Spring源码阅读5-IOC容器-BeanDefinition的注册

在上一节，我们得到了BeanDefinitionHolder对象，即将Bean的解析和载入完成了，下面将会对这个对象进行注册到容器中。 在前面我们提到过，IOC容器实际上是一个Map, 在这里将得到验证。

我们从前面的分析得到，在DefaultBeanDefinitionDocumentReader类中的processBeanDefinition方法中：

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

接着我们会看到在DefaultListableBeanFactory类中的registerBeanDefinition方法：

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 {
				((AbstractBeanDefinition) beanDefinition).validate();
			}
			catch (BeanDefinitionValidationException ex) {
				throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
						"Validation of bean definition failed", ex);
			}
		}
        // 检查是否有相同名字的Bean, 如果该Bean不允许被覆盖，则抛出异常
		BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
		if (existingDefinition != null) {
			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 + "]");
				}
			}
            // 注册BeanDefinition
			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 {
				// Still in startup registration phase
				this.beanDefinitionMap.put(beanName, beanDefinition);
				this.beanDefinitionNames.add(beanName);
				this.manualSingletonNames.remove(beanName);
			}
			this.frozenBeanDefinitionNames = null;
		}

		if (existingDefinition != null || containsSingleton(beanName)) {
			resetBeanDefinition(beanName);
		}
	}

在此，我们需要特别注意在类变量中，有如下的定义：

private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<>(256);

这也是IOC底层数据结构的定义地方。

自此，所有的BeanDefinition可以被容器使用了，这些信息是依赖反转的基础。调用时序图如下：

对此，IOC容器对于BeanDefinition的载入和注册完成，后面的文章我们将分析依赖注入的过程。