BeanDefinitionReader 的作用是读取 Spring 配置文件中的内容，将其转换为 IOC 容器内部的数据结构：BeanDefinition，就是使用 ResouceLoad 将配置信息解析成一个个 BeanDefinition, 最终借助 BeanDefinitionRegistry 接口将 BeanDefinition 注册到容器当中。
BeanDefinitionReader.java 接口源代码如下：
/**
* Simple interface for bean definition readers.
* Specifies load methods with Resource and String location parameters.
*
* <p>Concrete bean definition readers can of course add additional
* load and register methods for bean definitions, specific to
* their bean definition format.
*
* <p>Note that a bean definition reader does not have to implement
* this interface. It only serves as suggestion for bean definition
* readers that want to follow standard naming conventions.
*
* @author Juergen Hoeller
* @see org.springframework.core.io.Resource
* @since 1.1
*/
public interface BeanDefinitionReader {
/**
* Return the bean factory to register the bean definitions with.
* <p>The factory is exposed through the BeanDefinitionRegistry interface,
* encapsulating the methods that are relevant for bean definition handling.
* 返回Bean工厂以向其注册Bean定义
*/
BeanDefinitionRegistry getRegistry();
/**
* Return the resource loader to use for resource locations.
* Can be checked for the <b>ResourcePatternResolver</b> interface and cast
* accordingly, for loading multiple resources for a given resource pattern.
* <p>A {@code null} return value suggests that absolute resource loading
* is not available for this bean definition reader.
* <p>This is mainly meant to be used for importing further resources
* from within a bean definition resource, for example via the "import"
* tag in XML bean definitions. It is recommended, however, to apply
* such imports relative to the defining resource; only explicit full
* resource locations will trigger absolute resource loading.
* <p>There is also a {@code loadBeanDefinitions(String)} method available,
* for loading bean definitions from a resource location (or location pattern).
* This is a convenience to avoid explicit ResourceLoader handling.
*
* @see #loadBeanDefinitions(String)
* @see org.springframework.core.io.support.ResourcePatternResolver
* <p>
* 返回资源加载器以用于资源位置
*/
ResourceLoader getResourceLoader();
/**
* Return the class loader to use for bean classes.
* <p>{@code null} suggests to not load bean classes eagerly
* but rather to just register bean definitions with class names,
* with the corresponding Classes to be resolved later (or never).
* 返回用于Bean类的类加载器
*/
ClassLoader getBeanClassLoader();
/**
* Return the BeanNameGenerator to use for anonymous beans
* (without explicit bean name specified).
* 返回BeanNameGenerator用于匿名Bean（未指定显式Bean名称）
*/
BeanNameGenerator getBeanNameGenerator();
/**
* Load bean definitions from the specified resource.
* <p>
* 从指定的资源加载bean定义
*
* @param resource the resource descriptor
* @return the number of bean definitions found
* @throws BeanDefinitionStoreException in case of loading or parsing errors
*/
int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException;
/**
* Load bean definitions from the specified resources.
*
* @param resources the resource descriptors
* @return the number of bean definitions found
* @throws BeanDefinitionStoreException in case of loading or parsing errors
*/
int loadBeanDefinitions(Resource... resources) throws BeanDefinitionStoreException;
/**
* Load bean definitions from the specified resource location.
* <p>The location can also be a location pattern, provided that the
* ResourceLoader of this bean definition reader is a ResourcePatternResolver.
* <p>
* 从指定的资源位置加载bean定义
* 该位置也可以是位置模式，前提是此bean定义读取器的ResourceLoader是ResourcePatternResolver
*
* @param location the resource location, to be loaded with the ResourceLoader
* (or ResourcePatternResolver) of this bean definition reader
* @return the number of bean definitions found
* @throws BeanDefinitionStoreException in case of loading or parsing errors
* @see #getResourceLoader()
* @see #loadBeanDefinitions(org.springframework.core.io.Resource)
* @see #loadBeanDefinitions(org.springframework.core.io.Resource[])
*/
int loadBeanDefinitions(String location) throws BeanDefinitionStoreException;
/**
* Load bean definitions from the specified resource locations.
*
* @param locations the resource locations, to be loaded with the ResourceLoader
* (or ResourcePatternResolver) of this bean definition reader
* @return the number of bean definitions found
* @throws BeanDefinitionStoreException in case of loading or parsing errors
*/
int loadBeanDefinitions(String... locations) throws BeanDefinitionStoreException;
}
Application 会适情况调用，BeanDefinitonReader 其中一个 load 方法，在 BeanDefinitionReader 的 load 方法当中会使用，ResourceLoad 读取资源，将解析出来的 BeanDefinition 注册到容器当中。
-
AbstractBeanDefinitionReader：实现了EnvironmentCapable，提供了获取/设置环境的方法。定义了一些通用方法，使用策略模式，将一些具体方法放到子类实现。 -
XmlBeanDefinitionReader：读取XML 文件定义的BeanDefinition。 -
PropertiesBeanDefinitionReader：可以从属性文件，Resource，Property 对象等读取BeanDefinition。
AbstractBeanDefinitionReader
该类是实现了 BeanDefinitionReader 和 EnvironmentCapable 接口的抽象类，提供常见属性：工作的 bean 工厂、资源加载器、用于加载 bean 类的类加载器、环境等。该类中最核心的方法就是 loadBeanDefinitions 方法。
当传入的参数为资源位置数组时，进入上述方法，如果为字符串数组，则挨个遍历调用 loadBeanDefinitions(location) 方法。
关于 loadBeanDefinitions(location)：根据资源加载器的不同，来处理资源路径，从而返回多个或一个资源，然后再将资源作为参数传递给 loadBeanDefinitions(resources) 方法。在该类中存在一个 loadBeanDefinitions(Resource. .. resources) 方法，该方法用于处理多个资源，归根结底，最后还是调用 loadBeanDefinitions((Resource)resource) 方法，该方法的具体实现在 XmlBeanDefinitionReader 中。
