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("[e-mail]");
 userdao.insert(user);

 相当于：

insert into user(id,username,password,email) values(101,'test','123456','[e-mail]');

 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("[e-mail]");
 userdao.updatebyprimarykey(user);

 相当于：

update user set username='joe',password='joe',email='[e-mail]' 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("[e-mail]");
 userdao.updatebyprimarykey(user);

 相当于：

update user set username='joe',password='joe',email='[e-mail]' 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。