Mybatis是如何实现SQL语句复用功能的？-轻识

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来源 | https://my.oschina.net/zudajun/blog/687326

今天，我们将分析Mybatis之sqlFragment，可以翻译为sql片段，它的存在价值在于可复用sql片段，避免到处重复编写。

在工作中，往往有这样的需求，对于同一个sql条件查询，首先需要统计记录条数，用以计算pageCount，然后再对结果进行分页查询显示，看下面一个例子。

<sql id="studentProperties">    select       stud_id as studId      , name, email      , dob      , phone    from students  sql>
  <select id="countAll" resultType="int">    select count(1) from (      <include refid="studentProperties">include>    ) tmp  select>
  <select id="findAll" resultType="Student" parameterType="map">    select * from (      <include refid="studentProperties">include>    ) tmp limit #{offset}, #{pagesize}  select>

这就是sqlFragment，它可以为select|insert|update|delete标签服务，可以定义很多sqlFragment，然后使用include标签引入多个sqlFragment。在工作中，也是比较常用的一个功能，它的优点很明显，复用sql片段，它的缺点也很明显，不能完整的展现sql逻辑，如果一个标签，include了四至五个sqlFragment，其可读性就非常差了。

sqlFragment里的内容是可以随意写的，它不需要是一个完整的sql，它可以是“,phone”这么简单的文本。

1.sqlFragment的解析过程

sqlFragment存储于Configuration内部。

protected final Map<String, XNode> sqlFragments = new StrictMap("XML fragments parsed from previous mappers");

解析sqlFragment的过程非常简单。

org.apache.ibatis.builder.xml.XMLMapperBuilder.configurationElement(XNode)方法部分源码。

// 解析sqlFragmentsqlElement(context.evalNodes("/mapper/sql"));// 为select|insert|update|delete提供服务buildStatementFromContext(context.evalNodes("select|insert|update|delete"));

sqlFragment存储于Map结构当中。其实最关键的，是它如何为select|insert|update|delete提供服务的。

2.select|insert|update|delete标签中，解析include标签的过程

org.apache.ibatis.builder.xml.XMLStatementBuilder.parseStatementNode()方法源码。

// Include Fragments before parsingXMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);// 重点关注的方法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);

注释“pre: and were parsed and removed”，含义为解析完，并移除。为什么要移除呢？秘密都隐藏在applyIncludes()方法内部了。

org.apache.ibatis.builder.xml.XMLIncludeTransformer.applyIncludes(Node, Properties)方法源码。

  /**   * Recursively apply includes through all SQL fragments.   * @param source Include node in DOM tree   * @param variablesContext Current context for static variables with values   */  private void applyIncludes(Node source, final Properties variablesContext) {    if (source.getNodeName().equals("include")) {      // new full context for included SQL - contains inherited context and new variables from current include node      Properties fullContext;
      String refid = getStringAttribute(source, "refid");      // replace variables in include refid value      refid = PropertyParser.parse(refid, variablesContext);      Node toInclude = findSqlFragment(refid);      Properties newVariablesContext = getVariablesContext(source, variablesContext);      if (!newVariablesContext.isEmpty()) {        // merge contexts        fullContext = new Properties();        fullContext.putAll(variablesContext);        fullContext.putAll(newVariablesContext);      } else {        // no new context - use inherited fully        fullContext = variablesContext;      }      // 递归调用      applyIncludes(toInclude, fullContext);      if (toInclude.getOwnerDocument() != source.getOwnerDocument()) {        toInclude = source.getOwnerDocument().importNode(toInclude, true);      }      // 将include节点，替换为sqlFragment节点      source.getParentNode().replaceChild(toInclude, source);      while (toInclude.hasChildNodes()) {        // 将sqlFragment的子节点（也就是文本节点），插入到sqlFragment的前面        toInclude.getParentNode().insertBefore(toInclude.getFirstChild(), toInclude);      }      // 移除sqlFragment节点      toInclude.getParentNode().removeChild(toInclude);    } else if (source.getNodeType() == Node.ELEMENT_NODE) {      NodeList children = source.getChildNodes();      for (int i=0; i        // 递归调用        applyIncludes(children.item(i), variablesContext);      }    } else if (source.getNodeType() == Node.ATTRIBUTE_NODE && !variablesContext.isEmpty()) {      // replace variables in all attribute values      source.setNodeValue(PropertyParser.parse(source.getNodeValue(), variablesContext));    } else if (source.getNodeType() == Node.TEXT_NODE && !variablesContext.isEmpty()) {      // replace variables ins all text nodes      source.setNodeValue(PropertyParser.parse(source.getNodeValue(), variablesContext));    }  }

上面是对源码的解读，为了便于理解，我们接下来采用图示的办法，演示其过程。

3.图示过程演示

①解析节点

<select id="countAll" resultType="int">    select count(1) from (      <include refid="studentProperties">include>    ) tmp  select>

②include节点替换为sqlFragment节点

<select id="countAll" resultType="int">    select count(1) from (        "studentProperties">          select             stud_id as studId            , name, email            , dob            , phone          from students        ) tmp  select>

③将sqlFragment的子节点（文本节点）insert到sqlFragment节点的前面。注意，对于dom来说，文本也是一个节点，叫TextNode。

<select id="countAll" resultType="int">    select count(1) from (        select             stud_id as studId            , name, email            , dob            , phone          from students        "studentProperties">          select             stud_id as studId            , name, email            , dob            , phone          from students        ) tmp  select>

④移除sqlFragment节点

<select id="countAll" resultType="int">    select count(1) from (        select             stud_id as studId            , name, email            , dob            , phone          from students) tmp  select>