基于Xml 的 IOC 容器的初始化（三）注册

以下内容是基于基于Xml 的 IOC 容器的初始化注册，我们知道IOC容器的初始化过程大概可以分成，定位，加载，注册。
IOC定位：基于 Xml 的 IOC 容器的初始化（一）定位
IOC加载：基于Xml 的 IOC 容器的初始化（二）加载

16、分配注册策略

让我们继续跟踪程序的执行顺序，接下来我们来分析 DefaultBeanDefinitionDocumentReader 对 Bean 定义转换的 Document 对象解析的流程中，在其 processBeanDefinition 方法中的 delegate.parseDefaultElement() 方法中完成对 Document 对象的解析后得到封装 BeanDefinition 的 BeanDefinitionHold 对象，源码如下：

//解析Bean定义资源Document对象的普通元素
protected void processBeanDefinition(Element ele, BeanDefinitionParserDelegate delegate) {
    BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
    // BeanDefinitionHolder是对BeanDefinition的封装，即Bean定义的封装类
    //对Document对象中<Bean>元素的解析由BeanDefinitionParserDelegate实现
    // BeanDefinitionHolder bdHolder = delegate.parseBeanDefinitionElement(ele);
    if (bdHolder != null) {
        bdHolder = delegate.decorateBeanDefinitionIfRequired(ele, bdHolder);
        try {
            // Register the final decorated instance.
            //向Spring IOC容器注册解析得到的Bean定义，这是Bean定义向IOC容器注册的入口
            BeanDefinitionReaderUtils.registerBeanDefinition(bdHolder, getReaderContext().getRegistry());
        }
        catch (BeanDefinitionStoreException ex) {
            getReaderContext().error("Failed to register bean definition with name '" +
                                     bdHolder.getBeanName() + "'", ele, ex);
        }
        // Send registration event.
        //在完成向Spring IOC容器注册解析得到的Bean定义之后，发送注册事件
        getReaderContext().fireComponentRegistered(new BeanComponentDefinition(bdHolder));
    }
}

然后调用BeanDefinitionReaderUtils 的 registerBeanDefinition() 方法向 IOC 容器注册解析的 Bean ， BeanDefinitionReaderUtils 的注册的源码如下：

public class BeanDefinitionReaderUtils {
    
    //将解析的BeanDefinitionHold注册到容器中
	public static void registerBeanDefinition(
			BeanDefinitionHolder definitionHolder, BeanDefinitionRegistry registry)
			throws BeanDefinitionStoreException {

		// Register bean definition under primary name.
		//获取解析的BeanDefinition的名称
		String beanName = definitionHolder.getBeanName();
		//向IOC容器注册BeanDefinition
		registry.registerBeanDefinition(beanName, definitionHolder.getBeanDefinition());

		// Register aliases for bean name, if any.
		//如果解析的BeanDefinition有别名，向容器为其注册别名
		String[] aliases = definitionHolder.getAliases();
		if (aliases != null) {
			for (String alias : aliases) {
				registry.registerAlias(beanName, alias);
			}
		}
	}
   
}

当调用 BeanDefinitionReaderUtils 向 IOC 容器注册解析的 BeanDefinition 时，真正完成注册功能的是 DefaultListableBeanFactory。

BeanDefinitionReaderUtils 的 registry.registerBeanDefinition() 调用的是 BeanDefinitionRegistry 接口，接着是调用 GenericApplicationContext 的 registerBeanDefinition() 方法，实际上最后是调用 DefaultListableBeanFactory 的 registerBeanDefinition();

17、向容器注册

DefaultListableBeanFactory 中使用一个 HashMap 的集合对象存放 IOC 容器中注册解析的 BeanDefinition，向 IOC 容器注册的主要源码如下：

//
public class DefaultListableBeanFactory extends AbstractAutowireCapableBeanFactory
		implements ConfigurableListableBeanFactory {
    
    //存储注册信息的BeanDefinition
	private final Map<String, BeanDefinition> beanDefinitionMap = new ConcurrentHashMap<>(256);
 
    //---------------------------------------------------------------------
	// Implementation of BeanDefinitionRegistry interface
	//---------------------------------------------------------------------
    
    //向IOC容器注册解析的BeanDefiniton
	@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");

		//校验解析的BeanDefiniton
		if (beanDefinition instanceof AbstractBeanDefinition) {
			try {
				((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);

		if (oldBeanDefinition != null) {
			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 + "]");
				}
			}
			this.beanDefinitionMap.put(beanName, beanDefinition);
		}
		else {
			if (hasBeanCreationStarted()) {
				// Cannot modify startup-time collection elements anymore (for stable iteration)
				//注册的过程中需要线程同步，以保证数据的一致性
				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;
		}

		//检查是否有同名的BeanDefinition已经在IOC容器中注册
		if (oldBeanDefinition != null || containsSingleton(beanName)) {
			//重置所有已经注册过的BeanDefinition的缓存
			resetBeanDefinition(beanName);
		}
	}
}