通过阅读源码对实现机制进行了解有利于陶冶情操，承接前文Spring mybatis源码篇章-Mybatis主文件加载
前话
前文主要讲解了Mybatis的主文件加载方式，本文则分析不使用主文件加载方式，即SqlSessionFactoryBean对象不配置configLocation属性而配置mapperLocations属性
<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
<property name="mapperLocations" value="classpath:com/du/wx/resources/mapper/*.xml" />
</bean>
其中每个mapper文件均类似如下样例
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="com.du.wx.mapper.joggle.ActivityBookingDao">
<insert id="addBookingInfo">
insert into booking_activity_infos(uid,activityid,isbooked)
values(#{uid},#{activityid},#{isbooked})
</insert>
<select id="queryCountBookingByAct" resultType="int">
select count(*) from booking_activity_infos
where
activityid=#{0}
</select>
<select id="queryCountBookingByUser" resultType="int">
select count(*) from booking_activity_infos
where
uid=#{0}
</select>
<select id="queryUsersByAct" resultType="BaseUser">
select a.uid,a.openid,a.nickname,a.phone,a.email,a.address
from booking_activity_infos b
right join
base_user a on(b.uid=a.uid)
where b.activityid=#{0} and b.isbooked=1
</select>
<select id="queryActByUser" resultType="Activity">
select a.id,a.title,a.content,a.url,a.address,a.time,a.type,a.status,a.limit,a.people
from activity a
left join
booking_activity_infos b
on(a.id=b.activityid)
where b.uid=#{0} and b.isbooked=1
</select>
<select id="queryActBooking" resultType="ActivityBooking">
select * from booking_activity_infos
where uid=#{0} and activityid=#{1}
</select>
<update id="updateBooking">
update booking_activity_infos
set
isbooked=#{2}
where uid=#{0} and activityid=#{1}
</update>
</mapper>
SqlSessionFactoryBean读取mapper配置文件集合
代码片段如下
if (!isEmpty(this.mapperLocations)) {
//具体的如何从string转为Resource[]，属于spring的基础操作了，有兴趣的读者可自行查阅
for (Resource mapperLocation : this.mapperLocations) {
if (mapperLocation == null) {
continue;
}
try {
//对扫描包及其子包下的每个mapper配置文件进行解析
XMLMapperBuilder xmlMapperBuilder = new XMLMapperBuilder(mapperLocation.getInputStream(),
configuration, mapperLocation.toString(), configuration.getSqlFragments());
xmlMapperBuilder.parse();
} catch (Exception e) {
throw new NestedIOException("Failed to parse mapping resource: '" + mapperLocation + "'", e);
} finally {
ErrorContext.instance().reset();
}
if (this.logger.isDebugEnabled()) {
this.logger.debug("Parsed mapper file: '" + mapperLocation + "'");
}
}
} else {
if (this.logger.isDebugEnabled()) {
this.logger.debug("Property 'mapperLocations' was not specified or no matching resources found");
}
}
主要是遍历mapper集合，然后对每个XML文件都通过XMLMapperBuilder对象来进行解析。这在前文也有提及
XMLMapperBuilder
笔者直接去观察其parse()方法，代码如下
public void parse() {
//对每个xml资源只加载一次
if (!configuration.isResourceLoaded(resource)) {
//解析xml配置，其中配置的根节点必须为mapper
configurationElement(parser.evalNode("/mapper"));
//已加载
configuration.addLoadedResource(resource);
//绑定mapper的工作区间
bindMapperForNamespace();
}
// 弥补措施
parsePendingResultMaps();
parsePendingChacheRefs();
parsePendingStatements();
}
紧接着观察XMLMapperBuilder#configurationElement()方法，其来解析mapper文件
private void configurationElement(XNode context) {
try {
//表明mapper根节点的namespace属性是必须的，且不为空
String namespace = context.getStringAttribute("namespace");
if (namespace.equals("")) {
throw new BuilderException("Mapper's namespace cannot be empty");
}
//设置工作区间
builderAssistant.setCurrentNamespace(namespace);
//解析相应的属性
cacheRefElement(context.evalNode("cache-ref"));
cacheElement(context.evalNode("cache"));
//解析<parameterMap>节点集合
parameterMapElement(context.evalNodes("/mapper/parameterMap"));
//解析<resultMap>节点集合
resultMapElements(context.evalNodes("/mapper/resultMap"));
//解析<sql>节点集合
sqlElement(context.evalNodes("/mapper/sql"));
//创建MappedStatement,这里与注解方式的加载方式还是类似的
buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
} catch (Exception e) {
throw new BuilderException("Error parsing Mapper XML. Cause: " + e, e);
}
}
1.mapper文件的namespace属性不可或缺，否则会抛异常
2.mapper文件的parameterMap/resultMap/sql节点一般都是用来充当模板使用的
3.mapper文件对应的数据库的增删改查节点分别为insert|delete|update|select
笔者稍微对上述的节点作下简单的浏览
resultMap
解析mapper节点下的resultMap节点，相关代码如下
private ResultMap resultMapElement(XNode resultMapNode, List<ResultMapping> additionalResultMappings) throws Exception {
ErrorContext.instance().activity("processing " + resultMapNode.getValueBasedIdentifier());
//读取id属性，最好配置以免不必要的错误
String id = resultMapNode.getStringAttribute("id",
resultMapNode.getValueBasedIdentifier());
//优先级为type>ofType>resultType>javaType
String type = resultMapNode.getStringAttribute("type",
resultMapNode.getStringAttribute("ofType",
resultMapNode.getStringAttribute("resultType",
resultMapNode.getStringAttribute("javaType"))));
String extend = resultMapNode.getStringAttribute("extends");
//是否开启自动映射，默认值为unset
Boolean autoMapping = resultMapNode.getBooleanAttribute("autoMapping");
Class<?> typeClass = resolveClass(type);
//<discriminator><case /><case/></discriminator>根据结果值进行结果类型的映射，类似java的switch-case语法
Discriminator discriminator = null;
//ResultMap节点信息转化为ResultMapping集合
List<ResultMapping> resultMappings = new ArrayList<ResultMapping>();
resultMappings.addAll(additionalResultMappings);
List<XNode> resultChildren = resultMapNode.getChildren();
for (XNode resultChild : resultChildren) {
if ("constructor".equals(resultChild.getName())) {
//<resultMap>节点下<constructor>节点处理
processConstructorElement(resultChild, typeClass, resultMappings);
} else if ("discriminator".equals(resultChild.getName())) {
//<resultMap>节点下<discriminator>节点处理
discriminator = processDiscriminatorElement(resultChild, typeClass, resultMappings);
} else {
//<id>/<result>/<collection>/<association>节点的解析
ArrayList<ResultFlag> flags = new ArrayList<ResultFlag>();
if ("id".equals(resultChild.getName())) {
flags.add(ResultFlag.ID);
}
resultMappings.add(buildResultMappingFromContext(resultChild, typeClass, flags));
}
}
ResultMapResolver resultMapResolver = new ResultMapResolver(builderAssistant, id, typeClass, extend, discriminator, resultMappings, autoMapping);
try {
//组装成ResultMap对象保存到Configuration对象的私有集合变量resultMaps
return resultMapResolver.resolve();
} catch (IncompleteElementException e) {
configuration.addIncompleteResultMap(resultMapResolver);
throw e;
}
}
具体的属性就不展开了，查阅相关的官方文档即可
sql
解析mapper节点下的sql节点。主要作用是将每个sql节点对象都保存到Configuration对象中的sqlFragments属性中(HashMap)。
private void sqlElement(List<XNode> list, String requiredDatabaseId) throws Exception {
for (XNode context : list) {
//sql节点的databaseId属性
String databaseId = context.getStringAttribute("databaseId");
//sql节点的id属性,id=${namespace}+"."+id
String id = context.getStringAttribute("id");
id = builderAssistant.applyCurrentNamespace(id, false);
//true的前提是主配置文件指定了databaseId属性或者主配置和sql节点的databaseId属性均不存在，但sql节点的id属性存在
if (databaseIdMatchesCurrent(id, databaseId, requiredDatabaseId)) sqlFragments.put(id, context);
}
}
sql节点在配置id属性时，可简单的配置别名而不用带上namespace前缀，代码会自动校验进行拼装~~
CRUD节点解析
即解析select/update/delete/insert节点对应的信息，笔者此处关注XMLStatementBuilder#parseStatementNode()方法的片段代码
//节点上支持的常见属性
Integer fetchSize = context.getIntAttribute("fetchSize");
Integer timeout = context.getIntAttribute("timeout");
String parameterMap = context.getStringAttribute("parameterMap");
String parameterType = context.getStringAttribute("parameterType");
Class<?> parameterTypeClass = resolveClass(parameterType);
String resultMap = context.getStringAttribute("resultMap");
String resultType = context.getStringAttribute("resultType");
String lang = context.getStringAttribute("lang");
LanguageDriver langDriver = getLanguageDriver(lang);
...
String resultSetType = context.getStringAttribute("resultSetType");
StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));
...
// Include Fragments before parsing 导入<include>标签内容，其内部可以含有<if>/<where>/<set>等标签
XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
includeParser.applyIncludes(context.getNode());
// Parse selectKey after includes and remove them.导入<selectKey>标签内容
processSelectKeyNodes(id, parameterTypeClass, langDriver);
// Parse the SQL (pre: <selectKey> and <include> were parsed and removed) 如何解析sql语句？放置下一章节讲解
SqlSource sqlSource = langDriver.createSqlSource(configuration, context, parameterTypeClass);
...
//创建MappedStatement对象，保存在Configuration的mappedStatement集合属性中
builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
resultSetTypeEnum, flushCache, useCache, resultOrdered,
keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
代码其实很长，不过跟前文的注解方式的解析是类似的，最终都是生成MappedStatement对象。
而其会保存在org.apache.ibatis.session.Configuration对象中~~~~
总结
1.不管是通过注解模式还是XML文件模式，都会生成MappedStatement对象保存到Configuration对象中
2.每个select|update|insert|delete标签均会被解析为单个MappedStatement对象，其中唯一ID为
${namespace}.${id}3.Spring官方以及笔者都建议通过此方式来加载指定的XML文件，但是细心的笔者发现这其实并没有与DAO接口关联起来，看来还是得借助org.mybatis.spring.mapper.MapperScannerConfigurer的力量