为什么MyBatis配置映射器只有四种

以下代码为MyBatis源码：

//解析mappers标签
mapperElement(root.evalNode("mappers"));

private void mapperElement(XNode parent) throws Exception {
    
    if (parent != null) {
      //循环遍历我们mappers标签下面的所有子标签
      for (XNode child : parent.getChildren()) {
          
        //方式一：通过package方式配置映射器
        if ("package".equals(child.getName())) {
          String mapperPackage = child.getStringAttribute("name");
          configuration.addMappers(mapperPackage);
        } else {
          
          //获取其他三种配置方式
          String resource = child.getStringAttribute("resource");
          String url = child.getStringAttribute("url");
          String mapperClass = child.getStringAttribute("class");
            
          //方式二：使用resource方式配置映射器
          if (resource != null && url == null && mapperClass == null) {
            ErrorContext.instance().resource(resource);
            try (InputStream inputStream = Resources.getResourceAsStream(resource)) {
              XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
              mapperParser.parse();
            }
              
          //方式三：使用url的方式配置映射器
          } else if (resource == null && url != null && mapperClass == null) {
            ErrorContext.instance().resource(url);
            try (InputStream inputStream = Resources.getUrlAsStream(url)) {
              XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
              mapperParser.parse();
            }
              
          //方式四：使用类名指定接口名称的方式配置映射器
          } else if (resource == null && url == null && mapperClass != null) {
            Class<?> mapperInterface = Resources.classForName(mapperClass);
            configuration.addMapper(mapperInterface);
          } else {
              
            //不属于上述四种方式则会抛出异常
            throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
          }
        }
      }
    }
  }

A mapper element may only specify a url, resource or class, but not more than one.

图示标签代码，为mybatis-config.xml配置文件中，mappers标签的底层定义

结论：

1. MyBatis配置映射器的方式只有四种方式，如果不属于四种之一就会抛出异常
2. package配置的方式和另外三种配置方式在代码逻辑上属于两类
3. mappers标签内部，可以使用不同的进行配置mapper映射器