spring技术内幕笔记：IoC容器的依赖注入

spring版本：4.3.8.RELEASE

在IoC容器初始化的过程中，并没有看到IoC容器对Bean依赖关系的注入。假设IoC容器已经载入了Bean信息，依赖注入的过程是用户第一次向IoC容器索要Bean时触发的，当然也有例外，也就是通过lazy-init属性让容器完成Bean的预初始化。预初始化也是一个完成依赖注入的过程，只不过它是在初始化的过程中完成的。

lazy-init可参考： http://www.cnblogs.com/wcyBlog/p/3756624.html

1.在IoC容器接口BeanFactory中，有一个getBean的接口定义，这个接口的实现就是触发依赖注入发生的地方。从DefaultListableBeanFactory的基类AbstractBeanFactory
入手看看getBean的实现。
AbstractBeanFactory实现了BeanFactory的getBean方法。

public abstract class AbstractBeanFactory extends FactoryBeanRegistrySupport implements ConfigurableBeanFactory {

	//---------------------------------------------------------------------
	// BeanFactory接口的实现类
	// 这些getBean接口方法最终是通过调用doGetBean来实现的
	//---------------------------------------------------------------------

	@Override
	public Object getBean(String name) throws BeansException {
		return doGetBean(name, null, null, false);
	}

	@Override
	public  T getBean(String name, Class requiredType) throws BeansException {
		return doGetBean(name, requiredType, null, false);
	}

	@Override
	public Object getBean(String name, Object... args) throws BeansException {
		return doGetBean(name, null, args, false);
	}

	public  T getBean(String name, Class requiredType, Object... args) throws BeansException {
		return doGetBean(name, requiredType, args, false);
	}

	/**
	 * 实际取得Bean的地方，也就是触发依赖注入发生的地方
	 * 
	 */
	@SuppressWarnings("unchecked")
	protected  T doGetBean(
			final String name, final Class requiredType, final Object[] args, boolean typeCheckOnly)
			throws BeansException {

		final String beanName = transformedBeanName(name);
		Object bean;

		// 从缓存中取得Bean，处理那些已经被创建过的单例模式的Bean，这种Bean的请求不需要重复的创建
		Object sharedInstance = getSingleton(beanName);
		if (sharedInstance != null && args == null) {
			if (logger.isDebugEnabled()) {
				if (isSingletonCurrentlyInCreation(beanName)) {
					logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
							"' that is not fully initialized yet - a consequence of a circular reference");
				}
				else {
					logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
				}
			}
			/*
			 *  这里的getObjectForBeanInstance完成的是FactoryBean的相关处理，以取得FactoryBean的生产结果
			 */
			bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
		}

		else {
			// Fail if we're already creating this bean instance:
			// We're assumably within a circular reference.
			if (isPrototypeCurrentlyInCreation(beanName)) {
				throw new BeanCurrentlyInCreationException(beanName);
			}

			// 对IoC容器中的BeanDefinition是否存在进行检查，检查是否能在当前的BeanFactory中取得需要的Bean,如果在当前的工厂中取不到，则到双亲BeanFactory中去取得需要的Bean
			// 如果在当前的工厂中取不到，则到双亲BeanFactory中去取，如果当前的双亲工厂取不到，就顺着双亲BeanFactory链一直向上查找
			BeanFactory parentBeanFactory = getParentBeanFactory();
			if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
				// Not found -> check parent.
				String nameToLookup = originalBeanName(name);
				if (args != null) {
					// Delegation to parent with explicit args.
					return (T) parentBeanFactory.getBean(nameToLookup, args);
				}
				else {
					// No args -> delegate to standard getBean method.
					return parentBeanFactory.getBean(nameToLookup, requiredType);
				}
			}

			if (!typeCheckOnly) {
				markBeanAsCreated(beanName);
			}

			try {
				//根据Bean的名字取得BeanDefinition
				final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
				checkMergedBeanDefinition(mbd, beanName, args);

				// 获取当前Bean的依赖的所有Bean,这样会触发getBean的递归调用，直到取到一个没有任何依赖的Bean为止
				String[] dependsOn = mbd.getDependsOn();
				if (dependsOn != null) {
					for (String dep : dependsOn) {
						if (isDependent(beanName, dep)) {
							throw new BeanCreationException(mbd.getResourceDescription(), beanName,
									"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
						}
						registerDependentBean(dep, beanName);
						getBean(dep);
					}
				}

				// 创建bean的实例
				// 通过调用createBean方法创建单例模式的bean实例，这里有一个回调函数getObject,会在getSingleton方法中调用ObjectFactory的createBean
				if (mbd.isSingleton()) {
					sharedInstance = getSingleton(beanName, new ObjectFactory