一、介紹
spring在解析xml中的bean標籤時，會把該標籤的所有信息封裝到一個GenericBeanDefinition實例中。然後把該GenericBeanDefinition實例註冊到一個beanDefinitionMap 中，供後面複雜的功能使用。
二、整體脈絡
<bean id="person" class="com.kaka.spring.beans.Person">
<property name="age" value="22"/>
</bean>
spring對以上bean標籤的處理步驟如下：
- 創建BeanDefinition實例
- 解析bean標籤上的屬性，並存儲到BeanDefinition實例中
- 解析bean標籤的子元素，並存儲到BeanDefinition實例中
- 把BeanDefinition實例註冊到beanDefinitionMap 中
- 通知監聽器解析及註冊完成
本篇主要分析第4個步驟，beanDefinitionMap 的所屬關係如下：![在這裏插入圖片描述]()
三、相關類、方法
- org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader#processBeanDefinition：解析及註冊bean標籤的整體過程
- org.springframework.beans.factory.support.BeanDefinitionReaderUtils#registerBeanDefinition：註冊BeanDefinition的入口
- org.springframework.beans.factory.support.DefaultListableBeanFactory#registerBeanDefinition：根據名稱註冊BeanDefinition
- org.springframework.core.SimpleAliasRegistry#registerAlias：註冊BeanDefinition的別名
四、源碼分析
- 解析及註冊bean的入口，DefaultBeanDefinitionDocumentReader的processBeanDefinition()方法
protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
// 1.解析bean標籤，封裝至BeanDefinitionHolder
BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
if (bdHolder != null) {
// 2.裝飾BeanDefinitionHolder
bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
try {
// 3.註冊BeanDefinition 點進去
BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
}
catch (BeanDefinitionStoreException ex) {
getReaderContext().error("Failed to register bean definition with name '" +
bdHolder.getBeanName() + "'", ele, ex);
}
// 發佈註冊完成的事件，通知監聽器
getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
}
}
- 使用BeanDefinitionReaderUtils工具類的registerBeanDefinition()方法註冊bean
public static void registerBeanDefinition(
BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
throws BeanDefinitionStoreException {
// 1.根據beanName註冊BeanDefinition
String beanName = definitionHolder.getBeanName();
registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());
// 2.循環註冊所有的bean名
String[] aliases = definitionHolder.getAliases();
if (aliases != null) {
for (String alias : aliases) {
registry.registerAlias(beanName, alias);
}
}
}
- 使用DefaultListableBeanFactory的registerBeanDefinition()方法，根據beanName註冊BeanDefinition
public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
throws BeanDefinitionStoreException {
Assert.hasText(beanName, "Bean name must not be empty");
Assert.notNull(beanDefinition, "BeanDefinition must not be null");
if (beanDefinition instanceof AbstractBeanDefinition) {
try {
// 校驗一些特殊的屬性，如methodOverrides
((AbstractBeanDefinition) beanDefinition).validate();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
"Validation of bean definition failed", ex);
}
}
BeanDefinition oldBeanDefinition;
oldBeanDefinition = this.beanDefinitionMap.get(beanName);
// 該beanName對應的BeanDefinition已存在的情況
if (oldBeanDefinition != null) {
// 不允許覆蓋，則拋出異常
if (!isAllowBeanDefinitionOverriding()) {
throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
"Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
"': There is already [" + oldBeanDefinition + "] bound.");
}
else if (oldBeanDefinition.getRole() < beanDefinition.getRole()) {
// e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
if (this.logger.isWarnEnabled()) {
this.logger.warn("Overriding user-defined bean definition for bean '" + beanName +
"' with a framework-generated bean definition: replacing [" +
oldBeanDefinition + "] with [" + beanDefinition + "]");
}
}
else if (!beanDefinition.equals(oldBeanDefinition)) {
if (this.logger.isInfoEnabled()) {
this.logger.info("Overriding bean definition for bean '" + beanName +
"' with a different definition: replacing [" + oldBeanDefinition +
"] with [" + beanDefinition + "]");
}
}
else {
if (this.logger.isDebugEnabled()) {
this.logger.debug("Overriding bean definition for bean '" + beanName +
"' with an equivalent definition: replacing [" + oldBeanDefinition +
"] with [" + beanDefinition + "]");
}
}
// 把BeanDefinition存入beanDefinitionMap
this.beanDefinitionMap.put(beanName, beanDefinition);
}
else {
if (hasBeanCreationStarted()) {
// Cannot modify startup-time collection elements anymore (for stable iteration)
synchronized (this.beanDefinitionMap) {
this.beanDefinitionMap.put(beanName, beanDefinition);
List<String> updatedDefinitions = new ArrayList<>(this.beanDefinitionNames.size() + 1);
updatedDefinitions.addAll(this.beanDefinitionNames);
updatedDefinitions.add(beanName);
this.beanDefinitionNames = updatedDefinitions;
if (this.manualSingletonNames.contains(beanName)) {
Set<String> updatedSingletons = new LinkedHashSet<>(this.manualSingletonNames);
updatedSingletons.remove(beanName);
this.manualSingletonNames = updatedSingletons;
}
}
}
else {
// 把BeanDefinition存入beanDefinitionMap
this.beanDefinitionMap.put(beanName, beanDefinition);
// 記錄beanName
this.beanDefinitionNames.add(beanName);
this.manualSingletonNames.remove(beanName);
}
this.frozenBeanDefinitionNames = null;
}
if (oldBeanDefinition != null || containsSingleton(beanName)) {
resetBeanDefinition(beanName);
}
}
- 使用SimpleAliasRegistry的registerAlias()方法來註冊別名
public void registerAlias(String name, String alias) {
Assert.hasText(name, "'name' must not be empty");
Assert.hasText(alias, "'alias' must not be empty");
synchronized (this.aliasMap) {
// 如果beanName與alias相同的話，刪除該alias(所以當beanName與其他bean的別名重複時，會覆蓋其他bean的別名)
if (alias.equals(name)) {
this.aliasMap.remove(alias);
if (logger.isDebugEnabled()) {
logger.debug("Alias definition '" + alias + "' ignored since it points to same name");
}
}
else {
String registeredName = this.aliasMap.get(alias);
// 該別名已存在的情況
if (registeredName != null) {
// 該別名已經指向這個beanName了，正常返回
if (registeredName.equals(name)) {
// An existing alias - no need to re-register
return;
}
// 不允許覆蓋別名，則拋出異常
if (!allowAliasOverriding()) {
throw new IllegalStateException("Cannot define alias '" + alias + "' for name '" +
name + "': It is already registered for name '" + registeredName + "'.");
}
if (this.logger.isInfoEnabled()) {
logger.info("Overriding alias '" + alias + "' definition for registered name '" +
registeredName + "' with new target name '" + name + "'");
}
}
// 當存在A->B時，若再次出現A->C->B的時候則會拋出異常
checkForAliasCircle(name, alias);
this.aliasMap.put(alias, name);
if (logger.isDebugEnabled()) {
logger.debug("Alias definition '" + alias + "' registered for name '" + name + "'");
}
}
}
}
五、總結
- 解析bean標籤的整體過程爲DefaultBeanDefinitionDocumentReader的processBeanDefinition()方法
- 其中註冊bean的工作，由BeanDefinitionReaderUtils工具類的registerBeanDefinition()方法處理
- 註冊BeanDefinition包含兩部分：根據beanName註冊BeanDefinition、註冊別名
- 根據beanName註冊BeanDefinition：使用DefaultListableBeanFactory的registerBeanDefinition()方法
- 註冊別名：使用SimpleAliasRegistry的registerAlias()方法