Mybatis目前最新版本为3.4.0,因此,我也将我的项目由3.3.1替换为3.4.0。在上一篇博文中,详细分析了Mybatis在使用foreach循环进行批量insert,返回主键id列表时,如果使用BatchExecutor,那么由于Mybatis存在bug,返回的id列表将是null值。很遗憾的告诉大家,Mybatis3.4.0依然是没有修复该bug的,该bug依然存在。
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
, phone
from 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。
1.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。
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 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: <selectKey> and <include> 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<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 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 (
<sql id="studentProperties">
select
stud_id as studId
, name, email
, dob
, phone
from students
</sql>
) 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
<sql id="studentProperties">
select
stud_id as studId
, name, email
, dob
, phone
from students
</sql>
) 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>
⑤最终结果如图所示
(Made In QQ截图及时编辑)
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<selectKey> and <include>节点的原因。
这就是Mybatis的sqlFragment,以上示例,均为静态sql,即static sql,有关动态sql,即dynamic sql,将在后续博文中进行仔细分析。
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