Mapper的內建方法
model層就是實體類，對應資料庫的表。controller層是Servlet，主要是負責業務模組流程的控制，呼叫service介面的方法，在struts2就是Action。Service層主要做邏輯判斷，Dao層是資料訪問層，與資料庫進行對接。至於Mapper是mybtis框架的對映用到,mapper對映檔案在dao層用。

下面是介紹一下Mapper的內建方法：

1、countByExample ===>根據條件查詢數量

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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

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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

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1 2	int deleteByPrimaryKey(Integer id); userDAO.deleteByPrimaryKey(101);

相當於：

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1 2	delete from user where id=101

4、insert===>插入資料

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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("[email protected]"); userDAO.insert(user);

 相當於：

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1	insert into user(ID,username,password,email) values(101,'test','123456','[email protected]');

 5、insertSelective===>插入資料

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1	int insertSelective(Account record);

6、selectByExample===>根據條件查詢資料

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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 生成的檔案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後的查詢條件。

 7、selectByPrimaryKey===>根據主鍵查詢資料

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1	Account selectByPrimaryKey(Integer id);//相當於select * from user where id = 變數id

 
8、updateByExampleSelective===>按條件更新值不為null的欄位

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int updateByExampleSelective(@Param("record") Account record, @Param("example") AccountExample example); //下面是一個完整的案列 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'

 
9、updateByExampleSelective===>按條件更新

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1	int updateByExample(@Param("record") Account record, @Param("example") AccountExample example);

10、updateByPrimaryKeySelective===>按條件更新

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int updateByPrimaryKeySelective(Account record); //下面是一個完整的案例 User user = new User(); user.setId(101); user.setPassword("joe"); userDAO.updateByPrimaryKeySelective(user);

相當於：

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1	update user set password='joe' where id=101

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int updateByPrimaryKeySelective(Account record); //下面是一個完整的案例 User user = new User(); user.setId(101); user.setPassword("joe"); userDAO.updateByPrimaryKeySelective(user);

相當於：update user set password='joe' where id=101

11、updateByPrimaryKey===>按主鍵更新

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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("[email protected]"); userDAO.updateByPrimaryKey(user);

 相當於：

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1	update user set username='joe',password='joe',email='[email protected]' where id=101

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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("[email protected]"); userDAO.updateByPrimaryKey(user);

 相當於：

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1	update user set username='joe',password='joe',email='[email protected]' where id=101

 
解析mapper的xml配置檔案
我們來看看mybatis是怎麼讀取mapper的xml配置檔案並解析其中的sql語句。

我們還記得是這樣配置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>

這裡配置了一個mapperLocations屬性，它是一個表示式，sqlSessionFactory會根據這個表示式讀取包com.xxx.mybaits.mapper下面的所有xml格式檔案，那麼具體是怎麼根據這個屬性來讀取配置檔案的呢？

答案就在SqlSessionFactoryBean類中的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類的例項來解析mapper配置檔案。

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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); } 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; }

接著系統呼叫xmlMapperBuilder的parse方法解析mapper。

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public void parse() { //如果configuration物件還沒載入xml配置檔案（避免重複載入，實際上是確認是否解析了mapper節點的屬性及內容， //為解析它的子節點如cache、sql、select、resultMap、parameterMap等做準備）， //則從輸入流中解析mapper節點，然後再將resource的狀態置為已載入 if (!configuration.isResourceLoaded(resource)) { configurationElement(parser.evalNode("/mapper")); configuration.addLoadedResource(resource); bindMapperForNamespace(); } //解析在configurationElement函式中處理resultMap時其extends屬性指向的父物件還沒被處理的<resultMap>節點 parsePendingResultMaps(); //解析在configurationElement函式中處理cache-ref時其指向的物件不存在的<cache>節點(如果cache-ref先於其指向的cache節點載入就會出現這種情況) parsePendingChacheRefs(); //同上，如果cache沒載入的話處理statement時也會丟擲異常 parsePendingStatements(); }

mybatis解析mapper的xml檔案的過程已經很明顯了，接下來我們看看它是怎麼解析mapper的：

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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"); } //設定當前namespace builderAssistant.setCurrentNamespace(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的物件解析mapper的<select>、<insert>、<update>、<delete>節點, //mybaits會使用MappedStatement.Builder類build一個MappedStatement物件, //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函式幾乎解析了mapper節點下所有子節點，至此mybaits解析了mapper中的所有節點，並將其加入到了Configuration物件中提供給sqlSessionFactory物件隨時使用。這裡我們需要補充講一下mybaits是怎麼使用XMLStatementBuilder類的物件的parseStatementNode函數借用MapperBuilderAssistant類物件builderAssistant的addMappedStatement解析MappedStatement並將其關聯到Configuration類物件的：

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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); }

由以上程式碼可以看出mybaits使用XPath解析mapper的配置檔案後將其中的resultMap、parameterMap、cache、statement等節點使用關聯的builder建立並將得到的物件關聯到configuration物件中，而這個configuration物件可以從sqlSession中獲取的，這就解釋了我們在使用sqlSession對資料庫進行操作時mybaits怎麼獲取到mapper並執行其中的sql語句的問題。