Mybatis3源码分析（三）：解析mapper的xml配置文件

    这一章我们来看看mybatis是怎么读取mapper的xml配置文件并解析其中的sql语句。

    我们还记得是这样配置sqlSessionFactory的：

<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">       <property name="dataSource" ref="dataSource" />     <property name="configLocation" value="classpath:configuration.xml"></property>      <property name="mapperLocations" value="classpath:com/xxx/mybatis/mapper/*.xml"/>       <property name="typeAliasesPackage" value="com.tiantian.mybatis.model" />    </bean>  

    这里配置了一个mapperLocations属性，它是一个表达式，sqlSessionFactory会根据这个表达式读取包com.xxx.mybaits.mapper下面的所有xml格式文件，那么具体是怎么根据这个属性来读取配置文件的呢？

    答案就在SqlSessionFactoryBean类中的buildSqlSessionFactory方法中：

if (!isEmpty(this.mapperLocations)) {      for (Resource mapperLocation : this.mapperLocations) {        if (mapperLocation == null) {          continue;        }        try {          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 (logger.isDebugEnabled()) {          logger.debug("Parsed mapper file: '" + mapperLocation + "'");        }      }    }

    mybatis使用XMLMapperBuilder类的实例来解析mapper配置文件。

public XMLMapperBuilder(Reader reader, Configuration configuration, String resource, Map<String, XNode> sqlFragments) {    this(new XPathParser(reader, true, configuration.getVariables(), new XMLMapperEntityResolver()),        configuration, resource, sqlFragments);  }

  

private XMLMapperBuilder(XPathParser parser, Configuration configuration, String resource, Map<String, XNode> sqlFragments) {    super(configuration);    this.builderAssistant = new MapperBuilderAssistant(configuration, resource);    this.parser = parser;    this.sqlFragments = sqlFragments;    this.resource = resource;  }

    接着系统调用xmlMapperBuilder的parse方法解析mapper。

public void parse() {    //如果configuration对象还没加载xml配置文件（避免重复加载，实际上是确认是否解析了mapper节点的属性及内容，    //为解析它的子节点如cache、sql、select、resultMap、parameterMap等做准备），    //则从输入流中解析mapper节点，然后再将resource的状态置为已加载    if (!configuration.isResourceLoaded(resource)) {      configurationElement(parser.evalNode("/mapper"));      configuration.addLoadedResource(resource);      bindMapperForNamespace();    }    //解析在configurationElement函数中处理resultMap时其extends属性指向的父对象还没被处理的<resultMap>节点    parsePendingResultMaps();    //解析在configurationElement函数中处理cache-ref时其指向的对象不存在的<cache>节点(如果cache-ref先于其指向的cache节点加载就会出现这种情况)    parsePendingChacheRefs();    //同上，如果cache没加载的话处理statement时也会抛出异常    parsePendingStatements();  }

     mybatis解析mapper的xml文件的过程已经很明显了，接下来我们看看它是怎么解析mapper的：

private void configurationElement(XNode context) {    try {      //获取mapper节点的namespace属性      String namespace = context.getStringAttribute("namespace");      if (namespace.equals("")) {        throw new BuilderException("Mapper's namespace cannot be empty");      }      //设置当前namespace      builderAssistant.setCurrentNamespace(namespace);      //解析mapper的<cache-ref>节点      cacheRefElement(context.evalNode("cache-ref"));      //解析mapper的<cache>节点      cacheElement(context.evalNode("cache"));      //解析mapper的<parameterMap>节点      parameterMapElement(context.evalNodes("/mapper/parameterMap"));      //解析mapper的<resultMap>节点      resultMapElements(context.evalNodes("/mapper/resultMap"));      //解析mapper的<sql>节点      sqlElement(context.evalNodes("/mapper/sql"));      //使用XMLStatementBuilder的对象解析mapper的<select>、<insert>、<update>、<delete>节点,      //mybaits会使用MappedStatement.Builder类build一个MappedStatement对象,      //所以mybaits中一个sql对应一个MappedStatement      buildStatementFromContext(context.evalNodes("select|insert|update|delete"));    } catch (Exception e) {      throw new BuilderException("Error parsing Mapper XML. Cause: " + e, e);    }  }

    configurationElement函数几乎解析了mapper节点下所有子节点，至此mybaits解析了mapper中的所有节点，并将其加入到了Configuration对象中提供给sqlSessionFactory对象随时使用。这里我们需要补充讲一下mybaits是怎么使用XMLStatementBuilder类的对象的parseStatementNode函数借用MapperBuilderAssistant类对象builderAssistant的addMappedStatement解析MappedStatement并将其关联到Configuration类对象的：

public void parseStatementNode() {//ID属性    String id = context.getStringAttribute("id");    //databaseId属性    String databaseId = context.getStringAttribute("databaseId");    if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {      return;    }    //fetchSize属性    Integer fetchSize = context.getIntAttribute("fetchSize");    //timeout属性    Integer timeout = context.getIntAttribute("timeout");    //parameterMap属性    String parameterMap = context.getStringAttribute("parameterMap");    //parameterType属性    String parameterType = context.getStringAttribute("parameterType");    Class<?> parameterTypeClass = resolveClass(parameterType);    //resultMap属性    String resultMap = context.getStringAttribute("resultMap");    //resultType属性    String resultType = context.getStringAttribute("resultType");    //lang属性    String lang = context.getStringAttribute("lang");    LanguageDriver langDriver = getLanguageDriver(lang);    Class<?> resultTypeClass = resolveClass(resultType);    //resultSetType属性    String resultSetType = context.getStringAttribute("resultSetType");    StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));    ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);    String nodeName = context.getNode().getNodeName();    SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH));    //是否是<select>节点    boolean isSelect = sqlCommandType == SqlCommandType.SELECT;    //flushCache属性    boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);    //useCache属性    boolean useCache = context.getBooleanAttribute("useCache", isSelect);    //resultOrdered属性    boolean resultOrdered = context.getBooleanAttribute("resultOrdered", false);    // Include Fragments before parsing    XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);    includeParser.applyIncludes(context.getNode());    // Parse selectKey after includes and remove them.    processSelectKeyNodes(id, parameterTypeClass, langDriver);        // Parse the SQL (pre: <selectKey> and <include> were parsed and removed)    SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);    //resultSets属性    String resultSets = context.getStringAttribute("resultSets");    //keyProperty属性    String keyProperty = context.getStringAttribute("keyProperty");    //keyColumn属性    String keyColumn = context.getStringAttribute("keyColumn");    KeyGenerator keyGenerator;    String keyStatementId = id + SelectKeyGenerator.SELECT_KEY_SUFFIX;    keyStatementId = builderAssistant.applyCurrentNamespace(keyStatementId, true);    if (configuration.hasKeyGenerator(keyStatementId)) {      keyGenerator = configuration.getKeyGenerator(keyStatementId);    } else {      //useGeneratedKeys属性      keyGenerator = context.getBooleanAttribute("useGeneratedKeys",          configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType))          ? new Jdbc3KeyGenerator() : new NoKeyGenerator();    }    builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,        fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,        resultSetTypeEnum, flushCache, useCache, resultOrdered,         keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);  }

     由以上代码可以看出mybaits使用XPath解析mapper的配置文件后将其中的resultMap、parameterMap、cache、statement等节点使用关联的builder创建并将得到的对象关联到configuration对象中，而这个configuration对象可以从sqlSession中获取的，这就解释了我们在使用sqlSession对数据库进行操作时mybaits怎么获取到mapper并执行其中的sql语句的问题。