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：

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

    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 sqlFragments) {
     this(new XPathParser(reader, true, configuration.getVariables(), new XMLMapperEntityResolver()),
         configuration, resource, sqlFragments);
   }

  

 private XMLMapperBuilder(XPathParser parser, Configuration configuration, String resource, Map 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的节点
       cacheRefElement(context.evalNode("cache-ref"));
       //解析mapper的节点
       cacheElement(context.evalNode("cache"));
       //解析mapper的节点
       parameterMapElement(context.evalNodes("/mapper/parameterMap"));
       //解析mapper的节点
       resultMapElements(context.evalNodes("/mapper/resultMap"));
       //解析mapper的节点
       sqlElement(context.evalNodes("/mapper/sql"));
       //使用XMLStatementBuilder的对象解析mapper的、、、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:  and  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。