（Mapper用於映射SQL語句，可以說是MyBatis操作數據庫的核心特性之一，這里我們討論java的MyBatis框架中Mapper映射配置的使用及原理解析，包括對mapper.xml配置文件的讀取流程解讀）
Mapper的內置方法
model層就是實體類，對應數據庫的表。controller層是Servlet，主要是負責業務模塊流程的控制，調用service接口的方法，在struts2就是Action。Service層主要做邏輯判斷，Dao層是數據訪問層（與數據庫進行對接）。至於Mapper是mybatis框架映射用到的，而mapper映射文件在dao層使用。
下面是介紹一下Mapper的內置方法：
1、countByExample => 根據條件查詢數量
int countByExample(UserExample example); //下面是一個完整的案列 UserExample example = new UserExample(); Criteria criteria = example.createCriteria(); criteria.andUsernameEqualTo("joe"); int count = userDAO.countByExample(example);
相當於：select count(*) from user where username='joe'
2、deleteByExample => 根據條件刪除多條
int deleteByExample(AccountExample example); //下面是一個完整的案例 UserExample example=new UserExample(); Criteria criteria=example.createCriteria(); criteria.andUsernameEqualTo("joe"); userDao.deleteByExample(example);
相當於：delete from user where username='joe'
3、deleteByPrimaryKey => 根據條件刪除單條
int deleteByPrimaryKey(Integer id); userDao.deleteByPrimaryKey(101);
相當於：delete from user where id=101
4、insert => 插入數據
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);
相當於：insert into user (id,username,password,email) values (101,'test','123456','1232qq.com')
5、selectByExample => 根據條件查詢數據
List<Account> selectByExample(AccountExample example); //下面是一個完整的案例 UserExample example=new UserExample(); Criteria criteria=example.createCriteria(); criteria.andUsernameEqualTo("joe"); criteria.andUsernameIsNull(); example.setOrderByClause("username asc,email desc"); List<?> list=userDao.selectByExample(example);
相當於：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中包含一個static的內部類Criteria，在Criteria中有很多方法，主要是定義SQL語句where后的查詢條件
6、updateByExampleSelective => 按條件更新不為null的字段
int updateByExampleSelective(@Param("record")Account record,@Param("example")AccountExample example); //下面是一個完整的案例 UserExample example = new UserExample(); Criteria criteria = example.createCriteria(); criteria.andUsernameEqualTo("joe"); User user = new User(); user.setPassword("123"); userDAO.updateByPrimaryKeySelective(user,example);
相當於：update user set password='123' where username='joe'
7、updateByPrimaryKeySelective => 按條件更新
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);
相當於：update user set password='joe' where id=101
8、updateByPrimaryKey => 按主鍵更新
int updateByPrimaryKey(Account record); //下面是一個完整的案例 User user =new User(); user.setId(101); user.setUsername("joe"); user.setPassword("joe"); user.setEmail("joe@163.com"); userDAO.updateByPrimaryKey(user);
相當於：update user set username='joe',password='joe',email='joe@163.com' where id=101
解析mapper的xml配置文件
我們來看看mybatis是怎么讀取mapper的xml配置文件並解析其中的sql語句。
我們還記得是這樣配置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>
這里配置了一個mapperLocation屬性，它是一個表達式，sqlSessionFactory會根據這個表達式讀取包com.xxx.mybatis.mapper下面的所有xml格式文件，那么具體是怎么根據這個屬性來讀取配置文件的呢？
答案就在SqlSessionFactoryBean類中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類的實例來解析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; }
接着系統調用xmlMapperBuilder的parse方法解析mapper
public void parse(){ //如果configuration對象還沒加載xml配置文件（避免重復加載，實際上是確認是否解析了mapper節點的屬性以及內容， //為解析它的子節點如cache、sql、select、resultMap、parameterMap等做准備）， //則從輸入流中解析mapper節點，然后再將resource的狀態置為已加載 if(!configuration.isResourceloaded(resource)){ configurationElement(parser.evalNode("/mapper")); configuration.addCloadedResource(respurce); bindMapperForNamespace(); } //解析在configurationElement函數中處理resultMap時其extends屬性指向的父對象還沒被處理的<resultMap>節點 parsePendingResultMaps(); //解析在configurationElement函數中處理cache-ref時其指向的對象不存在的<cache>節點(如果cache-ref先於其指向的cache節點加載就會出現這種情況) parsePendingChacheRefs(); //同上，如果cache沒加載的話處理statement時也會拋出異常 parsePendingStatements(); }
mybatis解析mapper的xml文件的過程已經很明顯了，接下來我們看看它是怎么解析mapper的：
private void configurationElement(XNode context) { try{ //獲取mapper節點的namespace屬性 String namespace=context.getStringAttribute("namespace"); if(namespace.equals("")){ throw new BuilderException("Mapper's namespace cannot be empty"); //設置當前的namespace builderAssistant.setCuttentNamespace(namespace); //解析mapper的<cache-ref>節點 cacheRefElement(context.evalNode("cache-ref")); //解析mapper的<cache>節點 cacheElement(context.evalNode("cache")); //解析mapper的<parameterMap>節點 parameterMapElement(context.evalNodes("/mapper/parameterMap")); //解析mapper的<resultMap>節點 resultMapElements(context.evalNodes("/mapper/resultMap")); //解析mapper的<sql>節點 sqlElement(context.evalNodes("/mapper/sql")); //使用XMLStatementBuilder的對象解析mapper的<select>、<insert>、<update>、<delete>節點, //mybaits會使用MappedStatement.Builder類build一個MappedStatement對象, //所以mybaits中一個sql對應一個MappedStatement buildStatementFromContext(context.evalNodes("select|insert|update|delete")); } }catch(Exception e){ throw new BuilderException("Error parsing Mapper XML. Cause: " + e, e); } }
configurationElement函數幾乎解析了mapper節點下所有子節點，至此mybatis解析了mapper中的所有節點，並將其加入到了Configuration對象中提供sqlSessionFactory對象隨時使用。這里我們需要補充講一下mybatis是怎么使用XMLStatementBuilder類的對象的parseStatementNode函數借用MapperBuilderAssistant類對象builderAssistant的addMappedStatement解析MappedStatement並將其關聯到Configuration類對象的
public void parseStatementNode() { //ID屬性 String id = context.getStringAttribute("id"); //databaseId屬性 String databaseId = context.getStringAttribute("databaseId"); if (!databaseIdMatchesCurrent(id, databaseId, this.requiredDatabaseId)) { return; } //fetchSize屬性 Integer fetchSize = context.getIntAttribute("fetchSize"); //timeout屬性 Integer timeout = context.getIntAttribute("timeout"); //parameterMap屬性 String parameterMap = context.getStringAttribute("parameterMap"); //parameterType屬性 String parameterType = context.getStringAttribute("parameterType"); Class<?> parameterTypeClass = resolveClass(parameterType); //resultMap屬性 String resultMap = context.getStringAttribute("resultMap"); //resultType屬性 String resultType = context.getStringAttribute("resultType"); //lang屬性 String lang = context.getStringAttribute("lang"); LanguageDriver langDriver = getLanguageDriver(lang); Class<?> resultTypeClass = resolveClass(resultType); //resultSetType屬性 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>節點 boolean isSelect = sqlCommandType == SqlCommandType.SELECT; //flushCache屬性 boolean flushCache = context.getBooleanAttribute("flushCache", !isSelect); //useCache屬性 boolean useCache = context.getBooleanAttribute("useCache", isSelect); //resultOrdered屬性 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屬性 String resultSets = context.getStringAttribute("resultSets"); //keyProperty屬性 String keyProperty = context.getStringAttribute("keyProperty"); //keyColumn屬性 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屬性 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); }
由以上代碼可以看出mybatis使用XPath解析mapper的配置文件后將其中的resultMap、parameterMap、cache、statement等節點使用關聯的builder創建並將得到的對象關聯到configuration對象中，而這個configuration對象可以從sqlSession中獲取的，這就解釋了我們在使用sqlSession對數據庫進行操作時mybatis怎么獲取到mapper並執行其中的sql語句的問題。