Spring mybatis源码篇章-Mybatis的XML文件加载

通过阅读源码对实现机制进行了解有利于陶冶情操，承接前文Spring mybatis源码篇章-Mybatis主文件加载

前话

前文主要讲解了Mybatis的主文件加载方式，本文则分析不使用主文件加载方式，即SqlSessionFactoryBean对象不配置configLocation属性而配置mapperLocations属性

<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
	<property name="mapperLocations" value="classpath:com/du/wx/resources/mapper/*.xml" />
</bean>

其中每个mapper文件均类似如下样例

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
        "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.du.wx.mapper.joggle.ActivityBookingDao">
	<insert id="addBookingInfo">
		insert into booking_activity_infos(uid,activityid,isbooked)
		values(#{uid},#{activityid},#{isbooked})
	</insert>

	<select id="queryCountBookingByAct" resultType="int">
		select count(*) from booking_activity_infos
		where
		activityid=#{0}
	</select>
	
	<select id="queryCountBookingByUser" resultType="int">
		select count(*) from booking_activity_infos
		where
		uid=#{0}
	</select>
	
	<select id="queryUsersByAct" resultType="BaseUser">
		select a.uid,a.openid,a.nickname,a.phone,a.email,a.address
		from booking_activity_infos b 
		right join 
		base_user a on(b.uid=a.uid)
		where b.activityid=#{0} and b.isbooked=1
	</select>
	
	<select id="queryActByUser" resultType="Activity">
		select a.id,a.title,a.content,a.url,a.address,a.time,a.type,a.status,a.limit,a.people
		from activity a
		left join 
		booking_activity_infos b
		on(a.id=b.activityid)
		where b.uid=#{0} and b.isbooked=1
	</select>
	
	<select id="queryActBooking" resultType="ActivityBooking">
		select * from booking_activity_infos
		where uid=#{0} and activityid=#{1}
	</select>
	
	<update id="updateBooking">
		update booking_activity_infos
		set
		isbooked=#{2}
		where uid=#{0} and activityid=#{1}
	</update>
</mapper>

SqlSessionFactoryBean读取mapper配置文件集合

代码片段如下

	if (!isEmpty(this.mapperLocations)) {
	  //具体的如何从string转为Resource[]，属于spring的基础操作了，有兴趣的读者可自行查阅
      for (Resource mapperLocation : this.mapperLocations) {
        if (mapperLocation == null) {
          continue;
        }

        try {
         //对扫描包及其子包下的每个mapper配置文件进行解析
          XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(),
              configuration, mapperLocation.toString(), configuration.getSqlFragments());
          xmlMapperBuilder.parse();
        } catch (Exception e) {
          throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e);
        } finally {
          ErrorContext.instance().reset();
        }

        if (this.logger.isDebugEnabled()) {
          this.logger.debug("Parsed mapper file: '" + mapperLocation + "'");
        }
      }
    } else {
      if (this.logger.isDebugEnabled()) {
        this.logger.debug("Property 'mapperLocations' was not specified or no matching resources found");
      }
    }

主要是遍历mapper集合，然后对每个XML文件都通过XMLMapperBuilder对象来进行解析。这在前文也有提及

XMLMapperBuilder

笔者直接去观察其parse()方法，代码如下

public void parse() {
	//对每个xml资源只加载一次
    if (!configuration.isResourceLoaded(resource)) {
      //解析xml配置，其中配置的根节点必须为mapper
      configurationElement(parser.evalNode("/mapper"));
      //已加载
      configuration.addLoadedResource(resource);
      //绑定mapper的工作区间
      bindMapperForNamespace();
    }
	
	// 弥补措施
    parsePendingResultMaps();
    parsePendingChacheRefs();
    parsePendingStatements();
  }

紧接着观察XMLMapperBuilder#configurationElement()方法，其来解析mapper文件

  private void configurationElement(XNode context) {
    try {
	  //表明mapper根节点的namespace属性是必须的，且不为空
      String namespace = context.getStringAttribute("namespace");
      if (namespace.equals("")) {
    	  throw new BuilderException("Mapper's namespace cannot be empty");
      }
	  //设置工作区间
      builderAssistant.setCurrentNamespace(namespace);
	  //解析相应的属性
      cacheRefElement(context.evalNode("cache-ref"));
      cacheElement(context.evalNode("cache"));
	  //解析<parameterMap>节点集合
      parameterMapElement(context.evalNodes("/mapper/parameterMap"));
	  //解析<resultMap>节点集合
      resultMapElements(context.evalNodes("/mapper/resultMap"));
	  //解析<sql>节点集合
      sqlElement(context.evalNodes("/mapper/sql"));
	  //创建MappedStatement,这里与注解方式的加载方式还是类似的  
      buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing Mapper XML. Cause: " + e, e);
    }
  }

1.mapper文件的namespace属性不可或缺，否则会抛异常

2.mapper文件的parameterMap/resultMap/sql节点一般都是用来充当模板使用的

3.mapper文件对应的数据库的增删改查节点分别为insert|delete|update|select

笔者稍微对上述的节点作下简单的浏览

resultMap

解析mapper节点下的resultMap节点，相关代码如下

  private ResultMap resultMapElement(XNode resultMapNode, List<ResultMapping> additionalResultMappings) throws Exception {
    ErrorContext.instance().activity("processing " + resultMapNode.getValueBasedIdentifier());
	//读取id属性，最好配置以免不必要的错误
    String id = resultMapNode.getStringAttribute("id",
    resultMapNode.getValueBasedIdentifier());
    //优先级为type>ofType>resultType>javaType
    String type = resultMapNode.getStringAttribute("type",
    resultMapNode.getStringAttribute("ofType",
    resultMapNode.getStringAttribute("resultType",
    resultMapNode.getStringAttribute("javaType"))));
    
    String extend = resultMapNode.getStringAttribute("extends");
    //是否开启自动映射，默认值为unset
    Boolean autoMapping = resultMapNode.getBooleanAttribute("autoMapping");
    Class<?> typeClass = resolveClass(type);
    
    //<discriminator><case /><case/></discriminator>根据结果值进行结果类型的映射，类似java的switch-case语法 
    Discriminator discriminator = null;
    //ResultMap节点信息转化为ResultMapping集合
    List<ResultMapping> resultMappings = new ArrayList<ResultMapping>();
    resultMappings.addAll(additionalResultMappings);
    List<XNode> resultChildren = resultMapNode.getChildren();
    for (XNode resultChild : resultChildren) {
        if ("constructor".equals(resultChild.getName())) {
			//<resultMap>节点下<constructor>节点处理
			processConstructorElement(resultChild, typeClass, resultMappings);
        } else if ("discriminator".equals(resultChild.getName())) {
            //<resultMap>节点下<discriminator>节点处理
            discriminator = processDiscriminatorElement(resultChild, typeClass, resultMappings);
        } else {
            //<id>/<result>/<collection>/<association>节点的解析
            ArrayList<ResultFlag> flags = new ArrayList<ResultFlag>();
            if ("id".equals(resultChild.getName())) {
              flags.add(ResultFlag.ID);
            }
            resultMappings.add(buildResultMappingFromContext(resultChild, typeClass, flags));
        }
    }
    ResultMapResolver resultMapResolver = new ResultMapResolver(builderAssistant, id, typeClass, extend, discriminator, resultMappings, autoMapping);
    try {
        //组装成ResultMap对象保存到Configuration对象的私有集合变量resultMaps
        return resultMapResolver.resolve();
    } catch (IncompleteElementException  e) {
        configuration.addIncompleteResultMap(resultMapResolver);
        throw e;
    }
  }

具体的属性就不展开了，查阅相关的官方文档即可

sql

解析mapper节点下的sql节点。主要作用是将每个sql节点对象都保存到Configuration对象中的sqlFragments属性中(HashMap)。

  private void sqlElement(List<XNode> list, String requiredDatabaseId) throws Exception {
    for (XNode context : list) {
	  //sql节点的databaseId属性
      String databaseId = context.getStringAttribute("databaseId");
	  //sql节点的id属性,id=${namespace}+"."+id
      String id = context.getStringAttribute("id");
      id = builderAssistant.applyCurrentNamespace(id, false);
	  //true的前提是主配置文件指定了databaseId属性或者主配置和sql节点的databaseId属性均不存在，但sql节点的id属性存在
      if (databaseIdMatchesCurrent(id, databaseId, requiredDatabaseId)) sqlFragments.put(id, context);
    }
  }  

sql节点在配置id属性时，可简单的配置别名而不用带上namespace前缀，代码会自动校验进行拼装~~

CRUD节点解析

即解析select/update/delete/insert节点对应的信息，笔者此处关注XMLStatementBuilder#parseStatementNode()方法的片段代码

	//节点上支持的常见属性
	Integer fetchSize = context.getIntAttribute("fetchSize");
    Integer timeout = context.getIntAttribute("timeout");
    String parameterMap = context.getStringAttribute("parameterMap");
    String parameterType = context.getStringAttribute("parameterType");
    Class<?> parameterTypeClass = resolveClass(parameterType);
    String resultMap = context.getStringAttribute("resultMap");
    String resultType = context.getStringAttribute("resultType");
    String lang = context.getStringAttribute("lang");
    LanguageDriver langDriver = getLanguageDriver(lang);
    ...
	String resultSetType = context.getStringAttribute("resultSetType");
	StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));
	...
	// Include Fragments before parsing 导入<include>标签内容，其内部可以含有<if>/<where>/<set>等标签
    XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
    includeParser.applyIncludes(context.getNode());


    // Parse selectKey after includes and remove them.导入<selectKey>标签内容
    processSelectKeyNodes(id, parameterTypeClass, langDriver);
    
    // Parse the SQL (pre: <selectKey> and <include> were parsed and removed) 如何解析sql语句？放置下一章节讲解
    SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);

    ...
    //创建MappedStatement对象，保存在Configuration的mappedStatement集合属性中
    builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
    fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
    resultSetTypeEnum, flushCache, useCache, resultOrdered, 
    keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);

代码其实很长，不过跟前文的注解方式的解析是类似的，最终都是生成MappedStatement对象。
而其会保存在org.apache.ibatis.session.Configuration对象中~~~~

总结

1.不管是通过注解模式还是XML文件模式，都会生成MappedStatement对象保存到Configuration对象中

2.每个select|update|insert|delete标签均会被解析为单个MappedStatement对象，其中唯一ID为${namespace}.${id}

3.Spring官方以及笔者都建议通过此方式来加载指定的XML文件，但是细心的笔者发现这其实并没有与DAO接口关联起来，看来还是得借助org.mybatis.spring.mapper.MapperScannerConfigurer的力量