1 BeanDefinition 合并
关于 Spring Bean 在初始化或实例化之前，有一个新的阶段就叫 BeanDefinition 合并阶段。
- 父子 BeanDefinitin 合并
- 当前 BeanFactory 查找
- 层次性 BeanFactory 查找
2 BeanDefinition 合并引用示例
2.1 存在父子关系的 Bean XML 定义
<?xml version="1.0" encoding="UTF-8"?>
<beans
xmlns="http://www.springframework.org/schema/beans"
xmlns:context="http://www.springframework.org/schema/context"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://www.springframework.org/schema/beans
https://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/context
https://www.springframework.org/schema/beans/spring-context.xsd">
<!-- <context:annotation-config />-->
<!-- <context:component-scan base-package="org.wmg.thinking.in.spring.ioc.overview.dependency.injection" />-->
<!-- Root BeanDefinition 不需要合并，不存在 parent -->
<!-- 普通 BeanDefinition GenericBeanDefinition -->
<!-- 经过合并后 GenericBeanDefinition 变成 RootBeanDefinition-->
<bean id="user" class="org.wmg.thinking.in.spring.ioc.overview.domain.User">
<property name="id" value="1" />
<property name="name" value="年糕" />
<property name="city" value="HANGZHOU" />
<property name="workCities" value="BEIJING,HANGZHOU" />
<property name="lifeCities">
<list>
<value>BEIJING</value>
<value>SHANGHAI</value>
</list>
</property>
<property name="configFileLocation" value="classpath:/META-INF/user-config.properties" />
</bean>
<!-- 普通 beanDefinition GenericBeanDefinition -->
<!-- 合并后 GenericBeanDefinition 变成 RootBeanDefinition，并且覆盖 parent 相关配置-->
<!-- primary = true，增加了 address 属性-->
<bean id="superUser" class="org.wmg.thinking.in.spring.ioc.overview.domain.SuperUser" parent="user" primary="true">
<property name="address" value="上海" />
</bean>
<bean id="objectFactory" class="org.springframework.beans.factory.config.ObjectFactoryCreatingFactoryBean">
<property name="targetBeanName" value="user" />
</bean>
</beans>
2.2 XML 读取加载之后，会进行 BeanDefinition 合并
...
public class MergedBeanDefinitionMergeDemo {
public static void main(String[] args) {
DefaultListableBeanFactory beanFactory = new DefaultListableBeanFactory();
// 基于 XML 资源 BeanDefinition 实现
XmlBeanDefinitionReader beanDefinitionReader = new XmlBeanDefinitionReader(beanFactory);
String location = "META-INF/dependency-lookup-context.xml";
// 基于 ClassPath 加载 XML 资源
Resource resource = new ClassPathResource(location);
// 指定字符集编码 UTF-8
EncodedResource encodedResource = new EncodedResource(resource, "UTF-8");
int beanNumbers = beanDefinitionReader.loadBeanDefinitions(encodedResource);
System.out.println("已加载 BeanDefinition 数量：" + beanNumbers);
// 通过 Bean Id 和类型进行依赖查找
User user = beanFactory.getBean("user", User.class);
System.out.println(user);
User superUser = beanFactory.getBean("superUser", User.class);
System.out.println(superUser);
}
3 BeanDefinition 合并（源码分析）
合并入口方法：AbstractBeanFactory#getMergedBeanDefinition(java.lang.String)
...
/**
* Return a 'merged' BeanDefinition for the given bean name,
* merging a child bean definition with its parent if necessary.
* <p>This {@code getMergedBeanDefinition} considers bean definition
* in ancestors as well.
* @param name the name of the bean to retrieve the merged definition for
* (may be an alias)
* @return a (potentially merged) RootBeanDefinition for the given bean
* @throws NoSuchBeanDefinitionException if there is no bean with the given name
* @throws BeanDefinitionStoreException in case of an invalid bean definition
*/
@Override
public BeanDefinition getMergedBeanDefinition(String name) throws BeansException {
String beanName = transformedBeanName(name);
// Efficiently check whether bean definition exists in this factory.
if (!containsBeanDefinition(beanName) && getParentBeanFactory() instanceof ConfigurableBeanFactory) {
return ((ConfigurableBeanFactory) getParentBeanFactory()).getMergedBeanDefinition(beanName);
}
// Resolve merged bean definition locally.
return getMergedLocalBeanDefinition(beanName);
}
...
4 总结
如果是根 BeanDefinition 即 RootBeanDefinition 不需要合并，即不存在 parent。如果是普通 BeanDefinition 即 GenericBeanDefinition 需要合并，即存在 parent。经过合并后 GenericBeanDefinition 变成 RootBeanDefinition。这样合并的好处是：子类不需要复写父类的属性，减少了代码的冗余。
