源码解析：mybatis调用链之XMLStatementBuilder解析解析sql语句节点

该过程由XMLMapperBuilder的configurationElement方法触发：

XMLMapperBuilder

private void configurationElement(XNode context) {
    try {
        //获取mapper节点的namespace属性
      String namespace = context.getStringAttribute("namespace");
      if (namespace == null || namespace.equals("")) {
        throw new BuilderException("Mapper's namespace cannot be empty");
      }
      //设置builderAssistant的namespace属性
      builderAssistant.setCurrentNamespace(namespace);
      //解析cache-ref节点
      cacheRefElement(context.evalNode("cache-ref"));
      //重点分析 ：解析cache节点----------------1-------------------
      cacheElement(context.evalNode("cache"));
      //解析parameterMap节点（已废弃）
      parameterMapElement(context.evalNodes("/mapper/parameterMap"));
      //重点分析 ：解析resultMap节点（基于数据结果去理解）----------------2-------------------
      resultMapElements(context.evalNodes("/mapper/resultMap"));
      //解析sql节点
      sqlElement(context.evalNodes("/mapper/sql"));
      //重点分析 ：解析select、insert、update、delete节点 ----------------3-------------------
      buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
    } catch (Exception e) {
      throw new BuilderException("Error parsing Mapper XML. The XML location is '" + resource + "'. Cause: " + e, e);
    }
  }

调用buildStatementFromContext(context.evalNodes("select|insert|update|delete"));

XMLMapperBuilder

 //解析select、insert、update、delete节点
  private void buildStatementFromContext(List<XNode> list) {
    if (configuration.getDatabaseId() != null) {
      buildStatementFromContext(list, configuration.getDatabaseId());
    }
    buildStatementFromContext(list, null);
  }

调用private void buildStatementFromContext(List list, String requiredDatabaseId)

XMLMapperBuilder

//处理所有的sql语句节点并注册至configuration对象
  private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) {
    for (XNode context : list) {
      //创建XMLStatementBuilder 专门用于解析sql语句节点
      final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
      try {
        //解析sql语句节点
        statementParser.parseStatementNode();
      } catch (IncompleteElementException e) {
        configuration.addIncompleteStatement(statementParser);
      }
    }
  }

该方法新建XMLStatementBuilder对象，对象构建过程：

首先XMLStatementBuilder类继承于BaseBuilder，与XMLStatementBuilder、XMLConfigBuilder一样调用BaseBuilder构造方法，初始化属性configuration、typeAliasRegistry、typeHandlerRegistry值。

然后初始化builderAssistant（该属性的值由XMLMapperBuilder传入，首次赋值在XMLMapperBuilder构造方法中）、context、requiredDatabaseId属性值。

XMLStatementBuilder对象初始化完成后，执行statementParser.parseStatementNode();解析过程：

XMLStatementBuilder

public void parseStatementNode() {
    //获取sql节点的id
    String id = context.getStringAttribute("id");
    String databaseId = context.getStringAttribute("databaseId");

    if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {
      return;
    }
    /*获取sql节点的各种属性*/
    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);

    Class<?> resultTypeClass = resolveClass(resultType);
    String resultSetType = context.getStringAttribute("resultSetType");
    StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));
    ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);

    
    
    //根据sql节点的名称获取SqlCommandType（INSERT, UPDATE, DELETE, SELECT）
    String nodeName = context.getNode().getNodeName();
    SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH));
    boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
    boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);
    boolean useCache = context.getBooleanAttribute("useCache", isSelect);
    boolean resultOrdered = context.getBooleanAttribute("resultOrdered", false);

    // Include Fragments before parsing
    //在解析sql语句之前先解析<include>节点
    XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
    includeParser.applyIncludes(context.getNode());

    // Parse selectKey after includes and remove them.
    //在解析sql语句之前，处理<selectKey>子节点，并在xml节点中删除
    processSelectKeyNodes(id, parameterTypeClass, langDriver);
    
    // Parse the SQL (pre: <selectKey> and <include> were parsed and removed)
    //解析sql语句是解析mapper.xml的核心，实例化sqlSource，使用sqlSource封装sql语句
    SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
    String resultSets = context.getStringAttribute("resultSets");//获取resultSets属性
    String keyProperty = context.getStringAttribute("keyProperty");//获取主键信息keyProperty
    String keyColumn = context.getStringAttribute("keyColumn");///获取主键信息keyColumn
    
    //根据<selectKey>获取对应的SelectKeyGenerator的id
    KeyGenerator keyGenerator;
    String keyStatementId = id + SelectKeyGenerator.SELECT_KEY_SUFFIX;
    keyStatementId = builderAssistant.applyCurrentNamespace(keyStatementId, true);
    
    
    //获取keyGenerator对象，如果是insert类型的sql语句，会使用KeyGenerator接口获取数据库生产的id；
    if (configuration.hasKeyGenerator(keyStatementId)) {
      keyGenerator = configuration.getKeyGenerator(keyStatementId);
    } else {
      keyGenerator = context.getBooleanAttribute("useGeneratedKeys",
          configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType))
          ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;
    }

    //通过builderAssistant实例化MappedStatement，并注册至configuration对象
    builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
        fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
        resultSetTypeEnum, flushCache, useCache, resultOrdered, 
        keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
  }

通过MapperBuilderAssistant实例化MappedStatement，并注册至configuration对象

MapperBuilderAssistant

public MappedStatement addMappedStatement(
      String id,
      SqlSource sqlSource,
      StatementType statementType,
      SqlCommandType sqlCommandType,
      Integer fetchSize,
      Integer timeout,
      String parameterMap,
      Class<?> parameterType,
      String resultMap,
      Class<?> resultType,
      ResultSetType resultSetType,
      boolean flushCache,
      boolean useCache,
      boolean resultOrdered,
      KeyGenerator keyGenerator,
      String keyProperty,
      String keyColumn,
      String databaseId,
      LanguageDriver lang,
      String resultSets) {

    if (unresolvedCacheRef) {
      throw new IncompleteElementException("Cache-ref not yet resolved");
    }

    id = applyCurrentNamespace(id, false);
    boolean isSelect = sqlCommandType == SqlCommandType.SELECT;

    MappedStatement.Builder statementBuilder = new MappedStatement.Builder(configuration, id, sqlSource, sqlCommandType)
        .resource(resource)
        .fetchSize(fetchSize)
        .timeout(timeout)
        .statementType(statementType)
        .keyGenerator(keyGenerator)
        .keyProperty(keyProperty)
        .keyColumn(keyColumn)
        .databaseId(databaseId)
        .lang(lang)
        .resultOrdered(resultOrdered)
        .resultSets(resultSets)
        .resultMaps(getStatementResultMaps(resultMap, resultType, id))
        .resultSetType(resultSetType)
        .flushCacheRequired(valueOrDefault(flushCache, !isSelect))
        .useCache(valueOrDefault(useCache, isSelect))
        .cache(currentCache);

    ParameterMap statementParameterMap = getStatementParameterMap(parameterMap, parameterType, id);
    if (statementParameterMap != null) {
      statementBuilder.parameterMap(statementParameterMap);
    }

    MappedStatement statement = statementBuilder.build();
    configuration.addMappedStatement(statement);
    return statement;
  }

此处可以看到，MappedStatement中使用了MapperBuilderAssistant对象的currentCache（二级缓存）。

在MappedStatement.Builder构建过程中，

.flushCacheRequired(valueOrDefault(flushCache, !isSelect)

此处flushCache的值表明了是否需要清除一级缓存。默认查询的时候，不需要清除，其他操作都要清除。在分布式系统中，若需要清控一级缓存，则配置flushCache为true即可。效果上相当于关闭了该sql的一级缓存。