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{
	
	@Override
	public String hello(String name){
		return "[student service] hello " + name;
	}
}

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

三、定义控制器

@RestController
public class TestController{
	
	@Autowired
	private PersonService studentService;
	
	@Autowired
	private 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bean
		instanceWrapper = 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#postProcessProperties
				PropertyValues 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studentService
	Field field = (Field) this.member;
	// 属性对应的value
	Object 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bean
		if (matchingBeans.size() > 1) {
			// 从已经匹配的bean中选择一个符合的bean
			autowiredBeanName = 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;
}