BeanDefinitionReader加载过程

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
    */
   @Nullable
   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
    */
   @Nullable
   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。