Spring源码分析：多个bean同时满足注入条件时,spring的选择

今天在翻阅Spring的源码时，看到了spring中对属性注入的处理，突然想到如果需要注入的接口有多个实现类，spring会如何解决的问题（Spring的版本为4.3.7）。

//如果对源码的分析没有兴趣，可以直接跳到最后总结部分

spring中的注入方式有两种：通过bean的名字进行注入或者根据类型进行注入。

Spring在创建Bean实例后，进行依赖注入的代码是：

protected void populateBean(String beanName, RootBeanDefinition mbd, BeanWrapper bw) {
		PropertyValues pvs = mbd.getPropertyValues();

		//省略：如果bw为null则不用注入，直接返回

		// Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
		// state of the bean before properties are set. This can be used, for example,
		// to support styles of field injection.
		boolean continueWithPropertyPopulation = true;

		if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
			for (BeanPostProcessor bp : getBeanPostProcessors()) {
				if (bp instanceof InstantiationAwareBeanPostProcessor) {//完成注解标注的属性注入
					InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
					if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
						continueWithPropertyPopulation = false;
						break;
					}
				}
			}
		}

		if (!continueWithPropertyPopulation) {
			return;
		}

		//在<bean></bean>标签中可以指定default-autowire来进行注入
		// 在不设置<beans default-autowire="byName/byType"/>的情况下是不会调用方法的
		if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME ||
				mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
			MutablePropertyValues newPvs = new MutablePropertyValues(pvs);

			// Add property values based on autowire by name if applicable.
			if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {
				autowireByName(beanName, mbd, bw, newPvs);
			}

			// Add property values based on autowire by type if applicable.
			if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
				autowireByType(beanName, mbd, bw, newPvs);
			}

			pvs = newPvs;
		}

		boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
		boolean needsDepCheck = (mbd.getDependencyCheck() != RootBeanDefinition.DEPENDENCY_CHECK_NONE);

		if (hasInstAwareBpps || needsDepCheck) {
			PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
			if (hasInstAwareBpps) {
				for (BeanPostProcessor bp : getBeanPostProcessors()) {
					if (bp instanceof InstantiationAwareBeanPostProcessor) {
						//通过注解进行标注的域在此处通过AutowiredAnnotationBeanPostProcessor完成注入
						InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
						pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
						if (pvs == null) {
							return;
						}
					}
				}
			}
			if (needsDepCheck) {
				checkDependencies(beanName, mbd, filteredPds, pvs);
			}
		}
		//将属性注入到实例中
		applyPropertyValues(beanName, mbd, bw, pvs);
	}

1、根据bean名称进行属性注入的方式(autowireByName)

例如：

@Resource(name = "userService")
    private UserService userService;

@Autowired
    @Qualifier(value = "articleService")
    private ArticleService articleService;

protected void autowireByName(
			String beanName, AbstractBeanDefinition mbd, BeanWrapper bw, MutablePropertyValues pvs) {

		//寻找BeanWrapper(对beanInstance的包装类)中需要依赖注入的属性
		String[] propertyNames = unsatisfiedNonSimpleProperties(mbd, bw);
		for (String propertyName : propertyNames) {
			if (containsBean(propertyName)) {
				//递归初始化相关的bean
				Object bean = getBean(propertyName);
				pvs.add(propertyName, bean);
				//注册依赖
				registerDependentBean(propertyName, beanName);
				if (logger.isDebugEnabled()) {
					logger.debug("Added autowiring by name from bean name '" + beanName +
							"' via property '" + propertyName + "' to bean named '" + propertyName + "'");
				}
			}
			else {
				if (logger.isTraceEnabled()) {
					logger.trace("Not autowiring property '" + propertyName + "' of bean '" + beanName +
							"' by name: no matching bean found");
				}
			}
		}
	}

2、根据bean的类型进行注入(autowireByType)

由于是根据bean的类型进行选择相应bean进行注入，所以就存在一个接口有多个实现类同时存在的情况。

//下面是跟踪Spring通过autowireByType()方法，搜索匹配类型的bean列表的源码，如果只关注如何选择bean的话可以直接掠过到最后的第5部分。

下面是根据属性注入的总体源码：

protected void autowireByType(
			String beanName, AbstractBeanDefinition mbd, BeanWrapper bw, MutablePropertyValues pvs) {

		TypeConverter converter = getCustomTypeConverter();
		if (converter == null) {
			converter = bw;
		}

		Set<String> autowiredBeanNames = new LinkedHashSet<String>(4);
		//寻找BeanWrapper中需要依赖注入的属性
		String[] propertyNames = unsatisfiedNonSimpleProperties(mbd, bw);
		for (String propertyName : propertyNames) {
			try {
				PropertyDescriptor pd = bw.getPropertyDescriptor(propertyName);
				// Don't try autowiring by type for type Object: never makes sense,
				// even if it technically is a unsatisfied, non-simple property.
				if (Object.class != pd.getPropertyType()) {
					//寻找属性的set方法
					MethodParameter methodParam = BeanUtils.getWriteMethodParameter(pd);
					// Do not allow eager init for type matching in case of a prioritized post-processor.
					boolean eager = !PriorityOrdered.class.isAssignableFrom(bw.getWrappedClass());
					DependencyDescriptor desc = new AutowireByTypeDependencyDescriptor(methodParam, eager);
					
					//在这一步完成对指定beanName的属性进行解析，并将解析到的属性名称存放在autowiredBeanNames中
					Object autowiredArgument = resolveDependency(desc, beanName, autowiredBeanNames, converter);
					
					
					if (autowiredArgument != null) {
						pvs.add(propertyName, autowiredArgument);
					}
					for (String autowiredBeanName : autowiredBeanNames) {