Mybatis之sqlFragment（可复用的sql片段）

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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">
<!--sql片段-->select stud_id as studId, name, email, dob, phonefrom students
</sql>

<select id="countAll" resultType="int">select count(1) from (<include refid="studentProperties"></include><!--复用-->) tmp
</select><select id="findAll" resultType="Student" parameterType="map">select * from (<include refid="studentProperties"></include><!--复用-->) 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。

`sqlFragment`的解析过程

sqlFragment存储于Configuration内部。

protected final Map<String, XNode> sqlFragments = new StrictMap<XNode>("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sqlFragment
sqlElement(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<String, XNode>结构当中。其实最关键的，是它如何为select|insert|update|delete提供服务的。

`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 parsing
XMLIncludeTransformer 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: <selectKey> and <include> 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 nodeProperties fullContext;String refid = getStringAttribute(source, "refid");// replace variables in include refid valuerefid = PropertyParser.parse(refid, variablesContext);Node toInclude = findSqlFragment(refid);Properties newVariablesContext = getVariablesContext(source, variablesContext);if (!newVariablesContext.isEmpty()) {// merge contextsfullContext = new Properties();fullContext.putAll(variablesContext);fullContext.putAll(newVariablesContext);} else {// no new context - use inherited fullyfullContext = 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<children.getLength(); i++) {// 递归调用applyIncludes(children.item(i), variablesContext);}} else if (source.getNodeType() == Node.ATTRIBUTE_NODE && !variablesContext.isEmpty()) {// replace variables in all attribute valuessource.setNodeValue(PropertyParser.parse(source.getNodeValue(), variablesContext));} else if (source.getNodeType() == Node.TEXT_NODE && !variablesContext.isEmpty()) {// replace variables ins all text nodessource.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。

图示过程演示

解析节点

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

include节点替换为sqlFragment节点

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

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 count(1) from (
select
stud_id as studId
, name, email
, dob
, phone
from students

select
stud_id as studId
, name, email
, dob
, phone
from students

) tmp
移除sqlFragment节点

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

最终结果如图所示

如此一来，TextNode1 + TextNode2 + TextNode3，就组成了一个完整的sql。遍历select的三个子节点，分别取出TextNode的value，append到一起，就是最终完整的sql。
这也是为什么要移除 and 节点的原因。
这就是Mybatis的sqlFragment