文章目录

• • 1 MergedBeanDefinition 后置处理
  • • 1.1 在哪里被调用执行？
    • 1.2 两个常用的实现类
    • • 1.2.1 AutowiredAnnotationBeanPostProcessor
      • 1.2.2 CommonAnnotationBeanPostProcessor
  • 2 Bean 属性赋值
  • • 2.1 实例化后阶段
    • 2.2 Bean 属性赋值前处理
    • 2.3 Bean 属性赋值
  • 后记

1 MergedBeanDefinition 后置处理

MergedBeanDefinitionPostProcessor接口的postProcessMergedBeanDefinition是spring给我们提供的又一BeanPostProcessor扩展点，用于处理合并后的BeanDefinition（RootBeanDefinition）。

1.1 在哪里被调用执行？

在上一阶段实例化Bean，我们知道是在AbstractAutowireCapableBeanFactory的doCreateBean()方法中完成，代码如下：

protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)throws BeanCreationException {// 省略实例化Object bean = instanceWrapper.getWrappedInstance();Class beanType = instanceWrapper.getWrappedClass();if (beanType != NullBean.class) {mbd.resolvedTargetType = beanType;}// Allow post-processors to modify the merged bean definition.synchronized (mbd.postProcessingLock) {if (!mbd.postProcessed) {try {applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);}// 省略 异常处理mbd.postProcessed = true;}}// 省略...
}

实例化之后开始执行applyMergedBeanDefinitionPostProcessors()方法，即使用MergedBeanDefinitionPostProcessor后置处理器处理合并后的BeanDefinition，方法源代码如下：

protected void applyMergedBeanDefinitionPostProcessors(RootBeanDefinition mbd, Class beanType, String beanName) {for (MergedBeanDefinitionPostProcessor processor : getBeanPostProcessorCache().mergedDefinition) {processor.postProcessMergedBeanDefinition(mbd, beanType, beanName);}
}

package org.springframework.beans.factory.support;import org.springframework.beans.factory.config.BeanPostProcessor;/*** 后置处理器回调接口*/
public interface MergedBeanDefinitionPostProcessor extends BeanPostProcessor {/*** 处理指定beanName合并后的BeanDefinition*/void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class beanType, String beanName);/*** 重置指定beanName的BeanDefinition*/default void resetBeanDefinition(String beanName) {}}

1.2 两个常用的实现类

那么具体应该干嘛呢？我们看下它的两个重要的实现类做了什么。

1.2.1 AutowiredAnnotationBeanPostProcessor

• AutowiredAnnotationBeanPostProcessor：自动注入注解bean后置处理器，比如@Autowired，@value

查看AutowiredAnnotationBeanPostProcessor类的postProcessMergedBeanDefinition()方法

	@Overridepublic void postProcessMergedBeanDefinition(RootBeanDefinition beanDefinition, Class beanType, String beanName) {InjectionMetadata metadata = findAutowiringMetadata(beanName, beanType, null);metadata.checkConfigMembers(beanDefinition);}

findAutowiringMetadata()查找自动注入的元数据

	private InjectionMetadata findAutowiringMetadata(String beanName, Class clazz, @Nullable PropertyValues pvs) {// Fall back to class name as cache key, for backwards compatibility with custom callers.String cacheKey = (StringUtils.hasLength(beanName) ? beanName : clazz.getName());// Quick check on the concurrent map first, with minimal locking.InjectionMetadata metadata = this.injectionMetadataCache.get(cacheKey);if (InjectionMetadata.needsRefresh(metadata, clazz)) {synchronized (this.injectionMetadataCache) {metadata = this.injectionMetadataCache.get(cacheKey);if (InjectionMetadata.needsRefresh(metadata, clazz)) {if (metadata != null) {metadata.clear(pvs);}metadata = buildAutowiringMetadata(clazz);this.injectionMetadataCache.put(cacheKey, metadata);}}}return metadata;}

injectionMetadataCache注入元数据缓存，默认是没有当前beanName对应的自动注解元数据，继续执行buildAutowiringMetadata()方法

private InjectionMetadata buildAutowiringMetadata(Class clazz) {if (!AnnotationUtils.isCandidateClass(clazz, this.autowiredAnnotationTypes)) {return InjectionMetadata.EMPTY;}List elements = new ArrayList<>();Class targetClass = clazz;do {final List currElements = new ArrayList<>();ReflectionUtils.doWithLocalFields(targetClass, field -> {MergedAnnotation ann = findAutowiredAnnotation(field);if (ann != null) {if (Modifier.isStatic(field.getModifiers())) {if (logger.isInfoEnabled()) {logger.info("Autowired annotation is not supported on static fields: " + field);}return;}boolean required = determineRequiredStatus(ann);currElements.add(new AutowiredFieldElement(field, required));}});// 省略...elements.addAll(0, currElements);targetClass = targetClass.getSuperclass();}while (targetClass != null && targetClass != Object.class);return InjectionMetadata.forElements(elements, clazz);
}public static void doWithLocalFields(Class clazz, FieldCallback fc) {for (Field field : getDeclaredFields(clazz)) {try {fc.doWith(field);}catch (IllegalAccessException ex) {throw new IllegalStateException("Not allowed to access field '" + field.getName() + "': " + ex);}}
}@FunctionalInterface
public interface FieldCallback {/*** Perform an operation using the given field.* @param field the field to operate on*/void doWith(Field field) throws IllegalArgumentException, IllegalAccessException;
}@Nullable
private MergedAnnotation findAutowiredAnnotation(AccessibleObject ao) {MergedAnnotations annotations = MergedAnnotations.from(ao);for (Class type : this.autowiredAnnotationTypes) {MergedAnnotation annotation = annotations.get(type);if (annotation.isPresent()) {return annotation;}}return null;
}

buildAutowiringMetadata()方法实现缓存@Autowired、@Value等标注的字段、方法的流程如下：

• 创建一个大的ArrayList容器elements

• 执行do循环

  • 创建当前存放当前类InjectionMetadata.InjectedElement的小ArrayList容器currElements

  • ReflectionUtils.doWithLocalFields()方法收集@Autowired、@Value等标注的字段、方法

    • doWithLocalFields()第二形参为FieldCallback，函数式接口，实际执行的是实参的箭头函数

    • 遍历获取目标class中Field字段，检测Field字段上注解方式包含在autowiredAnnotationTypes中。如果是把Field放入currElements集合中

      • private final Set> autowiredAnnotationTypes = new LinkedHashSet<>(4);public AutowiredAnnotationBeanPostProcessor() {this.autowiredAnnotationTypes.add(Autowired.class);this.autowiredAnnotationTypes.add(Value.class);try {this.autowiredAnnotationTypes.add((Class)ClassUtils.forName("jakarta.inject.Inject", AutowiredAnnotationBeanPostProcessor.class.getClassLoader()));logger.trace("'jakarta.inject.Inject' annotation found and supported for autowiring");}catch (ClassNotFoundException ex) {// jakarta.inject API not available - simply skip.}try {this.autowiredAnnotationTypes.add((Class)ClassUtils.forName("javax.inject.Inject", AutowiredAnnotationBeanPostProcessor.class.getClassLoader()));logger.trace("'javax.inject.Inject' annotation found and supported for autowiring");}catch (ClassNotFoundException ex) {// javax.inject API not available - simply skip.}}
        

      • autowiredAnnotationTypes中有我们常用的@Autowired、@Value注解

  • 遍历玩Fields，会用相同逻辑处理Methods

  • 把currElements中元素全部放入大集合elements

• 如果父类不为Object或者null，继续循环，即会把父类中标记了对应注解的字段、方法也放入集合

AutowiredAnnotationBeanPostProcessor在 postProcessMergedBeanDefinition 方法中对 @Autowired、@Value 等标注的方法、字段进行缓存

1.2.2 CommonAnnotationBeanPostProcessor

• CommonAnnotationBeanPostProcessor：通用注解bean后置处理器，比如@Resource

CommonAnnotationBeanPostProcessor的postProcessMergedBeanDefinition()方法执行和上述相同的逻辑，不同的是标记元素的注解类型：@Resource和jakarta.ejb.EJB

CommonAnnotationBeanPostProcessor
在 postProcessMergedBeanDefinition 方法中对 @Resource 等标注的字段、方法进行缓存

2 Bean 属性赋值

容器在执行完MergedBeanPostProcessors相关方法后，开始执行Bean属性赋值，具体就是执行AbstractAutowiredCapableBeanFactory类的doCreateBean()方法第603行populateBean()方法

2.1 实例化后阶段

​ populateBean()方法如下：

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {// 省略...if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {if (!bp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {return;}}}
// 省略...}

InstantiationAwareBeanPostProcessor我们在之前学习实例化前阶段的时候讲解过。实例化前阶段扩展点为该接口的 postProcessBeforeInstantiation()方法，这里实例化后阶段用到该接口的 postProcessAfterInstantiation()方法。

default boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException {return true;
}

postProcessAfterInstantiation()方法默认返回true；如果返回false，在populateBean()方法中，不会继续执行后续属性赋值。该方法是自定义属性注入提供一个合适的扩展点。

下面做一个小测试，看看该方法返回false会怎么样？

@Testpublic void testAfter() {DefaultListableBeanFactory factory = new DefaultListableBeanFactory();factory.addBeanPostProcessor(new InstantiationAwareBeanPostProcessor() {@Overridepublic boolean postProcessAfterInstantiation(Object bean, String beanName) throws BeansException {if ("person1".equals(beanName)) {return false;}return InstantiationAwareBeanPostProcessor.super.postProcessAfterInstantiation(bean, beanName);}});factory.registerBeanDefinition("person1",BeanDefinitionBuilder.genericBeanDefinition(Person.class).addPropertyValue("name", "张三").addPropertyValue("age", 25).getBeanDefinition());factory.registerBeanDefinition("person2",BeanDefinitionBuilder.genericBeanDefinition(Person.class).addPropertyValue("name", "李四").addPropertyValue("age", 28).getBeanDefinition());Person person1 = factory.getBean("person1", Person.class);Person person2 = factory.getBean("person2", Person.class);System.out.println(person1);System.out.println(person2);}

测试结果如下:

Person{name='null', age=null}
Person{name='李四', age=28}

2.2 Bean 属性赋值前处理

Bean属性赋值前操作，会调用InstantiationAwareBeanPostProcessor的提供的扩展点方法postProcessProperties()方法，可能会调用postProcessPropertyValues()方法。

我们先来看下populateBean()中调用逻辑，代码如下：

protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {// 省略。。。PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);// 注册自动注入非简单类型依赖关系int resolvedAutowireMode = mbd.getResolvedAutowireMode();if (resolvedAutowireMode == AUTOWIRE_BY_NAME || resolvedAutowireMode == AUTOWIRE_BY_TYPE) {MutablePropertyValues newPvs = new MutablePropertyValues(pvs);// Add property values based on autowire by name if applicable.if (resolvedAutowireMode == AUTOWIRE_BY_NAME) {autowireByName(beanName, mbd, bw, newPvs);}// Add property values based on autowire by type if applicable.if (resolvedAutowireMode == AUTOWIRE_BY_TYPE) {autowireByType(beanName, mbd, bw, newPvs);}pvs = newPvs;}boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);PropertyDescriptor[] filteredPds = null;if (hasInstAwareBpps) {if (pvs == null) {pvs = mbd.getPropertyValues();}for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {PropertyValues pvsToUse = bp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);if (pvsToUse == null) {if (filteredPds == null) {filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);}pvsToUse = bp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);if (pvsToUse == null) {return;}}pvs = pvsToUse;}}// 省略。。。}

解析：

• 在获取属性值后，会根据自动注入类型，注册非简单类型属性的依赖关系。
• 判断是否有InstantiationAwareBeanPostProcessor实现
  • 执行接口的postProcessProperties()方法
  • 判断返回的属性值集合是否为空
    • 执行接口的破碎tProcessPropertyValues()方法
    • 判断返回属性值集合为空，直接返回

说明：

• 如果接口的两个方法都返回空的情况下，不在执行后续的属性赋值操作

下面我们来下接口中这两个方法源代码：

@Nullabledefault PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName)throws BeansException {return null;}@Deprecated@Nullabledefault PropertyValues postProcessPropertyValues(PropertyValues pvs, PropertyDescriptor[] pds, Object bean, String beanName) throws BeansException {return pvs;}

• 前者默认返回null，后者默认返回传入的属性值集合。即默认情况下，会继续执行后续的属性赋值。

实例代码如下：

@Testpublic void testPostProcessProperties() {DefaultListableBeanFactory factory = new DefaultListableBeanFactory();factory.addBeanPostProcessor(new InstantiationAwareBeanPostProcessor() {@Overridepublic PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) throws BeansException {if ("person1".equals(beanName)) {if (pvs == null) {pvs = new MutablePropertyValues();}MutablePropertyValues mpvs  = (MutablePropertyValues) pvs;mpvs.addPropertyValue("name", "李四");mpvs.addPropertyValue("age", 25);return mpvs;}return InstantiationAwareBeanPostProcessor.super.postProcessProperties(pvs, bean, beanName);}});factory.registerBeanDefinition("person1",BeanDefinitionBuilder.genericBeanDefinition(Person.class).addPropertyValue("name", "张三").getBeanDefinition());Person person1 = factory.getBean("person1", Person.class);System.out.println(person1);}

测试结果：

Person{name='李四', age=25}

• 同名属性值覆盖，非同名新增。

2.3 Bean 属性赋值

Bean属性赋值，我们先看下执行逻辑，继续回到AbstractAutowireCapableBeanFactory的populateBean()方法

if (pvs != null) {applyPropertyValues(beanName, mbd, bw, pvs);}

applyPropertyValues()完成赋值，有兴趣的可自行查阅相关文档，这里不深入讲解。

后记

如果小伙伴什么问题或者指教，欢迎交流。

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⭐️源代码仓库地址：https://gitee.com/gaogzhen/spring6-study

参考：

[1]Spring系列之Bean生命周期详解[CP/OL]