多個(gè)對象對應(yīng)一個(gè)對象時(shí)，應(yīng)該如何進(jìn)行查詢？

例如

關(guān)鍵字：association : 聯(lián)系 ，關(guān)聯(lián) 多個(gè)人可以關(guān)聯(lián)一個(gè)人。

首先做一些準(zhǔn)備,如:實(shí)體類,工具類和Mybatis核心文件

實(shí)體類：

//老師實(shí)體類package com.MLXH.pojo;public class Teacher { private int id; private String name; public Teacher() { } public Teacher(int id, String name) {this.id = id;this.name = name; } public int getId() {return id; } public void setId(int id) {this.id = id; } public String getName() {return name; } public void setName(String name) {this.name = name; } @Override public String toString() {return 'Teacher{' +'id=' + id +', name=’' + name + ’’’ +’}’; }}

//學(xué)生實(shí)體類package com.MLXH.pojo;public class Student { private int id; private String name; private Teacher teacher; public Student() { } public Student(int id, String name, Teacher teacher) {this.id = id;this.name = name;this.teacher = teacher; } public int getId() {return id; } public void setId(int id) {this.id = id; } public String getName() {return name; } public void setName(String name) {this.name = name; } public Teacher getTeacher() {return teacher; } public void setTeacher(Teacher teacher) {this.teacher = teacher; } @Override public String toString() {return 'Student{' +'id=' + id +', name=’' + name + ’’’ +', teacher=' + teacher +’}’; }}

database.properties配置文件數(shù)據(jù)庫需要的數(shù)據(jù)

driver = com.mysql.jdbc.Driverurl = jdbc:mysql://localhost:3306/mybatis?useSSL=true&useUnicode=true&characterEncoding=utf-8username = rootpassword = 123456

Mybatis工具類:mybatis-config.xml

<?xml version='1.0' encoding='UTF-8' ?><!DOCTYPE configurationPUBLIC '-//mybatis.org//DTD Config 3.0//EN''http://mybatis.org/dtd/mybatis-3-config.dtd'><configuration> <!--配置文件修改--> <properties resource='database.properties'/> <!--Mybatis設(shè)置--> <settings><!--默認(rèn)日志實(shí)現(xiàn)--><!--<setting name='logImpl' value='STDOUT_LOGGING'/>--><!--Log4j實(shí)現(xiàn)--><setting name='logImpl' value='LOG4J'/> </settings> <!--配置別名--> <typeAliases><package name='com.MLXH.pojo'/> </typeAliases> <environments default='development'><environment id='development'> <transactionManager type='JDBC'/> <dataSource type='POOLED'><property name='driver' value='${driver}'/><property name='url' value='${url}'/><property name='username' value='${username}'/><property name='password' value='${password}'/> </dataSource></environment> </environments> <mappers><!--class對應(yīng)的是一個(gè)接口類--><!--resource對應(yīng)的是一個(gè)接口類的映射文件--><mapper resource='com/MLXH/dao/StudentMapper.xml'/> </mappers></configuration>

工具類:主要是為了使獲得sqlsession更為簡單方便

package com.MLXH.utils;import org.apache.ibatis.io.Resources;import org.apache.ibatis.session.SqlSession;import org.apache.ibatis.session.SqlSessionFactory;import org.apache.ibatis.session.SqlSessionFactoryBuilder;import java.io.IOException;import java.io.InputStream;//mybatis的工具類，重復(fù)的代碼的提純public class MyBatisUtils { //類變量不需要設(shè)置默認(rèn)值； private static SqlSessionFactory sqlSessionFactory; static {//在maven中，所有的資源文件一般都放在resources目錄下，我們可以直接拿到。try { String resource = 'mybatis-config.xml'; InputStream inputStream = Resources.getResourceAsStream(resource); sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);} catch (IOException e) { e.printStackTrace();} } //設(shè)置SqlSessionFactory公共的方法 public static SqlSessionFactory getSqlSessionFactory(){return sqlSessionFactory; } //獲得一個(gè)帶事務(wù)自動(dòng)提交功能的SqlSession公共的方法 public static SqlSession getSqlSession(){//自動(dòng)提交事務(wù)return sqlSessionFactory.openSession(true); }}

StudentDao接口

package com.MLXH.dao;import com.MLXH.pojo.Student;import java.util.List;public interface StudentDao { //獲得全部學(xué)生的信息以及對應(yīng)的老師 List<Student> getStudents(); //獲得全部學(xué)生的信息以及對應(yīng)的老師 List<Student> getStudentsTwo();}

針對于StudentDao接口的實(shí)現(xiàn)mapper文件:我們使用如下的方式來進(jìn)行查詢

1.模擬數(shù)據(jù)庫思想:連表查詢

StudentMapper.xml

<?xml version='1.0' encoding='UTF-8' ?><!DOCTYPE mapperPUBLIC '-//mybatis.org//DTD Mapper 3.0//EN''http://mybatis.org/dtd/mybatis-3-mapper.dtd'><!--namespace不能寫別名--><mapper namespace='com.MLXH.dao.StudentDao'> <!--遇到問題：學(xué)生類中關(guān)聯(lián)老師： 多個(gè)學(xué)生對應(yīng)一個(gè)老師 --><!--解決問題方式一:按查詢結(jié)果嵌套處理，模擬數(shù)據(jù)庫思想；--> <select resultMap='StudentTeacher'>select * from mybatis.student </select> <resultMap type='Student'><id column='id' property='id'/><result column='name' property='name'/><!--屬性和字段對應(yīng) ， 類和表對應(yīng) ， 對象和記錄關(guān)聯(lián)一個(gè)字段需求：拿到老師這個(gè)類的屬性association ： 關(guān)聯(lián)，多對一 column ： 數(shù)據(jù)庫對應(yīng)的列名 property ： 對應(yīng)屬性名 javaType ： 多對一字段對應(yīng)的Java類型 select ： 關(guān)聯(lián)一個(gè)語句--><association column='tid' property='teacher' javaType='Teacher' select='getTeacher'/> </resultMap> <select resultType='Teacher'>select * from mybatis.teacher where id = #{id} </select></mapper>

測試類

@Test public void getStudents(){SqlSession sqlSession = MyBatisUtils.getSqlSession();StudentDao mapper = sqlSession.getMapper(StudentDao.class);List<Student> students = mapper.getStudents();for (Student student : students) { System.out.println('學(xué)生姓名：'+student.getName()+'t老師姓名：'+student.getTeacher().getName());} }2.模擬面向?qū)ο蟮乃枷?p><?xml version='1.0' encoding='UTF-8' ?><!DOCTYPE mapperPUBLIC '-//mybatis.org//DTD Mapper 3.0//EN''http://mybatis.org/dtd/mybatis-3-mapper.dtd'><!--namespace不能寫別名!!!!!--><mapper namespace='com.MLXH.dao.StudentDao'> <!-- 解決方式二：一個(gè)resultMap解決 ， 模擬面向?qū)ο蟮乃枷?-> <select resultMap='StudentTeacher2'>select s.id,s.name,t.id as tid,t.name as tnamefrom mybatis.student as s, mybatis.teacher as twhere s.tid = t.id </select> <!--設(shè)置結(jié)果集映射ResultMap --> <resultMap type='Student'><id property='id' column='id'/><result property='name' column='name'/><!--直接關(guān)聯(lián)一個(gè)老師--><association property='teacher' javaType='Teacher'> <id property='id' column='tid'/> <result property='name' column='tname'/></association> </resultMap></mapper>

測試

@Testpublic void getStudentsTwo(){ SqlSession sqlSession = MyBatisUtils.getSqlSession(); StudentDao mapper = sqlSession.getMapper(StudentDao.class); List<Student> students = mapper.getStudentsTwo(); for (Student student : students) {System.out.println('學(xué)生姓名：'+student.getName()+'t老師姓名：'+student.getTeacher().getName()); }}

總結(jié)：

mybatis中遇到多對一的情況，要使用關(guān)聯(lián)映射處理：使用association

兩種處理思路：

數(shù)據(jù)庫思想 ： 聯(lián)表查詢 OOP思想 ：關(guān)聯(lián)對象Mybatis同時(shí)傳入多個(gè)對象及普通參數(shù)

當(dāng)傳入多個(gè)文件時(shí)，mapper接口文件的方法參數(shù)要使用@param（“xx”）注釋。

例子：

mapper:

//Student是對象，age是String類型。int getPojo(@param('student') Student student, @param('age') String age );

xml：

<select resultMap='BaseResultMap'> select <include refid='Base_Column_List' /> from student where 1 = 1 <!-- 使用參數(shù)一（是個(gè)自己的對象） --> <if test='student.id != null'>and id = #{student.id} </if> <!-- 使用參數(shù)二 String類型 --> <if test='age!= null'>and age = #{age} </if></select>

以上為個(gè)人經(jīng)驗(yàn)，希望能給大家一個(gè)參考，也希望大家多多支持好吧啦網(wǎng)。