Mybatis 源码学习(一) Mapper 注册

绝地灬酷狼 2022-05-23 07:10 203阅读 0赞

问题：mybatis中mapper接口和xml文件是如何对应起来的。

在mybatis-config.xml中配置一个mapper

是最简单最直接的方式了。这样我们就可以通过SqlSession来获取这个mapper了。

表明上我们获取mapper是通过sqlSession的getMapper函数，稍微看一下源码，发现实际sqlSession调用的Configuration的getMapper函数。继续跟下去，Configuration又调用MapperRegistry类的getMapper函数。

看来MapperRegistry才是最终管理mapper注册和获取的类。

那么mybatis是怎么通过一个xml文件，来注册一个对应的mapper的呢。

研究最简单的加载方式：

public class App 
    {
        public static void main( String[] args )
        {
            String resource = "mybatis-config.xml";
            try {
                InputStream inputStream = Resources.getResourceAsStream(resource);
                SqlSessionFactoryBuilder sqlSessionFactoryBuilder = new SqlSessionFactoryBuilder();
                SqlSessionFactory sqlSessionFactory = sqlSessionFactoryBuilder.build(inputStream);
                SqlSession sqlSession = sqlSessionFactory.openSession();
                UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

来看一下调用栈：

![20180604165744302][]

当我们看到 下面的这个函数实现，心里就豁然开朗了。xml的各种配置方式都在这。

private void mapperElement(XNode parent) throws Exception {
        if (parent != null) {
          for (XNode child : parent.getChildren()) {
            if ("package".equals(child.getName())) {
              String mapperPackage = child.getStringAttribute("name");
              configuration.addMappers(mapperPackage);
            } else {
              String resource = child.getStringAttribute("resource");
              String url = child.getStringAttribute("url");
              String mapperClass = child.getStringAttribute("class");
              if (resource != null && url == null && mapperClass == null) {
                ErrorContext.instance().resource(resource);
                InputStream inputStream = Resources.getResourceAsStream(resource);
                XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
                mapperParser.parse();
              } else if (resource == null && url != null && mapperClass == null) {
                ErrorContext.instance().resource(url);
                InputStream inputStream = Resources.getUrlAsStream(url);
                XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, url, configuration.getSqlFragments());
                mapperParser.parse();
              } else if (resource == null && url == null && mapperClass != null) {
                Class<?> mapperInterface = Resources.classForName(mapperClass);
                configuration.addMapper(mapperInterface);
              } else {
                throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
              }
            }
          }
        }
      }

再往下看 当resource不为空的时候，最终会去执行mapperParser.parse();这个函数中关键的一个地方，bindMapperForNamespace();

private void bindMapperForNamespace() {
        String namespace = builderAssistant.getCurrentNamespace();
        if (namespace != null) {
          Class<?> boundType = null;
          try {
            boundType = Resources.classForName(namespace);
          } catch (ClassNotFoundException e) {
            //ignore, bound type is not required
          }
          if (boundType != null) {
            if (!configuration.hasMapper(boundType)) {
              // Spring may not know the real resource name so we set a flag
              // to prevent loading again this resource from the mapper interface
              // look at MapperAnnotationBuilder#loadXmlResource
              configuration.addLoadedResource("namespace:" + namespace);
              configuration.addMapper(boundType);
            }
          }
        }
      }

"namespace" 是关键，就是通过这个命名空间来找到对应的接口的，所以要注册一个mapper最终还有要找到这个接口。

原来怀疑是可以通过xml文件的内容直接反射来注册一个mapper的想法是错误的！

最后看一下完整的调用栈：

![20180604171053784][]

[20180604165744302]: /images/20220523/e5c5aa2854e243249993a23307bc9c1c.png
[20180604171053784]: /images/20220523/6542cd91f371455aa1cd61a962f97d18.png