前言
上一篇文章创建bean讲述了创建bean的第一二个步骤，这篇文章接着讲述创建bean的第三个步骤----属性注入。
属性注入
在了解循环依赖和创建bean的时候，我们曾经经常反复的提到了populateBean这个函数，这个方法的作用就是对属性进行填充，那么究竟是如何实现填充的呢？
protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) { if (bw == null) { if (mbd.hasPropertyValues()) { throw new BeanCreationException( mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance"); } else { //没有任何属性需要填充 return; } } //给InstantiationAwareBeanPostProcessors 最后一次机会在属性设置前来改变bean比如：可以用来支持属性注入的类型 boolean continueWithPropertyPopulation = true; if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) { for (BeanPostProcessor bp : getBeanPostProcessors()) { if (bp instanceof InstantiationAwareBeanPostProcessor) { InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp; //返回值为是否继续填充bean if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) { continueWithPropertyPopulation = false; break; } } } } //如果后处理器发出停止填充命令则终止后续的执行 if (!continueWithPropertyPopulation) { return; } PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null); if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME || mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) { MutablePropertyValues newPvs = new MutablePropertyValues(pvs); //根据名称自动注入 if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_NAME) { autowireByName(beanName, mbd, bw, newPvs); } //根据类型自动注入 if (mbd.getResolvedAutowireMode() == AUTOWIRE_BY_TYPE) { autowireByType(beanName, mbd, bw, newPvs); } pvs = newPvs; } //后处理器已经初始化 boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors(); //需要依赖检查 boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE); PropertyDescriptor[] filteredPds = null; if (hasInstAwareBpps) { if (pvs == null) { pvs = mbd.getPropertyValues(); } for (BeanPostProcessor bp : getBeanPostProcessors()) { if (bp instanceof InstantiationAwareBeanPostProcessor) { InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp; PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName); if (pvsToUse == null) { if (filteredPds == null) { filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching); } //对所有需要依赖检查的属性进行后处理 pvsToUse = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName); if (pvsToUse == null) { return; } } pvs = pvsToUse; } } } if (needsDepCheck) { //依赖检查，对应depends-on属性，3.0已经弃用此属性 if (filteredPds == null) { filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching); } checkDependencies(beanName, mbd, filteredPds, pvs); } if (pvs != null) { //将属性应用到bean中 applyPropertyValues(beanName, mbd, bw, pvs); } }
分析上述代码的逻辑：
（1）InstantiationAwareBeanPostProcessor处理器的postProcessAfterInstantiation函数的应用，此函数可以控制程序是否继续进行属性填充；
（2）根据注入类型（byName/byType），提取依赖的bean，并统一存入PropertyValues中；
（3）应用InstantiationAwareBeanPostProcessor处理器postProcessPropertyValues方法，对属性获取完毕填充前对属性的再次处理，典型应用是RequiredAnnotationBeanPostProcessor类中对属性的验证；
（4）将所有PropertyValues中的属性填充至BeanWrapper中。
将上述代码中的重点方法再进行进一步的分析：
1.autowireByName（根据名称注入）
protected void autowireByName( String beanName, AbstractBeanDefinition mbd, BeanWrapper bw, MutablePropertyValues pvs) { //寻找bw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.isTraceEnabled()) { logger.trace("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"); } } } }
上述代码并没有什么复杂的逻辑：在传入的参数pvs中找出已经加载的bean，并递归实例化，进而加入到pvs中。
2.autowireByType（根据类型注入）
protected void autowireByType( String beanName, AbstractBeanDefinition mbd, BeanWrapper bw, MutablePropertyValues pvs) { TypeConverter converter = getCustomTypeConverter(); if (converter == null) { converter = bw; } Set<String> autowiredBeanNames = new LinkedHashSet<>(4); //寻找bw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()) { //探测指定属性的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.isInstance(bw.getWrappedInstance()); DependencyDescriptor desc = new AutowireByTypeDependencyDescriptor(methodParam, eager); //解析指定beanName的属性所匹配的值，并把解析到的属性名称存储在autowireBeanNames中 Object autowiredArgument = resolveDependency(desc, beanName, autowiredBeanNames, converter); if (autowiredArgument != null) { pvs.add(propertyName, autowiredArgument); } for (String autowiredBeanName : autowiredBeanNames) { //注册依赖 registerDependentBean(autowiredBeanName, beanName); if (logger.isTraceEnabled()) { logger.trace("Autowiring by type from bean name '" + beanName + "' via property '" + propertyName + "' to bean named '" + autowiredBeanName + "'"); } } autowiredBeanNames.clear(); } } catch (BeansException ex) { throw new UnsatisfiedDependencyException(mbd.getResourceDescription(), beanName, propertyName, ex); } } }
上述代码的逻辑：
根据类型匹配的第一步和根据名称匹配的第一步是一样的，都是从参数bw中寻找需要依赖注入的属性，然后遍历这些属性并寻找类型匹配的bean，其中最复杂的就是寻找类型匹配的bean。同时，Spring中提供了对集合的类型注入的支持，如使用注解的方式：
@Autowired private List<Test> tests;
Spring将会把所有与Test相匹配的类型找出来并注入到tests属性中，正是由于这一因素，所以在autowireByType函数中，新建了局部遍历autowireBeanNames，用于存储所有依赖的bean，如果只是对非集合类的属性注入来说，此属性并无用处。
对于寻找类型匹配的逻辑实现封装在了resolveDependency函数中：
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) { //处理逻辑 result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter); } return result; } }
public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName, @Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException { InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor); try { Object shortcut = descriptor.resolveShortcut(this); if (shortcut != null) { return shortcut; } Class<?> type = descriptor.getDependencyType(); //用于Spring中新增的注解@value Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor); if (value != null) { if (value instanceof String) { String strVal = resolveEmbeddedValue((String) value); BeanDefinition bd = (beanName != null && containsBean(beanName) ? getMergedBeanDefinition(beanName) : null); value = evaluateBeanDefinitionString(strVal, bd); } TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter()); return (descriptor.getField() != null ? converter.convertIfNecessary(value, type, descriptor.getField()) : converter.convertIfNecessary(value, type, descriptor.getMethodParameter())); } Object multipleBeans = resolveMultipleBeans(descriptor, beanName, autowiredBeanNames, typeConverter); if (multipleBeans != null) { return multipleBeans; } //根据属性类型找到BeanFactory中所有类型匹配的bean，返回值的构成为：key=匹配的beanName，value=beanName对应的实例化后的bean Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor); if (matchingBeans.isEmpty()) { //根据autowire的require属性为true而找到的匹配项却为空则只能抛出异常 if (isRequired(descriptor)) { raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor); } return null; } String autowiredBeanName; Object instanceCandidate; if (matchingBeans.size() > 1) { autowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor); if (autowiredBeanName == null) { 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; } } 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()); } return result; } finally { ConstructorResolver.setCurrentInjectionPoint(previousInjectionPoint); } }