通过纯代码构建 MyBatis 的 Configuration 时，如何实现类似 mapper locations 的配置？

本文基于 MyBatis 3.5.5 进行

问题描述

使用纯代码创建 Configuration 时，如何配置 XML Mapper 文件？

Mapper Interface 的 SQL 语句主要的配置方式常用的方式主要有 2 种：

1. 通过 @Select 等注解配置
2. 通过 Mapper XML 文件配置

本文内容与方式 2 有关。

平时使用 SpringBoot + MyBatis (Plus) 指定 XML Mapper 的位置一般利用配置文件指定，如下：

# MyBatis
mybatis.mapperLocations = classpath*:/mapper/**/*.xml

# MyBatis Plus
mybatis-plus.mapperLocations = classpath*:/mapper/**/*.xml

如果使用 MyBatis Configuration XML 指定 XML Mapper 的位置，一般配置如下：

<mappers>
  <mapper resource="org/mybatis/builder/AuthorMapper.xml"/>
  <mapper resource="org/mybatis/builder/BlogMapper.xml"/>
  <mapper resource="org/mybatis/builder/PostMapper.xml"/>
mappers>


<mappers>
  <mapper url="file:///var/mappers/AuthorMapper.xml"/>
  <mapper url="file:///var/mappers/BlogMapper.xml"/>
  <mapper url="file:///var/mappers/PostMapper.xml"/>
mappers>

如果使用纯代码配置 Configuration，又该如何指定 XML Mapper 配置？
本人未能在官方文档上找到相关配置方法，可能是我遗漏了或者文档中没有说明。

有个 Mapper Interface 如下：

public interface OrderMapper {
    @Select("select * from t_order")
    List<Order> findAll();
    Order findById(Long id);
}

有个 XML Mapper 如下：

<mapper namespace="vip.wuweijie.hello.mybatis.mapper.OrderMapper">
    <select id="findById" resultType="vip.wuweijie.hello.mybatis.entity.Order">
        select * from t_order where id = #{id}
    select>
mapper>

通过代码构建 Configuration 配置 MyBatis 的 SqlSessionFactory：

        DataSource dataSource = new PooledDataSource("org.postgresql.Driver", "jdbc:postgresql://pg.sia.lo:5432/hello_quarkus", "postgres", "postgres");
        Environment environment = new Environment("dev", new JdbcTransactionFactory(), dataSource);
        Configuration configuration = new Configuration(environment);
        configuration.addMappers("vip.wuweijie.hello.mybatis.mapper");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(configuration);

问题来了：configuration 中有 addMapper, addMappers 等方法，但是参数只能传入 package 或者 Class。

如果此时执行以下代码，只能执行 mapper.findAll()，执行 mapper.findById(1L) 将会找不到 SQL：

        try (SqlSession sqlSession = sqlSessionFactory.openSession()) {
            OrderMapper mapper = sqlSession.getMapper(OrderMapper.class);
            System.out.println(mapper.findAll());
            System.out.println(mapper.findById(1L));
        }

执行结果：

[Order{id=1, orderTime=null, personName='null', orderType='null', remark='null'}, Order{id=2, orderTime=null, personName='null', orderType='null', remark='Double'}, Order{id=3, orderTime=null, personName='null', orderType='null', remark='From Post'}]

Exception in thread "main" org.apache.ibatis.binding.BindingException: Invalid bound statement (not found): vip.wuweijie.hello.mybatis.mapper.OrderMapper.findById
	at org.apache.ibatis.binding.MapperMethod$SqlCommand.(MapperMethod.java:235)
	at org.apache.ibatis.binding.MapperMethod.(MapperMethod.java:53)
	at org.apache.ibatis.binding.MapperProxy.lambda$cachedInvoker$0(MapperProxy.java:115)
	at java.base/java.util.concurrent.ConcurrentHashMap.computeIfAbsent(ConcurrentHashMap.java:1705)
	at org.apache.ibatis.binding.MapperProxy.cachedInvoker(MapperProxy.java:102)
	at org.apache.ibatis.binding.MapperProxy.invoke(MapperProxy.java:85)
	at com.sun.proxy.$Proxy18.findById(Unknown Source)
	at vip.wuweijie.hello.mybatis.MybatisApplication.main(MybatisApplication.java:45)

这个报错很简单，明显就是 XML Mapper 文件没有跟 Mapper 绑定，使用 Configuration XML 或者其他框架的封装解决仅仅是 加一行配置 的事情。

但现在要使用代码构建 Configuration，具体应该怎么做？
configuration 没有配置类似于 mapperLocations 的方法，也没有指定 XML Mapper 的方法，难道一定要通过 XMLConfig 配置 MyBatis 才能使用 XML Mapper 吗？

---

分析过程

先去看看 XMLConfigBuilder 是怎么做的。

org.apache.ibatis.builder.xml.XMLConfigBuilder代码节选：

  public Configuration parse() {
    if (parsed) {
      throw new BuilderException("Each XMLConfigBuilder can only be used once.");
    }
    parsed = true;
    parseConfiguration(parser.evalNode("/configuration"));
    return configuration;
  }

  private void parseConfiguration(XNode root) {
    try {
      // issue #117 read properties first
      propertiesElement(root.evalNode("properties"));
      Properties settings = settingsAsProperties(root.evalNode("settings"));
      loadCustomVfs(settings);
      loadCustomLogImpl(settings);
      typeAliasesElement(root.evalNode("typeAliases"));
      pluginElement(root.evalNode("plugins"));
      objectFactoryElement(root.evalNode("objectFactory"));
      objectWrapperFactoryElement(root.evalNode("objectWrapperFactory"));
      reflectorFactoryElement(root.evalNode("reflectorFactory"));
      settingsElement(settings);
      // read it after objectFactory and objectWrapperFactory issue #631
      environmentsElement(root.evalNode("environments"));
      databaseIdProviderElement(root.evalNode("databaseIdProvider"));
      typeHandlerElement(root.evalNode("typeHandlers"));
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }

parseConfiguration 方法最底下的 mapperElement 是解析 mappers 标签下内容的方法，即 XML Configuration 的这部分内容：

<mappers>
  <mapper resource="mapper/OrderMapper.xml"/>
  <mapper url="file:///var/mapper/OrderMapper.xml"/>
  <mapper class="vip.wuweijie.hello.mybatis.mapper.OrderMapper"/>
mappers>

继续跟进：

  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) {
          // 解析属性为 resource 的 mapper
            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) {
          // 解析属性为 url 的 mapper
            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) {
          // 解析属性为 class 的 mapper
            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.");
          }
        }
      }
    }
  }

了解了 XMLConfigBuilder 是怎么做的之后，我们构建 Configuration 时可以使用同样的方法：

最后将配置补充完整并执行：

        DataSource dataSource = new PooledDataSource("org.postgresql.Driver", "jdbc:postgresql://pg.sia.lo:5432/hello_quarkus", "postgres", "postgres");
        Environment environment = new Environment("dev", new JdbcTransactionFactory(), dataSource);
        Configuration configuration = new Configuration(environment);
        configuration.addMappers("vip.wuweijie.hello.mybatis.mapper");

		// 解析 XML Mapper 文件
        XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(MybatisApplication.class.getResourceAsStream("/mapper/OrderMapper.xml"), configuration, "mapper/OrderMapper.xml", configuration.getSqlFragments());
        xmlMapperBuilder.parse();

        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(configuration);
        OrderMapper mapper;
        try (SqlSession sqlSession = sqlSessionFactory.openSession()) {
            mapper = sqlSession.getMapper(OrderMapper.class);
            System.out.println(mapper.findAll());
            System.out.println(mapper.findById(1L));
        }

执行结果：

[Order{id=1, orderTime=null, personName='null', orderType='null', remark='null'}, Order{id=2, orderTime=null, personName='null', orderType='null', remark='Double'}, Order{id=3, orderTime=null, personName='null', orderType='null', remark='From Post'}]
Order{id=1, orderTime=null, personName='null', orderType='null', remark='null'}

---

总结

使用 XML Configuration 配置 XML Mapper 的方式：

<mappers>
  <mapper resource="mapper/OrderMapper.xml"/>
mappers>

以下代码等价于在 XML Configuration 中配置 XML Mapper 的位置：

XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(MybatisApplication.class.getResourceAsStream("/mapper/OrderMapper.xml"), configuration, "mapper/OrderMapper.xml", configuration.getSqlFragments());
xmlMapperBuilder.parse();