一、相关类与对象说明

• XMLMapperBuilder

  专门用来解析mapper映射文件

public XMLMapperBuilder(Reader reader, Configuration configuration, String resource, Map<String, XNode> sqlFragments, String namespace);
public XMLMapperBuilder(Reader reader, Configuration configuration, String resource, Map<String, XNode> sqlFragments) ;
public XMLMapperBuilder(InputStream inputStream, Configuration configuration, String resource, Map<String, XNode> sqlFragments, String namespace);
public XMLMapperBuilder(InputStream inputStream, Configuration configuration, String resource, Map<String, XNode> sqlFragments);

• XPathParser

  用来使用XPath语法解析XML的解析器

• MapperRegistry

  mapper的注册中心，用于注册mapper

• MapperBuilderAssistant

  用于构建MappedStatement对象的

• XMLStatementBuilder

  专门用来解析MappedStatement

• MappedStatement对象

  MyBatis的mapper文件最终会被解析器,解析成MappedStatement,其中insert|update|delete|select每一个标签分别对应一个MappedStatement

  private String resource;
  private Configuration configuration;
  //节点中的id属性加要命名空间
  private String id;
  //尝试影响驱动程序每次批量返回的结果行数和这个设置值相等
  private Integer fetchSize;
  //SQL超时时间
  private Integer timeout;
  //Statement的类型，STATEMENT/PREPARE/CALLABLE
  private StatementType statementType;
  //结果集类型，FORWARD_ONLY/SCROLL_SENSITIVE/SCROLL_INSENSITIVE
  private ResultSetType resultSetType;
  //表示解析出来的SQL
  private SqlSource sqlSource;
  //缓存
  private Cache cache;
  //已废弃
  private ParameterMap parameterMap;
  //对应的ResultMap
  private List<ResultMap> resultMaps;
  private boolean flushCacheRequired;
  private boolean useCache;
  private boolean resultOrdered;
  //SQL类型，INSERT/SELECT/DELETE
  private SqlCommandType sqlCommandType;
  //和SELECTKEY标签有关
  private KeyGenerator keyGenerator;
  private String[] keyProperties;
  private String[] keyColumns;
  private boolean hasNestedResultMaps;
  //数据库ID，用来区分不同环境
  private String databaseId;
  private Log statementLog;
  private LanguageDriver lang;
  //多结果集时
  private String[] resultSets;

  MappedStatement() {
    // constructor disabled
  }

二、XML映射文件

MyBatis 的真正强大在于它的语句映射,减少使用成本，让用户能更专注于 SQL 代码。

SQL 映射文件顶级元素.png

• select

select 元素允许你配置很多属性来配置每条语句的行为细节。

<select
  id="selectPerson"
  parameterType="int"
  parameterMap="deprecated"
  resultType="hashmap"
  resultMap="personResultMap"
  flushCache="false"
  useCache="true"
  timeout="10"
  fetchSize="256"
  statementType="PREPARED"
  resultSetType="FORWARD_ONLY">

select的属性.png

• insert, update 和 delete

  数据变更语句 insert，update 和 delete 的实现非常接近

<insert
  id="insertAuthor"
  parameterType="domain.blog.Author"
  flushCache="true"
  statementType="PREPARED"
  keyProperty=""
  keyColumn=""
  useGeneratedKeys=""
  timeout="20">

<update
  id="updateAuthor"
  parameterType="domain.blog.Author"
  flushCache="true"
  statementType="PREPARED"
  timeout="20">

<delete
  id="deleteAuthor"
  parameterType="domain.blog.Author"
  flushCache="true"
  statementType="PREPARED"
  timeout="20">

Insert, Update, Delete 元素的属性.png

• 动态sql

借助功能强大的基于 OGNL 的表达式，MyBatis 3 替换了之前的大部分元素，大大精简了元素种类

• if
• choose (when, otherwise)
  MyBatis 提供了 choose 元素，它有点像 Java 中的 switch 语句

<select id="findActiveBlogLike"
     resultType="Blog">
  SELECT * FROM BLOG WHERE state = ‘ACTIVE’
  <choose>
    <when test="title != null">
      AND title like #{title}
    </when>
    <when test="author != null and author.name != null">
      AND author_name like #{author.name}
    </when>
    <otherwise>
      AND featured = 1
    </otherwise>
  </choose>
</select>

• trim (where, set)

<trim prefix="WHERE" prefixOverrides="AND |OR ">
  ...
</trim>

<trim prefix="SET" suffixOverrides=",">
  ...
</trim>

• foreach
  动态 SQL 的另一个常见使用场景是对集合进行遍历（尤其是在构建 IN 条件语句的时候）。比如：

<select id="selectPostIn" resultType="domain.blog.Post">
  SELECT *
  FROM POST P
  WHERE ID in
  <foreach item="item" index="index" collection="list"
      open="(" separator="," close=")">
        #{item}
  </foreach>
</select>

• script
  要在带注解的映射器接口类中使用动态 SQL，可以使用 script 元素。比如:

@Update({"<script>",
      "update Author",
      "  <set>",
      "    <if test='username != null'>username=#{username},</if>",
      "    <if test='password != null'>password=#{password},</if>",
      "    <if test='email != null'>email=#{email},</if>",
      "    <if test='bio != null'>bio=#{bio}</if>",
      "  </set>",
      "where id=#{id}",
      "</script>"})
    void updateAuthorValues(Author author);

• bind
  bind 元素允许你在 OGNL 表达式以外创建一个变量，并将其绑定到当前的上下文。比如：

<select id="selectBlogsLike" resultType="Blog">
  <bind name="pattern" value="'%' + _parameter.getTitle() + '%'" />
  SELECT * FROM BLOG
  WHERE title LIKE #{pattern}
</select>

三、加载映射文件流程

入口：XMLConfigBuilder#mapperElement

解析全局配置文件中的<mappers>标签

/**
   * 解析<mappers>标签
   * @param parent  mappers标签对应的XNode对象
   * @throws Exception
   */
  private void mapperElement(XNode parent) throws Exception {
    if (parent != null) {
      // 获取<mappers>标签的子标签
      for (XNode child : parent.getChildren()) {
        // <package>子标签
        if ("package".equals(child.getName())) {
          // 获取mapper接口和mapper映射文件对应的package包名
          String mapperPackage = child.getStringAttribute("name");
          // 将包下所有的mapper接口以及它的代理对象存储到一个Map集合中，key为mapper接口类型，value为代理对象工厂
          configuration.addMappers(mapperPackage);
        } else {// <mapper>子标签
          // 获取<mapper>子标签的resource属性
          String resource = child.getStringAttribute("resource");
          // 获取<mapper>子标签的url属性
          String url = child.getStringAttribute("url");
          // 获取<mapper>子标签的class属性
          String mapperClass = child.getStringAttribute("class");
          // 它们是互斥的
          if (resource != null && url == null && mapperClass == null) {
            ErrorContext.instance().resource(resource);
            InputStream inputStream = Resources.getResourceAsStream(resource);
            // 专门用来解析mapper映射文件
            XMLMapperBuilder mapperParser = new XMLMapperBuilder(inputStream, configuration, resource, configuration.getSqlFragments());
            // 通过XMLMapperBuilder解析mapper映射文件
            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());
            // 通过XMLMapperBuilder解析mapper映射文件
            mapperParser.parse();
          } else if (resource == null && url == null && mapperClass != null) {
            Class<?> mapperInterface = Resources.classForName(mapperClass);
            // 将指定mapper接口以及它的代理对象存储到一个Map集合中，key为mapper接口类型，value为代理对象工厂
            configuration.addMapper(mapperInterface);
          } else {
            throw new BuilderException("A mapper element may only specify a url, resource or class, but not more than one.");
          }
        }
      }
    }
  }

流程图

加载映射文件流程.png

加载映射文件流程

一、<package>子标签

• 流程图

package子标签解析.png

• 1.Configuration#addMappers

将包下所有的mapper接口以及它的代理对象存储到一个Map集合中，key为mapper接口类型，value为代理对象工厂

public void addMappers(String packageName) {
    mapperRegistry.addMappers(packageName);
}

• 1.1MapperRegistry#addMappers

将Mapper接口添加到MapperRegistry中

//1
public void addMappers(String packageName) {
    addMappers(packageName, Object.class);
}

//2
public void addMappers(String packageName, Class<?> superType) {
    ResolverUtil<Class<?>> resolverUtil = new ResolverUtil<>();
    // 根据package名称，加载该包下Mapper接口文件（不是映射文件）
    resolverUtil.find(new ResolverUtil.IsA(superType), packageName);
    // 获取加载的Mapper接口
    Set<Class<? extends Class<?>>> mapperSet = resolverUtil.getClasses();
    for (Class<?> mapperClass : mapperSet) {
      // 将Mapper接口添加到MapperRegistry中
      addMapper(mapperClass);
    }
  }

//3
public <T> void addMapper(Class<T> type) {
    if (type.isInterface()) {
      // 如果Map集合中已经有该mapper接口的映射，就不需要再存储了
      if (hasMapper(type)) {
        throw new BindingException("Type " + type + " is already known to the MapperRegistry.");
      }
      boolean loadCompleted = false;
      try {
        // 将mapper接口以及它的代理对象存储到一个Map集合中，key为mapper接口类型，value为代理对象工厂
        knownMappers.put(type, new MapperProxyFactory<T>(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.
        
        // 用来解析注解方式的mapper接口
        MapperAnnotationBuilder parser = new MapperAnnotationBuilder(config, type);
        // 解析注解方式的mapper接口
        parser.parse();
        loadCompleted = true;
      } finally {
        if (!loadCompleted) {
          knownMappers.remove(type);
        }
      }
    }
  }

• 1.1.1.MapperAnnotationBuilder#parse

解析注解方式的mapper接口

public void parse() {
    // 获取mapper接口的全路径
    String resource = type.toString();
    // 是否解析过该mapper接口
    if (!configuration.isResourceLoaded(resource)) {
      // 先解析mapper映射文件
      loadXmlResource();
      // 设置解析标识
      configuration.addLoadedResource(resource);
      // Mapper构建者助手
      assistant.setCurrentNamespace(type.getName());
      // 解析CacheNamespace注解
      parseCache();
      // 解析CacheNamespaceRef注解
      parseCacheRef();
      Method[] methods = type.getMethods();
      for (Method method : methods) {
        try {
          // issue #237
          if (!method.isBridge()) {
            // 每个mapper接口中的方法，都解析成MappedStatement对象
            parseStatement(method);
          }
        } catch (IncompleteElementException e) {
          configuration.addIncompleteMethod(new MethodResolver(this, method));
        }
      }
    }
    //去检查所有的incompleteMethods,如果可以解析了.那就移除
    parsePendingMethods();
  }

• 1.1.1.1 MapperAnnotationBuilder#parseStatement

每个mapper接口中的方法，都解析成MappedStatement对象

void parseStatement(Method method) {
    // 获取Mapper接口的形参类型
    Class<?> parameterTypeClass = getParameterType(method);
    // 解析Lang注解
    LanguageDriver languageDriver = getLanguageDriver(method);
    // 
    SqlSource sqlSource = getSqlSourceFromAnnotations(method, parameterTypeClass, languageDriver);
    if (sqlSource != null) {
      Options options = method.getAnnotation(Options.class);
      // 组装mappedStatementId
      final String mappedStatementId = type.getName() + "." + method.getName();
      Integer fetchSize = null;
      Integer timeout = null;
      StatementType statementType = StatementType.PREPARED;
      ResultSetType resultSetType = null;
      // 获取该mapper接口中的方法是CRUD操作的哪一种
      SqlCommandType sqlCommandType = getSqlCommandType(method);
      // 是否是SELECT操作
      boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
      boolean flushCache = !isSelect;
      boolean useCache = isSelect;

      // 主键生成器，用于主键返回
      KeyGenerator keyGenerator;
      String keyProperty = null;
      String keyColumn = null;
      if (SqlCommandType.INSERT.equals(sqlCommandType) || SqlCommandType.UPDATE.equals(sqlCommandType)) {
        // first check for SelectKey annotation - that overrides everything else
        SelectKey selectKey = method.getAnnotation(SelectKey.class);
        if (selectKey != null) {
          keyGenerator = handleSelectKeyAnnotation(selectKey, mappedStatementId, getParameterType(method), languageDriver);
          keyProperty = selectKey.keyProperty();
        } else if (options == null) {
          keyGenerator = configuration.isUseGeneratedKeys() ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;
        } else {
          keyGenerator = options.useGeneratedKeys() ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;
          keyProperty = options.keyProperty();
          keyColumn = options.keyColumn();
        }
      } else {
        keyGenerator = NoKeyGenerator.INSTANCE;
      }

      if (options != null) {
        if (FlushCachePolicy.TRUE.equals(options.flushCache())) {
          flushCache = true;
        } else if (FlushCachePolicy.FALSE.equals(options.flushCache())) {
          flushCache = false;
        }
        useCache = options.useCache();
        fetchSize = options.fetchSize() > -1 || options.fetchSize() == Integer.MIN_VALUE ? options.fetchSize() : null; //issue #348
        timeout = options.timeout() > -1 ? options.timeout() : null;
        statementType = options.statementType();
        resultSetType = options.resultSetType();
      }

      // 处理ResultMap注解
      String resultMapId = null;
      ResultMap resultMapAnnotation = method.getAnnotation(ResultMap.class);
      if (resultMapAnnotation != null) {
        String[] resultMaps = resultMapAnnotation.value();
        StringBuilder sb = new StringBuilder();
        for (String resultMap : resultMaps) {
          if (sb.length() > 0) {
            sb.append(",");
          }
          sb.append(resultMap);
        }
        resultMapId = sb.toString();
      } else if (isSelect) {
        resultMapId = parseResultMap(method);
      }

      // 通过Mapper构建助手，创建一个MappedStatement对象，封装信息
      assistant.addMappedStatement(
          mappedStatementId,
          sqlSource,
          statementType,
          sqlCommandType,
          fetchSize,
          timeout,
          // ParameterMapID
          null,
          parameterTypeClass,
          resultMapId,
          getReturnType(method),
          resultSetType,
          flushCache,
          useCache,
          // TODO gcode issue #577
          false,
          keyGenerator,
          keyProperty,
          keyColumn,
          // DatabaseID
          null,
          languageDriver,
          // ResultSets
          options != null ? nullOrEmpty(options.resultSets()) : null);
    }
  }

• 1.1.1.1.2 MapperBuilderAssistant#addMappedStatement

通过Mapper构建助手，创建一个MappedStatement对象，封装信息

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;

    //利用构建者模式，去创建MappedStatement.Builder,用于创建MappedStatement对象
    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);
    }

    // 通过MappedStatement.Builder，构建一个MappedStatement
    MappedStatement statement = statementBuilder.build();
    // 将MappedStatement对象存储到Configuration中的Map集合中，key为statement的id，value为MappedStatement对象
    configuration.addMappedStatement(statement);
    return statement;
  }

二、<mapper>子标签

• 流程图

mapper子标签解析.png

• 1.XMLMapperBuilder#构造函数

  专门用来解析mapper映射文件

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

• 1.1XPathParser#构造函数

  用来使用XPath语法解析XML的解析器

public XPathParser(InputStream inputStream, boolean validation, Properties variables, EntityResolver entityResolver) {
    commonConstructor(validation, variables, entityResolver);
    // 解析XML文档为Document对象
    this.document = createDocument(new InputSource(inputStream));
  }

• 1.1.1 XPathParser#createDocument

  创建Mapper映射文件对应的Document对象

private Document createDocument(InputSource inputSource) {
    // important: this must only be called AFTER common constructor
    try {
      DocumentBuilderFactory factory = DocumentBuilderFactory.newInstance();
      // 进行dtd或者Schema校验
      factory.setValidating(validation);

      factory.setNamespaceAware(false);
      // 设置忽略注释为true
      factory.setIgnoringComments(true);
      // 设置是否忽略元素内容中的空白
      factory.setIgnoringElementContentWhitespace(false);
      factory.setCoalescing(false);
      factory.setExpandEntityReferences(true);

      DocumentBuilder builder = factory.newDocumentBuilder();
      builder.setEntityResolver(entityResolver);
      builder.setErrorHandler(new ErrorHandler() {
        @Override
        public void error(SAXParseException exception) throws SAXException {
          throw exception;
        }

        @Override
        public void fatalError(SAXParseException exception) throws SAXException {
          throw exception;
        }

        @Override
        public void warning(SAXParseException exception) throws SAXException {
        }
      });
      // 通过dom解析，获取Document对象
      return builder.parse(inputSource);
    } catch (Exception e) {
      throw new BuilderException("Error creating document instance.  Cause: " + e, e);
    }
  }

• 1.2 XMLMapperBuilder#构造函数

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;
  }

• 1.2.1MapperBuilderAssistant#构造函数

  用于构建MappedStatement对象的

public MapperBuilderAssistant(Configuration configuration, String resource) {
    super(configuration);
    ErrorContext.instance().resource(resource);
    this.resource = resource;
  }

• 2.XMLMapperBuilder#parse

通过XMLMapperBuilder解析mapper映射文件

public void parse() {
   // mapper映射文件是否已经加载过
   if (!configuration.isResourceLoaded(resource)) {
     // 从映射文件中的<mapper>根标签开始解析，直到完整的解析完毕
     configurationElement(parser.evalNode("/mapper"));
     // 标记已经解析
     configuration.addLoadedResource(resource);
     bindMapperForNamespace();
   }

   parsePendingResultMaps();
   parsePendingCacheRefs();
   parsePendingStatements();
 }

• 2.1 XMLMapperBuilder#configurationElement

  从映射文件中的<mapper>根标签开始解析，直到完整的解析完毕

 /**
   * 解析映射文件
   * @param context 映射文件根节点<mapper>对应的XNode
   */
  private void configurationElement(XNode context) {
    try {
      // 获取<mapper>标签的namespace值，也就是命名空间
      String namespace = context.getStringAttribute("namespace");
      // 命名空间不能为空
      if (namespace == null || namespace.equals("")) {
        throw new BuilderException("Mapper's namespace cannot be empty");
      }
      
      // 设置当前的命名空间为namespace的值
      builderAssistant.setCurrentNamespace(namespace);
      // 解析<cache-ref>子标签
      cacheRefElement(context.evalNode("cache-ref"));
      // 解析<cache>子标签
      cacheElement(context.evalNode("cache"));
      
      // 解析<parameterMap>子标签
      parameterMapElement(context.evalNodes("/mapper/parameterMap"));
      // 解析<resultMap>子标签
      resultMapElements(context.evalNodes("/mapper/resultMap"));
      // 解析<sql>子标签，也就是SQL片段
      sqlElement(context.evalNodes("/mapper/sql"));
      // 解析<select>\<insert>\<update>\<delete>子标签
      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);
    }
  }

• 2.1.1 XMLMapperBuilder#buildStatementFromContext

  用来创建MappedStatement对象的

//1、构建MappedStatement
private void buildStatementFromContext(List<XNode> list) {
    if (configuration.getDatabaseId() != null) {
      buildStatementFromContext(list, configuration.getDatabaseId());
    }
    // 构建MappedStatement
    buildStatementFromContext(list, null);
  }

//2、专门用来解析MappedStatement
private void buildStatementFromContext(List<XNode> list, String requiredDatabaseId) {
    for (XNode context : list) {
      // MappedStatement解析器
      final XMLStatementBuilder statementParser = new XMLStatementBuilder(configuration, builderAssistant, context, requiredDatabaseId);
      try {
        // 解析select等4个标签，创建MappedStatement对象
        statementParser.parseStatementNode();
      } catch (IncompleteElementException e) {
        configuration.addIncompleteStatement(statementParser);
      }
    }
  }

• 2.1.1.1 XMLStatementBuilder#构造函数

  专门用来解析MappedStatement

public XMLStatementBuilder(Configuration configuration, MapperBuilderAssistant builderAssistant, XNode context, String databaseId) {
    super(configuration);
    this.builderAssistant = builderAssistant;
    this.context = context;
    this.requiredDatabaseId = databaseId;
  }

• 2.1.1.2 XMLStatementBuilder#parseStatementNode

  解析<select><insert><update><delete>子标签

/**
   * 解析<select>\<insert>\<update>\<delete>子标签
   */
  public void parseStatementNode() {
    // 获取statement的id属性（特别关键的值）
    String id = context.getStringAttribute("id");
    String databaseId = context.getStringAttribute("databaseId");

    if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {
      return;
    }

    Integer fetchSize = context.getIntAttribute("fetchSize");
    Integer timeout = context.getIntAttribute("timeout");
    String parameterMap = context.getStringAttribute("parameterMap");
    // 获取入参类型
    String parameterType = context.getStringAttribute("parameterType");
    // 别名处理，获取入参对应的Java类型
    Class<?> parameterTypeClass = resolveClass(parameterType);
    // 获取ResultMap
    String resultMap = context.getStringAttribute("resultMap");
    // 获取结果映射类型
    String resultType = context.getStringAttribute("resultType");
    String lang = context.getStringAttribute("lang");
    LanguageDriver langDriver = getLanguageDriver(lang);
    
    // 别名处理，获取返回值对应的Java类型
    Class<?> resultTypeClass = resolveClass(resultType);
    String resultSetType = context.getStringAttribute("resultSetType");
    
    // 设置默认StatementType为Prepared，该参数指定了后面的JDBC处理时，采用哪种Statement
    StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));
    ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);

    String nodeName = context.getNode().getNodeName();
    // 解析SQL命令类型是什么？确定操作是CRUD中的哪一种
    SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH));
    //是否查询语句
    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
    // <include>标签解析
    XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
    includeParser.applyIncludes(context.getNode());

    // Parse selectKey after includes and remove them.
    // 解析<selectKey>标签
    processSelectKeyNodes(id, parameterTypeClass, langDriver);
    
    // Parse the SQL (pre: <selectKey> and <include> were parsed and removed)
    // 创建SqlSource，解析SQL，封装SQL语句（未参数绑定）和入参信息
    SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
   
    String resultSets = context.getStringAttribute("resultSets");
    String keyProperty = context.getStringAttribute("keyProperty");
    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 {
      keyGenerator = context.getBooleanAttribute("useGeneratedKeys",
          configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType))
          ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;
    }

    // 通过构建者助手，创建MappedStatement对象
    builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
        fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
        resultSetTypeEnum, flushCache, useCache, resultOrdered, 
        keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
  }

• 2.1.1.2.1 MapperBuilderAssistant#addMappedStatement

  通过构建者助手，创建MappedStatement对象

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;

    //利用构建者模式，去创建MappedStatement.Builder,用于创建MappedStatement对象
    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);
    }

    // 通过MappedStatement.Builder，构建一个MappedStatement
    MappedStatement statement = statementBuilder.build();
    // 将MappedStatement对象存储到Configuration中的Map集合中，key为statement的id，value为MappedStatement对象
    configuration.addMappedStatement(statement);
    return statement;
  }

• 2.1.1.2.1.1 MappedStatement.Builder#构造函数

  利用构建者模式，去创建MappedStatement.Builder,用于创建MappedStatement对象

public Builder(Configuration configuration, String id, SqlSource sqlSource, SqlCommandType sqlCommandType) {
      mappedStatement.configuration = configuration;
      mappedStatement.id = id;
      mappedStatement.sqlSource = sqlSource;
      mappedStatement.statementType = StatementType.PREPARED;
      mappedStatement.resultSetType = ResultSetType.DEFAULT;
      mappedStatement.parameterMap = new ParameterMap.Builder(configuration, "defaultParameterMap", null, new ArrayList<>()).build();
      mappedStatement.resultMaps = new ArrayList<>();
      mappedStatement.sqlCommandType = sqlCommandType;
      mappedStatement.keyGenerator = configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType) ? Jdbc3KeyGenerator.INSTANCE : NoKeyGenerator.INSTANCE;
      String logId = id;
      if (configuration.getLogPrefix() != null) {
        logId = configuration.getLogPrefix() + id;
      }
      mappedStatement.statementLog = LogFactory.getLog(logId);
      mappedStatement.lang = configuration.getDefaultScriptingLanguageInstance();
    }

• 2.1.1.2.1.2 MappedStatement#build

  通过MappedStatement.Builder，构建一个MappedStatement

public MappedStatement build() {
      assert mappedStatement.configuration != null;
      assert mappedStatement.id != null;
      assert mappedStatement.sqlSource != null;
      assert mappedStatement.lang != null;
      mappedStatement.resultMaps = Collections.unmodifiableList(mappedStatement.resultMaps);
      return mappedStatement;
    }