java中mapper层作用_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

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7int 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

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8int 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

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2int deleteByPrimaryKey(Integer id);

userDAO.deleteByPrimaryKey(101);

相当于：

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2delete from user where id=101

4、insert===>插入数据

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9int 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);

相当于：

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

5、insertSelective===>插入数据

1int insertSelective(Account record);

6、selectByExample===>根据条件查询数据

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12List 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 isnull 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

1Account 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

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10int 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

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

10、updateByPrimaryKeySelective===>按条件更新

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8int 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：

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

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8int 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

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9int 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("[email protected]");

userDAO.updateByPrimaryKey(user);

相当于：

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

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9int 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("[email protected]");

userDAO.updateByPrimaryKey(user);

相当于：

1update user set username='joe',password='joe',email='[email protected]' 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：

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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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21if (!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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12public XMLMapperBuilder(Reader reader, Configuration configuration, String resource, Map sqlFragments) {

this(new XPathParser(reader,true, configuration.getVariables(),new XMLMapperEntityResolver()),

configuration, resource, sqlFragments);

}

private XMLMapperBuilder(XPathParser parser, Configuration configuration, String resource, Map 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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16public 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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27private 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的节点

cacheRefElement(context.evalNode("cache-ref"));

//解析mapper的节点

cacheElement(context.evalNode("cache"));

//解析mapper的节点

parameterMapElement(context.evalNodes("/mapper/parameterMap"));

//解析mapper的节点

resultMapElements(context.evalNodes("/mapper/resultMap"));

//解析mapper的节点

sqlElement(context.evalNodes("/mapper/sql"));

//使用XMLStatementBuilder的对象解析mapper的、、、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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75public 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: and 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。