Spring通过<import>标签导入外部配置文件

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="http://www.springframework.org/schema/beans
       http://www.springframework.org/schema/beans/spring-beans.xsd">
   <import resource="spring-config.xml"/>
   <bean id="innerPerson" class="com.john.aop.Person">
   </bean>
</beans>

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
      xsi:schemaLocation="http://www.springframework.org/schema/beans
       http://www.springframework.org/schema/beans/spring-beans.xsd">
   <bean id="outerPerson" class="com.john.aop.Person">
          <property name="firstName" value="john"/>
            <property name="lastName" value="wonder"/>
   </bean>
   <bean id="ChineseFemaleSinger" class="com.john.beanFactory.Singer" abstract="true" >
      <property name="country" value="中国"/>
      <property name="gender" value="女"/>
   </bean>
</beans>

public static void main(String[] args) {
   ApplicationContext context=new ClassPathXmlApplicationContext("spring-import.xml");
   Person outerPerson=(Person)context.getBean("outerPerson");
   System.out.println(outerPerson);
}

Person [0, john wonder]

public static final String IMPORT_ELEMENT = "import";

private void parseDefaultElement(Element ele, BeanDefinitionParserDelegate delegate) {
   //todo  对 import 标签的解析 2020-11-17
   if (delegate.nodeNameEquals(ele, IMPORT_ELEMENT)) {
      importBeanDefinitionResource(ele);
   }
}

/**
 * Parse an "import" element and load the bean definitions
 * from the given resource into the bean factory.
 */
protected void importBeanDefinitionResource(Element ele) {
   //// 获取 resource 的属性值
   String location = ele.getAttribute(RESOURCE_ATTRIBUTE);
   //// 为空，直接退出
   if (!StringUtils.hasText(location)) {
      getReaderContext().error("Resource location must not be empty", ele);
      return;
   }
   // Resolve system properties: e.g. "${user.dir}"
   // // 解析系统属性，格式如 ："${user.dir}"
   location = getReaderContext().getEnvironment().resolveRequiredPlaceholders(location);
   Set<Resource> actualResources = new LinkedHashSet<>(4);
   // Discover whether the location is an absolute or relative URI
   //// 判断 location 是相对路径还是绝对路径
   boolean absoluteLocation = false;
   try {
      //以 classpath*: 或者 classpath: 开头为绝对路径
      absoluteLocation = ResourcePatternUtils.isUrl(location) || ResourceUtils.toURI(location).isAbsolute();
   }
   catch (URISyntaxException ex) {
      // cannot convert to an URI, considering the location relative
      // unless it is the well-known Spring prefix "classpath*:"
   }

   // Absolute or relative?
   //绝对路径 也会调用loadBeanDefinitions
   if (absoluteLocation) {
      try {
         int importCount = getReaderContext().getReader().loadBeanDefinitions(location, actualResources);
         if (logger.isTraceEnabled()) {
            logger.trace("Imported " + importCount + " bean definitions from URL location [" + location + "]");
         }
      }
      catch (BeanDefinitionStoreException ex) {
         getReaderContext().error(
               "Failed to import bean definitions from URL location [" + location + "]", ele, ex);
      }
   }
   else {
      //如果是相对路径，则先计算出绝对路径得到 Resource，然后进行解析
      // No URL -> considering resource location as relative to the current file.
      try {
         int importCount;
         Resource relativeResource = getReaderContext().getResource().createRelative(location);

         //如果相对路径 这个资源存在 那么就加载这个bean 定义
         if (relativeResource.exists()) {
            importCount = getReaderContext().getReader().loadBeanDefinitions(relativeResource);
            actualResources.add(relativeResource);
         }
         else {
            String baseLocation = getReaderContext().getResource().getURL().toString();
            //todo import节点 内部会调用loadBeanDefinitions 操作 2020-10-17
            importCount = getReaderContext().getReader().loadBeanDefinitions(
                  StringUtils.applyRelativePath(baseLocation, location), actualResources);
         }
         if (logger.isTraceEnabled()) {
            logger.trace("Imported " + importCount + " bean definitions from relative location [" + location + "]");
         }
      }
   }
}

/**
 * Load bean definitions from the specified XML file.
 * @param resource the resource descriptor for the XML file
 * @return the number of bean definitions found
 * @throws BeanDefinitionStoreException in case of loading or parsing errors
 */
@Override
public int loadBeanDefinitions(Resource resource) throws BeanDefinitionStoreException {
   //load中去注册BeanDefinition
   return loadBeanDefinitions(new EncodedResource(resource));
}

/**
 * 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.
 * @param location the resource location, to be loaded with the ResourceLoader
 * (or ResourcePatternResolver) of this bean definition reader
 * @param actualResources a Set to be filled with the actual Resource objects
 * that have been resolved during the loading process(要填充加载过程中已解析的实际资源对象*的集合). May be {@code null}
 * to indicate that the caller is not interested in those Resource objects.
 */
public int loadBeanDefinitions(String location, @Nullable Set<Resource> actualResources) throws BeanDefinitionStoreException {