Java的MyBatis框架中Mapper映射配置的使用及原理解析

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mybtis框架的映射用到,mapper映射文件在dao层用。

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("674531003@qq.com");
 userDAO.insert(user);

相当于：

insert into user(ID,username,password,email) values(101,'test','123456','674531003@qq.com');

5、insertSelective===>插入数据

int insertSelective(Account record);

6、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。

 7、selectByPrimaryKey===>根据主键查询数据

Account selectByPrimaryKey(Integer id);//相当于select * from user where id = 变量id

8、updateByExampleSelective===>按条件更新值不为null的字段

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

//下面是一个完整的案列
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'

9、updateByExampleSelective===>按条件更新

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

10、updateByPrimaryKeySelective===>按条件更新

int updateByPrimaryKeySelective(Account record);

//下面是一个完整的案例

 User user = new User();
user.setId(101);
user.setPassword("joe");
userDAO.updateByPrimaryKeySelective(user);

相当于：

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

int updateByPrimaryKeySelective(Account record);

//下面是一个完整的案例

 User user = new User();
user.setId(101);
user.setPassword("joe");
userDAO.updateByPrimaryKeySelective(user);

相当于：update user set password='joe' where id=101

11、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

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.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：

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。

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：

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：

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。