<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-2.5.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context.xsd"> <alias name="resourceHolder" alias="resourceHolder1"/>
<alias name="resourceHolder" alias="resourceHolder2"/>
<alias name="resourceHolder" alias="resourceHolder3"/>
<bean id="resourceHolder" class="com.zk.demo1025.ResourceHolder">
<property name="resource" value="classpath:com/zk/demo1025/applicationContext.xml"></property>
</bean>
</beans>
DefaultBeanDefinitionDocumentReader类中的processAliasRegistration()方法：
/**
* Process the given alias element, registering the alias with the registry.
* 处理alias元素，并且注册到注册器中
*/
protected void processAliasRegistration(Element ele) {
String name = ele.getAttribute(NAME_ATTRIBUTE);得到name值
String alias = ele.getAttribute(ALIAS_ATTRIBUTE);得到 alias值
boolean valid = true;
if (!StringUtils.hasText(name)) {
getReaderContext().error("Name must not be empty", ele);
valid = false;
}
if (!StringUtils.hasText(alias)) {
getReaderContext().error("Alias must not be empty", ele);
valid = false;
}
if (valid) {
try {
getReaderContext().getRegistry().registerAlias(name, alias);//这里实际执行的是SimpleAliasRegistry中的registerAlias()方法
}
catch (Exception ex) {
getReaderContext().error("Failed to register alias '" + alias +
"' for bean with name '" + name + "'", ele, ex);
}
getReaderContext().fireAliasRegistered(name, alias, extractSource(ele));
}
}
SimpleAliasRegistry中的registerAlias()方法：最终放入HashMap中。
private final Map<String, String> aliasMap = new ConcurrentHashMap<>(16);
@Override
public void registerAlias(String name, String alias) {synchronized (this.aliasMap) {
if (alias.equals(name)) {
this.aliasMap.remove(alias); 如果别名与名字相同，则在Map中移除
}
else {
String registeredName = this.aliasMap.get(alias);
if (registeredName != null) {
if (registeredName.equals(name)) { 如果相同，则无动于衷
return;
}
}
checkForAliasCircle(name, alias);检查别名回环
this.aliasMap.put(alias, name);放入Map中，key是alias，value是name
}
}
}接下来是取值：AbstractBeanFactory接口的getAlias()方法：
@Override
public String[] getAliases(String name) {
String beanName = transformedBeanName(name);转换名字，这个方法与FactoryBean前缀，别名转换为真名有关
List<String> aliases = new ArrayList<>();
boolean factoryPrefix = name.startsWith(FACTORY_BEAN_PREFIX);是否是FactoryBean
String fullBeanName = beanName;
if (factoryPrefix) {
fullBeanName = FACTORY_BEAN_PREFIX + beanName;
}
if (!fullBeanName.equals(name)) {
aliases.add(fullBeanName);
}
String[] retrievedAliases = super.getAliases(beanName);去父类中aliasMap取值
for (String retrievedAlias : retrievedAliases) {
String alias = (factoryPrefix ? FACTORY_BEAN_PREFIX : "") + retrievedAlias;
if (!alias.equals(name)) {
aliases.add(alias);
}
}
if (!containsSingleton(beanName) && !containsBeanDefinition(beanName)) {
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null) {
aliases.addAll(Arrays.asList(parentBeanFactory.getAliases(fullBeanName)));
}
}
return StringUtils.toStringArray(aliases);
}