很多時候我們有這樣的需求丹泉，數(shù)據(jù)庫的字段名與實體類的屬性名不一致，這個時候我們需要怎么做呢鸭蛙？有兩種解決方案摹恨，第一種：直接在查詢的時候使用別名，將別名設(shè)置成與實體類的屬性名一致娶视。第二種：使用resultType晒哄，自己定義映射關(guān)系。
整個項目的目錄如下：
image
首先肪获，我們需要搭建數(shù)據(jù)庫mysql環(huán)境(test.sql),id我們寫成了sid,name我們寫成了sname,age我們寫成了sage：
#創(chuàng)建數(shù)據(jù)庫
CREATE DATABASE `test` DEFAULT CHARACTER SET utf8 COLLATE utf8_general_ci;
#創(chuàng)建數(shù)據(jù)表
CREATE TABLE `student` ( `sid` INT NOT NULL AUTO_INCREMENT , `sname` VARCHAR(20) NOT NULL ,
`sage` INT NOT NULL , `score` DOUBLE NOT NULL , PRIMARY KEY (`sid`)) ENGINE = MyISAM;
Student.class實體類：
public class Student {
private Integer id;
private String name;
private int age;
private double score;
public Student(){
}
public Student(String name, int age, double score) {
super();
this.name = name;
this.age = age;
this.score = score;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public int getAge() {
return age;
}
public void setAge(int age) {
this.age = age;
}
public double getScore() {
return score;
}
public void setScore(double score) {
this.score = score;
}
@Override
public String toString() {
return "Student [id=" + id + ", name=" + name + ", age=" + age
+ ", score=" + score + "]";
}
}
pom.xml:
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.test</groupId>
<artifactId>test</artifactId>
<version>1.0-SNAPSHOT</version>
<dependencies>
<!-- mybatis核心包 -->
<dependency>
<groupId>org.mybatis</groupId>
<artifactId>mybatis</artifactId>
<version>3.3.0</version>
</dependency>
<!-- mysql驅(qū)動包 -->
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>8.0.21</version>
</dependency>
<!-- junit測試包 -->
<dependency>
<groupId>junit</groupId>
<artifactId>junit</artifactId>
<version>4.11</version>
<scope>test</scope>
</dependency>
<!-- 日志文件管理包 -->
<dependency>
<groupId>log4j</groupId>
<artifactId>log4j</artifactId>
<version>1.2.17</version>
</dependency>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-api</artifactId>
<version>1.7.12</version>
</dependency>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-log4j12</artifactId>
<version>1.7.12</version>
</dependency>
</dependencies>
</project>
主配置文件mybatis.xml:
<?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>
<!-- 配置數(shù)據(jù)庫文件 -->
<properties resource="jdbc_mysql.properties">
</properties>
<!-- 別名寝凌，對數(shù)據(jù)對象操作全名太長，需要使用別名 -->
<typeAliases>
<!--<typeAlias type="bean.Student" alias="Student"/>-->
<!--直接使用類名即可孝赫，對于整個包的路徑配置（別名）较木，簡單快捷 -->
<package name="bean"/>
</typeAliases>
<!-- 配置運行環(huán)境 -->
<!-- default 表示默認使用哪一個環(huán)境，可以配置多個青柄，比如開發(fā)時的測試環(huán)境伐债，上線后的正式環(huán)境等 -->
<environments default="mysqlEM">
<environment id="mysqlEM">
<transactionManager type="JDBC">
</transactionManager>
<dataSource type="POOLED">
<property name="driver" value="${jdbc.driver}"/>
<property name="url" value="${jdbc.url}"/>
<property name="username" value="${jdbc.user}"/>
<property name="password" value="${jdbc.password}"/>
</dataSource>
</environment>
</environments>
<!-- 注冊映射文件 -->
<mappers>
<mapper resource="mapper/mapper.xml"/>
</mappers>
</configuration>
數(shù)據(jù)庫配置文件(jdbc_mysql.properties)：
jdbc.driver=com.mysql.jdbc.Driver
jdbc.url=jdbc:mysql://localhost:3306/test?characterEncoding=utf-8&serverTimezone=UTC
jdbc.user=root
jdbc.password=123456
日志配置文件 log4j.prperties:
log4j.prpp
log4j.rootLogger=DEBUG, stdout
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=[service] %d - %c -%-4r [%t] %-5p %c %x - %m%n
log4j.logger.java.sql.Statement = debug
log4j.logger.java.sql.PreparedStatement = debug
log4j.logger.java.sql.ResultSet =debug
使用到的工具類（MyBatisUtils.java）：
public class MyBatisUtils {
static private SqlSessionFactory sqlSessionFactory;
static public SqlSession getSqlSession() {
InputStream is;
try {
is = Resources.getResourceAsStream("mybatis.xml");
if (sqlSessionFactory == null) {
sqlSessionFactory = new SqlSessionFactoryBuilder().build(is);
}
return sqlSessionFactory.openSession();
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
return null;
}
}
接口定義（IStudentDao.java）：
public interface IStudentDao {
// 返回所有學生的信息List
public List<Student> selectAllStudents();
// 根據(jù)id查找學生
public Student selectStudentById(int id);
}
接口實現(xiàn)類(StudentDaoImpl.class)：
public class StudentDaoImpl implements IStudentDao {
private SqlSession sqlSession;
public List<Student> selectAllStudents() {
List<Student> students ;
try {
sqlSession = MyBatisUtils.getSqlSession();
students = sqlSession.selectList("selectAllStudents");
//查詢不用修改预侯，所以不用提交事務(wù)
} finally {
if (sqlSession != null) {
sqlSession.close();
}
}
return students;
}
public Student selectStudentById(int id) {
Student student=null;
try {
sqlSession=MyBatisUtils.getSqlSession();
student=sqlSession.selectOne("selectStudentById",id);
sqlSession.commit();
} finally{
if(sqlSession!=null){
sqlSession.close();
}
}
return student;
}
}
最主要的mapper文件：
可以直接使用別名：
<?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="abc">
<select id="selectAllStudents" resultType="Student">
select sid as id,sname as name,sage as age,score from student
</select>
<select id="selectStudentById" resultType="Student">
select sid as id,sname as name,sage as age,score from student where sid=${value}
</select>
</mapper>
或者可以自己定義映射：
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE mapper
PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
"http://mybatis.org/dtd/mybatis-3-mapper.dtd">
<mapper namespace="abc">
<resultMap id="StudentMapper" type="Student">
<id column="sid" property="id"/>
<result column="sname" property="name"/>
<result column="sage" property="age"/>
</resultMap>
<select id="selectAllStudents" resultMap="StudentMapper">
select sid as id,sname as name,sage as age,score from student
</select>
<select id="selectStudentById" resultMap="StudentMapper">
select sid as id,sname as name,sage as age,score from student where sid=${value}
</select>
</mapper>
image
需要注意的點：
- <resultMap></resultMap>有一個id屬性，這個是在其他地方使用的時候的id
- Type - 實體類泳赋，可以寫別名雌桑，要不就要寫帶全路徑的類名
- id - 標簽是為了標記出作為 ID 的結(jié)果可以幫助提高整體性能
- result – 注入到字段或 JavaBean 屬性的普通結(jié)果
- association – 一個復雜類型的關(guān)聯(lián);許多結(jié)果將包裝成這種類型嵌套結(jié)果映射 – 關(guān)聯(lián)可以指定為一個 resultMap 元素，或者引用一個
- collection – 一個復雜類型的集合
嵌套結(jié)果映射 – 集合可以指定為一個 resultMap 元素祖今，或者引用一個- discriminator – 使用結(jié)果值來決定使用哪個 resultMap
case – 基于某些值的結(jié)果映射
嵌套結(jié)果映射 – 一個 case 也是一個映射它本身的結(jié)果,因此可以包含很多相 同的元素校坑，或者它可以參照一個外部的 resultMap。
如果對象名與屬性名一致千诬，我們可以不把它寫入<resultMap></resultMap>
測試類MyTest.class:
public class MyTest {
private IStudentDao dao;
@Before
public void Before(){
dao=new StudentDaoImpl();
}
/*
* 查詢列表
*
*/
@Test
public void testselectList(){
List<Student> students=dao.selectAllStudents();
if(students.size()>0){
for(Student student:students){
System.out.println(student);
}
}
}
/*
* 通過id來查詢student
*
*/
@Test
public void testselectStudentById(){
Student student=dao.selectStudentById(1);
System.out.println(student);
}
}
【作者簡介】：
秦懷耍目，公眾號【秦懷雜貨店】作者，技術(shù)之路不在一時徐绑，山高水長邪驮，縱使緩慢，馳而不息傲茄。這個世界希望一切都很快毅访，更快，但是我希望自己能走好每一步盘榨，寫好每一篇文章喻粹，期待和你們一起交流。
