protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
		// 解析xml元素
		BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
		if (bdHolder != null) {
			// 装饰bean
			bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
			try {
				// Register the final decorated instance.
				// 注册bean到容器中
				BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
			}
			catch (BeanDefinitionStoreException ex) {
				getReaderContext().error("Failed to register bean definition with name '" +
						bdHolder.getBeanName() + "'", ele, ex);
			}
			// Send registration event.
			// 发送事件
			getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
		}
	}

public static void registerBeanDefinition(
			BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
			throws BeanDefinitionStoreException {
		// 注册 bean
		// Register bean definition under primary name.
		// 获取 bean 的名称
		String beanName = definitionHolder.getBeanName();
		// 注册
		registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());

		//注册 bean 的所有别名
		// Register aliases for bean name, if any.
		String[] aliases = definitionHolder.getAliases();
		if (aliases != null) {
			for (String alias : aliases) {
				registry.registerAlias(beanName, alias);
			}
		}
	}

// XmlReaderContext
public final BeanDefinitionRegistry getRegistry() {
		return this.reader.getRegistry();
	}

// DefaultListableBeanFactory
@Override
	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 {
				// 校验 bean 的定义是否合法
				((AbstractBeanDefinition) beanDefinition).validate();
			}
			catch (BeanDefinitionValidationException ex) {
				throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
						"Validation of bean definition failed", ex);
			}
		}
		// 从缓存中取出 BeanDefinition
		BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
		// bean 已被注册过
		if (existingDefinition != null) {
			// 如果不允许重复注册则抛出异常
			if (!isAllowBeanDefinitionOverriding()) {
				throw new BeanDefinitionOverrideException(beanName, beanDefinition, existingDefinition);
			}
			else if (existingDefinition.getRole() < beanDefinition.getRole()) {
				// e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
				if (logger.isInfoEnabled()) {
					logger.info("Overriding user-defined bean definition for bean '" + beanName +
							"' with a framework-generated bean definition: replacing [" +
							existingDefinition + "] with [" + beanDefinition + "]");
				}
			}
			else if (!beanDefinition.equals(existingDefinition)) {
				if (logger.isDebugEnabled()) {
					logger.debug("Overriding bean definition for bean '" + beanName +
							"' with a different definition: replacing [" + existingDefinition +
							"] with [" + beanDefinition + "]");
				}
			}
			else {
				if (logger.isTraceEnabled()) {
					logger.trace("Overriding bean definition for bean '" + beanName +
							"' with an equivalent definition: replacing [" + existingDefinition +
							"] with [" + beanDefinition + "]");
				}
			}
			// 放入到 map 中
			// 覆盖
			this.beanDefinitionMap.put(beanName, beanDefinition);
		}
		else {
			// 判断是否有创建中的bean
			if (hasBeanCreationStarted()) {
				// Cannot modify startup-time collection elements anymore (for stable iteration)
				// 加锁
				// 不能修改启动时的集合,需要重新创建一个集合
				synchronized (this.beanDefinitionMap) {
					// 放入map中
					this.beanDefinitionMap.put(beanName, beanDefinition);
					List<String> updatedDefinitions = new ArrayList<>(this.beanDefinitionNames.size() + 1);
					updatedDefinitions.addAll(this.beanDefinitionNames);
					updatedDefinitions.add(beanName);
					this.beanDefinitionNames = updatedDefinitions;
					// 如果这个已经 bean 在存在 则移除
					removeManualSingletonName(beanName);
				}
			}
			else {
				// Still in startup registration phase
				this.beanDefinitionMap.put(beanName, beanDefinition);
				this.beanDefinitionNames.add(beanName);
        // 如果这个已经 bean 在存在 则移除
				removeManualSingletonName(beanName);
			}
			this.frozenBeanDefinitionNames = null;
		}
		//注册成功,如果是重复注册的则销毁之前注册的bean
		if (existingDefinition != null || containsSingleton(beanName)) {
			resetBeanDefinition(beanName);
		}
	}

// SimpleAliasRegistry
@Override
	public void registerAlias(String name, String alias) {
		Assert.hasText(name, "'name' must not be empty");
		Assert.hasText(alias, "'alias' must not be empty");
		// 加锁 并发控制
		synchronized (this.aliasMap) {
			// 判断 bean 名称是否与别名相同,如果相同则忽略
			if (alias.equals(name)) {
				// 移除别名
				this.aliasMap.remove(alias);
				if (logger.isDebugEnabled()) {
					logger.debug("Alias definition '" + alias + "' ignored since it points to same name");
				}
			}
			else {
				// 判断别名是否已存在
				String registeredName = this.aliasMap.get(alias);
				if (registeredName != null) {
					// 如果别名对应的bean的名称与name相同则忽略
					if (registeredName.equals(name)) {
						// An existing alias - no need to re-register
						return;
					}
					// 是否允许覆盖,如果不允许则报错
					if (!allowAliasOverriding()) {
						throw new IllegalStateException("Cannot define alias '" + alias + "' for name '" +
								name + "': It is already registered for name '" + registeredName + "'.");
					}
					if (logger.isDebugEnabled()) {
						logger.debug("Overriding alias '" + alias + "' definition for registered name '" +
								registeredName + "' with new target name '" + name + "'");
					}
				}
				//检查是否有循环引用别名 例如:A-B C-B A-C
				checkForAliasCircle(name, alias);
				//映射别名和名称到map中
				this.aliasMap.put(alias, name);
				if (logger.isTraceEnabled()) {
					logger.trace("Alias definition '" + alias + "' registered for name '" + name + "'");
				}
			}
		}
	}