<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>

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

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

// 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);

/*** 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));}}

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

<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>

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

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