);}else {// In case of an optional Collection/Map, silently ignore a non-unique case:// possibly it was meant to be an empty collection of multiple regular beans// (before 4.3 in particular when we didn't even look for collection beans).return null;}}// 根据bean名称获取对应的示例instanceCandidate = matchingBeans.get(autowiredBeanName);}else {// We have exactly one match.Map.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();autowiredBeanName = entry.getKey();instanceCandidate = entry.getValue();}if (autowiredBeanNames != null) {autowiredBeanNames.add(autowiredBeanName);}if (instanceCandidate instanceof Class) {instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);}Object result = instanceCandidate;if (result instanceof NullBean) {if (isRequired(descriptor)) {raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);}result = null;}if (!ClassUtils.isAssignableValue(type, result)) {throw new BeanNotOfRequiredTypeException(autowiredBeanName, type, instanceCandidate.getClass());}// 返回对应的示例对象return result;}finally {ConstructorResolver.setCurrentInjectionPoint(previousInjectionPoint);}
}

此处主要根据类型获取所有匹配的bean，如果匹配的bean有多个，那么最后会选择一个符合条件的bean名称，然后将对应的bena实例返回，调用set方法进行进行注入，到此注入的原理本该结束了。

选择符合条件的bean DefaultListableBeanFactory#determineAutowireCandidate

• 依据Bean的primary属性
• 依据javax.annotation.Priority
• 依据注入属性的名称

protected String determineAutowireCandidate(Map<String, Object> candidates, DependencyDescriptor descriptor) {Class<?> requiredType = descriptor.getDependencyType();// 如果bean对应的primary属性为true，则返回bean对应的名称String primaryCandidate = determinePrimaryCandidate(candidates, requiredType);if (primaryCandidate != null) {return primaryCandidate;}// 如果候选bean使用javax.annotation.Priority标注，返回高优先级bean对应的名称String priorityCandidate = determineHighestPriorityCandidate(candidates, requiredType);if (priorityCandidate != null) {return priorityCandidate;}// Fallback// 如果匹配bean的名称和需要注入的属性名称一致，则返回匹配bean的名称for (Map.Entry<String, Object> entry : candidates.entrySet()) {String candidateName = entry.getKey();Object beanInstance = entry.getValue();if ((beanInstance != null && this.resolvableDependencies.containsValue(beanInstance)) ||matchesBeanName(candidateName, descriptor.getDependencyName())) {return candidateName;}}return null;
}