Mybatis如何加载Mapper.xml

Mybatis如何加载Mapper.xml

hb0730 113 2021-08-02

在根据Mybatis入门中有这么一段

String resource = "org/mybatis/example/mybatis-config.xml";
InputStream inputStream = Resources.getResourceAsStream(resource);
SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

通过mybatis-config.xml读取Mybatis相关配置，其中有一个<maperrs></mappers>节点则是对对应> 我们相关SQL映射文件
今天我们来研究Mybatis是如何加载一系列sql xml

映射器（mappers）

在 <mappers></mappers>节点当中可以使对应的具体的<mapper></mapper>也可以是一个包路径package如

<!-- 使用相对于类路径的资源引用 -->
<mappers>
  <mapper resource="org/mybatis/builder/AuthorMapper.xml"/>
  <mapper resource="org/mybatis/builder/BlogMapper.xml"/>
  <mapper resource="org/mybatis/builder/PostMapper.xml"/>
</mappers>

<!-- 将包内的映射器接口实现全部注册为映射器 -->
<mappers>
  <package name="org.mybatis.builder"/>
</mappers>

而对应的包路径则与java mapper文件一一对应 ，我们以包路径package为例相关路径

image

image

SqlSessionFactoryBuilder#build

在我们根据mybatis启动流程时会发现这么一段代码

  //75
  public SqlSessionFactory build(InputStream inputStream, String environment, Properties properties) {
    try {
      XMLConfigBuilder parser = new XMLConfigBuilder(inputStream, environment, properties);
    //xml解析
      return build(parser.parse());
    } catch (Exception e) {
      throw ExceptionFactory.wrapException("Error building SqlSession.", e);
    } finally {
      ErrorContext.instance().reset();
      try {
        inputStream.close();
      } catch (IOException e) {
        // Intentionally ignore. Prefer previous error.
      }
    }
  }

在 xmlConfigBuilder解析时会得到 Configuration配置类，所以我们进入解析看一下解析了啥东西

XMLConfigBuilder#parse

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

我们可以具体关注 parseConfiguration,至于 parser.evalNode("/configuration")这是对于的config.xml的configuration节点，这个就和DocumentBuilderFactory解析差不多

 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"));
    //插件
      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"));
    //对应的sql mapper xml
      mapperElement(root.evalNode("mappers"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing SQL Mapper Configuration. Cause: " + e, e);
    }
  }

这个方法几乎是解析了configuration节点所有的信息

  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.");
          }
        }
      }
    }
  }

这里就是对应的mappers节点内容具体解析,可以看出一个就是package包节点和其他节点(取其他节点名应该无所谓，具体得有resource,url,class等属性)

所以我们继续看一看Configuration.addMappers(String packageName)方法

Configuration

public void addMappers(String packageName) {
    mapperRegistry.addMappers(packageName);
  }
  • mapperRegistry 其内部就是一个map

MapperRegistry

  /**
   * Adds the mappers.
   *
   * @param packageName
   *          the package name
   * @param superType
   *          the super type
   * @since 3.2.2
   */
  public void addMappers(String packageName, Class<?> superType) {
    ResolverUtil<Class<?>> resolverUtil = new ResolverUtil<>();
    resolverUtil.find(new ResolverUtil.IsA(superType), packageName);
    Set<Class<? extends Class<?>>> mapperSet = resolverUtil.getClasses();
    for (Class<?> mapperClass : mapperSet) {
      addMapper(mapperClass);
    }
  }

  /**
   * Adds the mappers.
   *
   * @param packageName
   *          the package name
   * @since 3.2.2
   */
  public void addMappers(String packageName) {
    addMappers(packageName, Object.class);
  }

其实我们可以看到这就是查询包下所有的xml

image

  public <T> void addMapper(Class<T> type) {
    if (type.isInterface()) {
      if (hasMapper(type)) {
        throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
      }
      boolean loadCompleted = false;
      try {
        knownMappers.put(type, new MapperProxyFactory<>(type));
        // It's important that the type is added before the parser is run
        // otherwise the binding may automatically be attempted by the
        // mapper parser. If the type is already known, it won't try.
        MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
        parser.parse();
        loadCompleted = true;
      } finally {
        if (!loadCompleted) {
          knownMappers.remove(type);
        }
      }
    }
  }

在这一步我们发现它还进行mapper解析，正如注释所说这很重要，我们瞧一瞧

MapperAnnotationBuilder

  public void parse() {
    String resource = type.toString();
     //1. 判断类是否加载过
    if (!configuration.isResourceLoaded(resource)) {
    //2. 加载对应的xml文件
      loadXmlResource();
      configuration.addLoadedResource(resource);
     //设置构建器助理的当前命名空间为type的包+类名
      assistant.setCurrentNamespace(type.getName());
    //3. 后面的都是解析接口中的注解
      parseCache();
      parseCacheRef();
      for (Method method : type.getMethods()) {
        if (!canHaveStatement(method)) {
          continue;
        }
        if (getAnnotationWrapper(method, false, Select.class, SelectProvider.class).isPresent()
            && method.getAnnotation(ResultMap.class) == null) {
          parseResultMap(method);
        }
        try {
    //构建MapperStatement对象，并添加到Mybatis全局配置信息中
          parseStatement(method);
        } catch (IncompleteElementException e) {
          configuration.addIncompleteMethod(new MethodResolver(this, method));
        }
      }
    }
    parsePendingMethods();
  }

在这里则是具体用来解析Mapper接口的xml文件和Mapper接口上的注解，感兴趣的可以具体研究研究

Java Mybatis