（Mapper用于映射SQL语句，可以说是MyBatis操作数据库的核心特性之一，这里我们讨论java的MyBatis框架中Mapper映射配置的使用及原理解析，包括对mapper.xml配置文件的读取流程解读）
Mapper的内置方法
model层就是实体类，对应数据库的表。controller层是Servlet，主要是负责业务模块流程的控制，调用service接口的方法，在struts2就是Action。Service层主要做逻辑判断，Dao层是数据访问层（与数据库进行对接）。至于Mapper是mybatis框架映射用到的，而mapper映射文件在dao层使用。
下面是介绍一下Mapper的内置方法：
1、countByExample => 根据条件查询数量
int countByExample(UserExample example); //下面是一个完整的案列 UserExample example = new UserExample(); Criteria criteria = example.createCriteria(); criteria.andUsernameEqualTo("joe"); int count = userDAO.countByExample(example);
相当于：select count(*) from user where username=’joe’
2、deleteByExample => 根据条件删除多条
int deleteByExample(AccountExample example); //下面是一个完整的案例 UserExample example=new UserExample(); Criteria criteria=example.createCriteria(); criteria.andUsernameEqualTo("joe"); userDao.deleteByExample(example);
相当于：delete from user where username=’joe’
3、deleteByPrimaryKey => 根据条件删除单条
int deleteByPrimaryKey(Integer id); userDao.deleteByPrimaryKey(101);
相当于：delete from user where id=101
4、insert => 插入数据
int insert(Account record); //下面是完整的案例 User user=new User(); user.setId(101); user.setUsername("test"); user.setPassword("123456"); user.setEmail("123@qq.com"); userDao.insert(user);
相当于：insert into user (id,username,password,email) values (101,’test’,’123456′,’1232qq.com’)
5、selectByExample => 根据条件查询数据
List<Account> selectByExample(AccountExample example); //下面是一个完整的案例 UserExample example=new UserExample(); Criteria criteria=example.createCriteria(); criteria.andUsernameEqualTo("joe"); criteria.andUsernameIsNull(); example.setOrderByClause("username asc,email desc"); List<?> list=userDao.selectByExample(example);
相当于：select * from user where username=’joe’ and username is null order by username asc,email desc
注：在iBator生成的文件UserExample.java中包含一个static的内部类Criteria，在Criteria中有很多方法，主要是定义SQL语句where后的查询条件
6、updateByExampleSelective => 按条件更新不为null的字段
int updateByExampleSelective(@Param("record")Account record,@Param("example")AccountExample example); //下面是一个完整的案例 UserExample example = new UserExample(); Criteria criteria = example.createCriteria(); criteria.andUsernameEqualTo("joe"); User user = new User(); user.setPassword("123"); userDAO.updateByPrimaryKeySelective(user,example);
相当于：update user set password=’123′ where username=’joe’
7、updateByPrimaryKeySelective => 按条件更新
int updateByPrimaryKeySelective(Account record); //下面是完整的案例 User user=new User(); user.setId(101); user.setPassword("joe"); userDAO.updateByPrimaryKeySelective(user);
相当于：update user set password=’joe’ where id=101
8、updateByPrimaryKey => 按主键更新
int updateByPrimaryKey(Account record); //下面是一个完整的案例 User user =new User(); user.setId(101); user.setUsername("joe"); user.setPassword("joe"); user.setEmail("joe@163.com"); userDAO.updateByPrimaryKey(user);
相当于：update user set username=’joe’,password=’joe’,email=’joe@163.com’ where id=101
解析mapper的xml配置文件
我们来看看mybatis是怎么读取mapper的xml配置文件并解析其中的sql语句。
我们还记得是这样配置sqlSessionFactory的：
<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.mybatis.model" />
</bean> 这里配置了一个mapperLocation属性，它是一个表达式，sqlSessionFactory会根据这个表达式读取包com.xxx.mybatis.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buildSqlSessionFactory方法中：
if(!isEmpty(this.mapperLocations)){ for(Resource mapperLocation:this.mapperLocations){ 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.isDebugEnablec()){ logger.debug("Parsed mapper file:'"+mapperLocation+"'"); } }
mybatis使用XMLMapperBuilder类的实例来解析mapper配置文件。
public XMLMapperBuilder(Reader reader,configuration configuration,String resource,Map<String,XNode> sqlFragments){ this(new XPathParser(reader,true,configuration.getVariables(),new XMLMapperEntityResolver()),configuration,resource,sqlFragments); } 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
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节点的属性以及内容， //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.addCloadedResource(respurce); 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：
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"); //设置当前的namespace builderAssistant.setCuttentNamespace(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mybatis解析了mapper中的所有节点，并将其加入到了Configuration对象中提供sqlSessionFactory对象随时使用。这里我们需要补充讲一下mybatis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
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mybatis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mybatis怎么获取到mapper并执行其中的sql语句的问题。
