Mybatis源码学习(三)基于@Mapper注解

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#mybatis #学习

本文介绍使用@Mapper注解配置MyBatis的方式，通过示例展示了如何定义接口及查询方法，并解析了其内部实现机制。

在上面的例子中，我们使用xml加载Mapper文件，在这一篇博客中，我们使用@Mapper注解加载sql映射

1 示例

我们修改Mybatis源码学习(一)中的代码，红色为修改部分：

MyBatisMain.java

public class MybatisMain {
  public static void main(String[] args) throws IOException
  {
    String resource = "mybatis-config.xml";
    InputStream inputStream = Resources.getResourceAsStream(resource);
    SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
    SqlSession sqlSession = sqlSessionFactory.openSession();
    BlogMapper blogMapper = sqlSession.getMapper(BlogMapper.class);
    Blog blog = blogMapper.query(101);
//    Blog blog = sqlSession.selectOne("org.mybatis.example.BlogMapper.selectBlog", 101);
    System.out.println(blog.getContext());
  }
}

新增接口BlogMapper.java

@Mapper
public interface BlogMapper {
  @Select("select * from blog where id=#{id}")
  Blog query(Integer id);
}

mybatis-config.xml

<configuration>
    <properties resource="jdbc.properties"/>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="${driver}"/>
                <property name="url" value="${url}"/>
                <property name="username" value="${username}"/>
                <property name="password" value="${password}"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <!--<mapper resource="mapper/BlogMapper.xml"/>-->
        <mapper class="org.apache.ibatis.demo.mapper.BlogMapper"/>
    </mappers>
</configuration>

2 分析

2.1 SqlSessionFactory

在 SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream); 中，build方法中经过若干步骤，执行如下方法

  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);
            try(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);
            try(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.");
          }
        }
      }
    }
  }

当使用注解方式时，执行如下分支，configuration中添加Mapper接口对应的Class。

else if (resource == null && url == null && mapperClass != null) {
            Class<?> mapperInterface = Resources.classForName(mapperClass);
            configuration.addMapper(mapperInterface);
}

2.2 通过BlogMapper接口查询

BlogMapper blogMapper = sqlSession.getMapper(BlogMapper.class);
Blog blog = blogMapper.query(101);

第一行代码SqlSession的getMapper方法

  public <T> T getMapper(Class<T> type) {
    return configuration.getMapper(type, this);
  }

Configuration的getMapper方法

  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    return mapperRegistry.getMapper(type, sqlSession);
  }

MapperRegistry的getMapper方法

  public <T> T getMapper(Class<T> type, SqlSession sqlSession) {
    final MapperProxyFactory<T> mapperProxyFactory = (MapperProxyFactory<T>) knownMappers.get(type);
    if (mapperProxyFactory == null) {
      throw new BindingException("Type " + type + " is not known to the MapperRegistry.");
    }
    try {
      return mapperProxyFactory.newInstance(sqlSession);
    } catch (Exception e) {
      throw new BindingException("Error getting mapper instance. Cause: " + e, e);
    }
  }

MapperProxyFactory的newInstance方法

  public T newInstance(SqlSession sqlSession) {
    final MapperProxy<T> mapperProxy = new MapperProxy<>(sqlSession, mapperInterface, methodCache);
    return newInstance(mapperProxy);
  }

  protected T newInstance(MapperProxy<T> mapperProxy) {
    return (T) Proxy.newProxyInstance(mapperInterface.getClassLoader(), new Class[] { mapperInterface }, mapperProxy);
  }

这里使用了JDK的Proxy动态代理方式，Proxy.newProxyInstance接收3个参数，第一个参数是ClassLoader，第二个参数是被代理接口BlogMapper，第三个参数是实现了 InvocationHandler 接口的MapperProxy类的实例。(这是Proxy代理的规范)。

总而言之，当我们使用@Mapper注解方式时，Mybatis帮助我们使用动态代理的方式代理Mapper接口，来实现接口中所声明的查询方法。 Blog blog = blogMapper.query(101); 执行时实际上是调用的代理对象mapperProxy中的invoke方法。