Question

怎么没有mybatis源码解析相关的文档

Answer 1

我们还记得是这样配置sqlSessionFactory的：
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<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
<property name="dataSource" ref="dataSource" />
<property name="configLocation" value="classpath:configuration.xml"></property>
<property name="mapperLocations" value="classpath:com/xxx/mybatis/mapper/*.xml"/>
<property name="typeAliasesPackage" value="com.tiantian.mybatis.model" />
</bean>
这里配置了一个mapperLocations属性，它是一个表达式，sqlSessionFactory会根据这个表达式读取包com.xxx.mybaits.mapper下面的所有xml格式文件，那么具体是怎么根据这个属性来读取配置文件的呢？
答案就在SqlSessionFactoryBean类中的buildSqlSessionFactory方法中：

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if (!isEmpty(this.mapperLocations)) {
for (Resource mapperLocation : this.mapperLocations) {
if (mapperLocation == null) {
continue;
}

try {
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 (logger.isDebugEnabled()) {
logger.debug("Parsed mapper file: '" + mapperLocation + "'");
}
}
}

mybatis使用XMLMapperBuilder类的实例来解析mapper配置文件。
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public XMLMapperBuilder(Reader reader, Configuration configuration, String resource, Map<String, XNode> sqlFragments) {
this(new XPathParser(reader, true, configuration.getVariables(), new XMLMapperEntityResolver()),
configuration, resource, sqlFragments);
}

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private XMLMapperBuilder(XPathParser parser, Configuration configuration, String resource, Map<String, XNode> sqlFragments) {
super(configuration);
this.builderAssistant = new MapperBuilderAssistant(configuration, resource);
this.parser = parser;
this.sqlFragments = sqlFragments;
this.resource = resource;
}
接着系统调用xmlMapperBuilder的parse方法解析mapper。

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public void parse() {
//如果configuration对象还没加载xml配置文件（避免重复加载，实际上是确认是否解析了mapper节点的属性及内容，
//为解析它的子节点如cache、sql、select、resultMap、parameterMap等做准备），
//则从输入流中解析mapper节点，然后再将resource的状态置为已加载
if (!configuration.isResourceLoaded(resource)) {
configurationElement(parser.evalNode("/mapper"));
configuration.addLoadedResource(resource);
bindMapperForNamespace();
}
//解析在configurationElement函数中处理resultMap时其extends属性指向的父对象还没被处理的<resultMap>节点
parsePendingResultMaps();
//解析在configurationElement函数中处理cache-ref时其指向的对象不存在的<cache>节点(如果cache-ref先于其指向的cache节点加载就会出现这种情况)
parsePendingChacheRefs();
//同上，如果cache没加载的话处理statement时也会抛出异常
parsePendingStatements();
}

mybatis解析mapper的xml文件的过程已经很明显了，接下来我们看看它是怎么解析mapper的：
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private void configurationElement(XNode context) {
try {
//获取mapper节点的namespace属性
String namespace = context.getStringAttribute("namespace");
if (namespace.equals("")) {
throw new BuilderException("Mapper's namespace cannot be empty");
}
//设置当前namespace
builderAssistant.setCurrentNamespace(namespace);
//解析mapper的<cache-ref>节点
cacheRefElement(context.evalNode("cache-ref"));
//解析mapper的<cache>节点
cacheElement(context.evalNode("cache"));
//解析mapper的<parameterMap>节点
parameterMapElement(context.evalNodes("/mapper/parameterMap"));
//解析mapper的<resultMap>节点
resultMapElements(context.evalNodes("/mapper/resultMap"));
//解析mapper的<sql>节点
sqlElement(context.evalNodes("/mapper/sql"));
//使用XMLStatementBuilder的对象解析mapper的<select>、<insert>、<update>、<delete>节点,
//mybaits会使用MappedStatement.Builder类build一个MappedStatement对象,
//所以mybaits中一个sql对应一个MappedStatement
buildStatementFromContext(context.evalNodes("select|insert|update|delete"));
} catch (Exception e) {
throw new BuilderException("Error parsing Mapper XML. Cause: " + e, e);
}
}
configurationElement函数几乎解析了mapper节点下所有子节点，至此mybaits解析了mapper中的所有节点，并将其加入到了Configuration对象中提供给sqlSessionFactory对象随时使用。这里我们需要补充讲一下mybaits是怎么使用XMLStatementBuilder类的对象的parseStatementNode函数借用MapperBuilderAssistant类对象builderAssistant的addMappedStatement解析MappedStatement并将其关联到Configuration类对象的：

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public void parseStatementNode() {
//ID属性
String id = context.getStringAttribute("id");
//databaseId属性
String databaseId = context.getStringAttribute("databaseId");

if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) {
return;
}
//fetchSize属性
Integer fetchSize = context.getIntAttribute("fetchSize");
//timeout属性
Integer timeout = context.getIntAttribute("timeout");
//parameterMap属性
String parameterMap = context.getStringAttribute("parameterMap");
//parameterType属性
String parameterType = context.getStringAttribute("parameterType");
Class<?> parameterTypeClass = resolveClass(parameterType);
//resultMap属性
String resultMap = context.getStringAttribute("resultMap");
//resultType属性
String resultType = context.getStringAttribute("resultType");
//lang属性
String lang = context.getStringAttribute("lang");
LanguageDriver langDriver = getLanguageDriver(lang);

Class<?> resultTypeClass = resolveClass(resultType);
//resultSetType属性
String resultSetType = context.getStringAttribute("resultSetType");
StatementType statementType = StatementType.valueOf(context.getStringAttribute("statementType", StatementType.PREPARED.toString()));
ResultSetType resultSetTypeEnum = resolveResultSetType(resultSetType);

String nodeName = context.getNode().getNodeName();
SqlCommandType sqlCommandType = SqlCommandType.valueOf(nodeName.toUpperCase(Locale.ENGLISH));
//是否是<select>节点
boolean isSelect = sqlCommandType == SqlCommandType.SELECT;
//flushCache属性
boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect);
//useCache属性
boolean useCache = context.getBooleanAttribute("useCache", isSelect);
//resultOrdered属性
boolean resultOrdered = context.getBooleanAttribute("resultOrdered", false);

// Include Fragments before parsing
XMLIncludeTransformer includeParser = new XMLIncludeTransformer(configuration, builderAssistant);
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);
//resultSets属性
String resultSets = context.getStringAttribute("resultSets");
//keyProperty属性
String keyProperty = context.getStringAttribute("keyProperty");
//keyColumn属性
String keyColumn = context.getStringAttribute("keyColumn");
KeyGenerator keyGenerator;
String keyStatementId = id + SelectKeyGenerator.SELECT_KEY_SUFFIX;
keyStatementId = builderAssistant.applyCurrentNamespace(keyStatementId, true);
if (configuration.hasKeyGenerator(keyStatementId)) {
keyGenerator = configuration.getKeyGenerator(keyStatementId);
} else {
//useGeneratedKeys属性
keyGenerator = context.getBooleanAttribute("useGeneratedKeys",
configuration.isUseGeneratedKeys() && SqlCommandType.INSERT.equals(sqlCommandType))
? new Jdbc3KeyGenerator() : new NoKeyGenerator();
}

builderAssistant.addMappedStatement(id, sqlSource, statementType, sqlCommandType,
fetchSize, timeout, parameterMap, parameterTypeClass, resultMap, resultTypeClass,
resultSetTypeEnum, flushCache, useCache, resultOrdered,
keyGenerator, keyProperty, keyColumn, databaseId, langDriver, resultSets);
}
由以上代码可以看出mybaits使用XPath解析mapper的配置文件后将其中的resultMap、parameterMap、cache、statement等节点使用关联的builder创建并将得到的对象关联到configuration对象中，而这个configuration对象可以从sqlSession中获取的，这就解释了我们在使用sqlSession对数据库进行操作时mybaits怎么获取到mapper并执行其中的sql语句的问题。