1.大家學習MyBatis時，可能會碰到實體類屬性跟數據庫字段不同的情況
如：數據庫 ------ 實體類
stuname ----> name
即: 數據庫中的stuname字段對應的事實體類裡的name屬性
如果這時，我們要用常規的查詢方法時是不能正確查詢到stuname的值的，它會顯示為null
這時，我們可以使用我們的resultMap來解決這一問題。。。
源碼介紹與對比:
1.Student.java (實體類)
package cn.zhang.entity;
import java.util.Date;
/**
* 學生實體類
*
*/
public class Student {
private Integer stuno;
private String name;
private Integer stuage;
private Date studate;
@Override
public String toString() {
return "Student [stuno=" + stuno + ", name=" + name + ", stuage="
+ stuage + ", studate=" + studate + "]";
}
public Integer getStuno() {
return stuno;
}
public void setStuno(Integer stuno) {
this.stuno = stuno;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public Integer getStuage() {
return stuage;
}
public void setStuage(Integer stuage) {
this.stuage = stuage;
}
public Date getStudate() {
return studate;
}
public void setStudate(Date studate) {
this.studate = studate;
}
}
View Code
2.mybatis-config.xml (MyBatis的配置文件)
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-config.dtd">
<configuration>
<!-- 配置別名 -->
<typeAliases>
<!--方式一: 按類型名定制別名 -->
<typeAlias type="cn.zhang.entity.Student" alias="Student" />
<!--方式二: 拿當前指定包下的簡單類名作為別名 -->
<!-- <package name="cn.zhang.entity"/> -->
</typeAliases>
<environments default="mysql">
<environment id="mysql">
<!-- 使用jdbc的事務 -->
<transactionManager type="JDBC" />
<!-- 使用自帶的連接池 -->
<dataSource type="POOLED">
<property name="driver" value="com.mysql.jdbc.Driver" />
<property name="url" value="jdbc:mysql://localhost:3306/y2161" />
<property name="username" value="root" />
<property name="password" value="root" />
</dataSource>
</environment>
</environments>
<mappers>
<mapper resource="cn/zhang/dao/StudentDAO.xml" />
</mappers>
</configuration>
View Code
3.MybatisUtil.java (獲得session的工具類)
package cn.zhang.util;
import java.io.IOException;
import java.io.Reader;
import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
/**
* 獲得session的工具類
*
*/
public class MybatisUtil {
private static String config = "mybatis-config.xml";
static Reader reader;
static {
try {
reader = Resources.getResourceAsReader(config);
} catch (IOException e) {
e.printStackTrace();
}
}
private static SqlSessionFactory factory = new SqlSessionFactoryBuilder()
.build(reader);
// 提供一個可以獲取到session的方法
public static SqlSession getSession() throws IOException {
SqlSession session = factory.openSession();
return session;
}
}
View Code
4.StudentDao.java (定義方法的接口)
package cn.zhang.dao;
import java.io.IOException;
import java.util.List;
import cn.zhang.entity.Student;
public interface StudentDao {
/**
* 查詢所有記錄
* @return
* @throws IOException
*/
public List<Student> findAll() throws IOException;
}
View Code
5.StudentDaoImpl.java (實現接口方法的實現類)
package cn.zhang.dao.impl;
import java.io.IOException;
import java.util.List;
import org.apache.ibatis.session.SqlSession;
import cn.zhang.dao.StudentDao;
import cn.zhang.entity.Student;
import cn.zhang.util.MybatisUtil;
public class StudentDaoImpl implements StudentDao {
SqlSession session;
public StudentDaoImpl() throws IOException {
session = MybatisUtil.getSession();
}
/**
* 查詢所有
*/
public java.util.List<Student> findAll() throws IOException {
List<Student> list = session.selectList("findAll");
session.close();
return list;
}
}
View Code
6.StudentDAO.xml (對應的映射文件)
最常規的配法:
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="cn.zhang.dao">
<!-- 查詢所有 -->
<select id="findAll" resultType="Student">
select * from student
</select>
</mapper>
用resultMap的配法:
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="cn.zhang.dao">
<!-- 結果映射，指定了數據庫和實體類中的對應值 -->
<resultMap type="Student" id="findstudent">
<result property="name" column="stuname" />
</resultMap>
<!-- 查詢所有 -->
<select id="findAll" resultMap="findstudent">
select * from student
</select>
</mapper>
7.log4j.properties
### direct log messages to stdout ###
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.Target=System.out
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d{ABSOLUTE} %5p %c{1}:%L - %m%n
### direct messages to file mylog.log ###
log4j.appender.file=org.apache.log4j.FileAppender
log4j.appender.file.File=c\:mylog.log
log4j.appender.file.layout=org.apache.log4j.PatternLayout
log4j.appender.file.layout.ConversionPattern=%d{ABSOLUTE} %5p %c{1}:%L - %m%n
### set log levels - for more verbose logging change 'info' to 'debug' ###
log4j.rootLogger=debug, stdout
View Code
8..MyTest.java (測試類)
package cn.zhang.test;
import java.io.IOException;
import java.util.List;
import org.junit.Before;
import org.junit.Test;
import cn.zhang.dao.StudentDao;
import cn.zhang.dao.impl.StudentDaoImpl;
import cn.zhang.entity.Student;
public class MyTest {
StudentDao dao;
@Before
public void initData() throws IOException{
dao=new StudentDaoImpl();
}
/**
* 查詢所有學生
* @throws IOException
*/
@Test
public void findAll() throws IOException{
List<Student> list = dao.findAll();
for (Student student : list) {
System.out.println("編號: "+student.getStuno()+"姓名："+student.getName());
}
}
}
View Code
對應StudentDAO.xml中常規配法的結果:
對應StudentDAO.xml中resultMap配法的結果:
這樣就可以成功的拿到值了，這就是resultMap的作用。
是不是很簡單,這裡只是記錄一下。。
