Mybatis【10】-- Mybatis 属性名和查询字段名不同怎么做?
很多时候我们有这样的需求,数据库的字段名与实体类的属性名不一致,这个时候我们需要怎么做呢?有两种解决方案,第一种:直接在查询的时候使用别名,将别名设置成与实体类的属性名一致。第二种:使用resultType,自己定义映射关系。
整个项目的目录如下:<br>
<br>
首先,我们需要搭建数据库mysql环境(test.sql),id我们写成了sid,name我们写成了sname,age我们写成了sage:
#创建数据库CREATE DATABASE `test` DEFAULT CHARACTER SET utf8 COLLATE utf8_general_ci;#创建数据表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实体类:
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驱动包 --> <dependency> <groupId>mysql</groupId> <artifactId>mysql-connector-java</artifactId> <version>8.0.21</version> </dependency> <!-- junit测试包 --> <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> <!-- 配置数据库文件 --> <properties resource="jdbc_mysql.properties"> </properties> <!-- 别名,对数据对象操作全名太长,需要使用别名 --> <typeAliases> <!--<typeAlias type="bean.Student" alias="Student"/>--> <!--直接使用类名即可,对于整个包的路径配置(别名),简单快捷 --> <package name="bean"/> </typeAliases> <!-- 配置运行环境 --> <!-- default 表示默认使用哪一个环境,可以配置多个,比如开发时的测试环境,上线后的正式环境等 --> <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> <!-- 注册映射文件 --> <mappers> <mapper resource="mapper/mapper.xml"/> </mappers></configuration>
数据库配置文件(jdbc_mysql.properties):
jdbc.driver=com.mysql.jdbc.Driverjdbc.url=jdbc:mysql://localhost:3306/test?characterEncoding=utf-8&serverTimezone=UTCjdbc.user=rootjdbc.password=123456
日志配置文件 log4j.prperties:
log4j.prpplog4j.rootLogger=DEBUG, stdoutlog4j.appender.stdout=org.apache.log4j.ConsoleAppenderlog4j.appender.stdout.layout=org.apache.log4j.PatternLayoutlog4j.appender.stdout.layout.ConversionPattern=[service] %d - %c -%-4r [%t] %-5p %c %x - %m%nlog4j.logger.java.sql.Statement = debuglog4j.logger.java.sql.PreparedStatement = debuglog4j.logger.java.sql.ResultSet =debug
使用到的工具类(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; }}
接口定义(IStudentDao.java):
public interface IStudentDao { // 返回所有学生的信息List public List<Student> selectAllStudents(); // 根据id查找学生 public Student selectStudentById(int id);}
接口实现类(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"); //查询不用修改,所以不用提交事务 } 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:<br>
可以直接使用别名:
<?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=${value} </select></mapper>
或者可以自己定义映射:
<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="abc"> <resultMap id="StudentMapper" type="Student"> <id column="sid" property="id"/> <result column="sname" property="name"/> <result column="sage" property="age"/> </resultMap> <select id="selectAllStudents" resultMap="StudentMapper"> select sid as id,sname as name,sage as age,score from student </select> <select id="selectStudentById" resultMap="StudentMapper"> select sid as id,sname as name,sage as age,score from student where sid=${value} </select></mapper>
<br>
需要注意的点:
* <resultMap></resultMap>有一个id属性,这个是在其他地方使用的时候的id
* Type - 实体类,可以写别名,要不就要写带全路径的类名
* id - 标签是为了标记出作为 ID 的结果可以帮助提高整体性能
* result – 注入到字段或 JavaBean 属性的普通结果
* association – 一个复杂类型的关联;许多结果将包装成这种类型嵌套结果映射 – 关联可以指定为一个 resultMap 元素,或者引用一个
* collection – 一个复杂类型的集合
嵌套结果映射 – 集合可以指定为一个 resultMap 元素,或者引用一个
* discriminator – 使用结果值来决定使用哪个 resultMap
case – 基于某些值的结果映射
嵌套结果映射 – 一个 case 也是一个映射它本身的结果,因此可以包含很多相 同的元素,或者它可以参照一个外部的 resultMap。
如果对象名与属性名一致,我们可以不把它写入<resultMap></resultMap>
测试类MyTest.class:
public class MyTest { private IStudentDao dao; @Before public void Before(){ dao=new StudentDaoImpl(); } /* * 查询列表 * */ @Test public void testselectList(){ List<Student> students=dao.selectAllStudents(); if(students.size()>0){ for(Student student:students){ System.out.println(student); } } } /* * 通过id来查询student * */ @Test public void testselectStudentById(){ Student student=dao.selectStudentById(1); System.out.println(student); }}
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