config.xml是全局配置文件,主要配置MyBatis的数据源(DataSource),事务管理(TransactionManager),以及打印SQL语句,开启二级缓存,设置实体类别名等功能。
Mapper.xml的作用是什么?我们之前介绍过,MyBatis是"半自动"的ORM框架,即SQL语句需要开发者自定义,MyBatis的关注点在POJO与SQL之间的映射关系。那么SQL语句在哪里配置自定义呢?就在Mapper.xml中配置。
首先来介绍Mapper.xml常用属性:
parameterType:参数数据类型
(1)基本数据类型,通过id查询User。
//通过id查找User public User getById(int id);
UserDao.xml:
<?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> <select id="getById" parameterType="int" resultType="com.xf.pojo.User"> SELECT * FROM t_user WHERE id=#{id} </select> </mapper>
(2)String类型,通过name查User
UserDao:
//通过name查询User public User getByName(String name);
UserDao.xml:
<select id="get2" parameterType="java.lang.String" resultType="com.xf.pojo.User"> select * from t_user where name = #{name} </select>
(3)包装类,通过id查询User。
UserDao:
//通过id查询User public User getById(Integer id);
UserDao.xml:
<select id="getById" parameterType="java.lang.Integer" resultType="com.xf.pojo.User"> select * from t_user where id=#{id} </select>
(4)多个参数,通过name和age查询User。两个参数分别是String类型和int类型,类型不一致,所以此时parameterType可以省略,通过参数下标取出参数值
UserDao:
//通过name和age查询User public User getByNameAge(int id,String name);
UserDao.xml:
<select id="getByNameAge" resultType="com.xf.pojo.User"> select * from t_user where name = #{0} and age = #{1} </select>
(5)POJO,很显然,当有多个参数时,一个一个写太麻烦了,这时候我们可以将参数列表进行封装,将封装对象作为parameterType的值。
UserDao:
//根据Usesr封装对象查询User public User getByUser(User user);
UserDao.xml:
<select id="getByUser" parameterType="com.xf.pojo.User" resultType="com.xf.pojo.User"> select * from t_user where name = #{name} and age = #{age} </select>
resultType:结果类型
(1)基本数据类型,统计User总数。
UserDao:
//通过User总数量 public int getCount();
UserDao.xml:
<select id="getCount" resultType="int"> select count(*) from t_user </select>
(2)包装类,统计User总数。
UserDao:
//通过User总数量 public Integer getCount();
UserDao.xml:
<select id="getCount" resultType="java.lang.Integer"> select count(*) from t_user </select>
(3)String类型,根据id查询User的name值。
UserDao:
//根据id查询User的name public String getNameById(int id);
UserDao.xml:
<select id="getNameById" parameterType="int" resultType="java.lang.String"> select name from user where id = #{name} </select>
(4)POJO,如通过id查询User,上面已经介绍过了,这里就不再重复了。
级联查询
一对多
我们现在查询的User是单表查询,如果是多表关联查询,比如查询Student同时级联对应的Classes,如何处理呢?
使用resultType无法完成,我们以通过id查询Student来举例。
SQL:
select * from student as s,classes as c where s.cid = c.c_id and s.id = 1;
查询结果:
实体类Student:
package com.xf.pojo; public class Student { private int id; private String name; private String address; private String tel; private int score; private Classes classes; 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 String getAddress() { return address; } public void setAddress(String address) { this.address = address; } public String getTel() { return tel; } public void setTel(String tel) { this.tel = tel; } public int getScore() { return score; } public void setScore(int score) { this.score = score; } public Classes getClasses() { return classes; } public void setClasses(Classes classes) { this.classes = classes; } }
Classes:
package com.xf.pojo; import java.util.List; public class Classes { private int id; private String name; private List<Student> students; 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 List<Student> getStudents() { return students; } public void setStudents(List<Student> students) { this.students = students; } }
MyBatis会自动将结果与实体类进行映射,将字段的值赋给对应的属性,若字段名与属性名一致,完成赋值,那么问题来了。
如图,id,name,address,tel,score属性可以对应字段,classes属性没有对应的字段,准确的讲,classes属性需要对应的对象为c_id,c_name封装起来的对象。
此时,需要使用resultMap来完成映射。
StudentDao:
//通过id查询Student public Student getById(int id);StudentDao.xml,使用association标签配置classes级联,因为一个Student只能对应一个Classes。
<resultMap type="student" id="studentMap"> <id property="id" column="id"/> <result property="name" column="name"/> <result property="address" column="address"/> <result property="tel" column="tel"/> <result property="score" column="score"/> <!-- 映射classes属性 --> <association property="classes" javaType="com.xf.pojo.Classes"> <id property="id" column="c_id"/> <result property="name" column="c_name"/> </association> </resultMap> <select id="getById" parameterType="int" resultMap="studentMap"> select * from student as s,classes as c where s.cid = c.c_id and s.id = #{id}; </select>
同理,反过来查询Classes,将级联的所有Student一并查询。
ClassesDao:
//根据id查询Classes public Classes getById(int id);ClassesDao.xml,使用collection标签配置students级联,因为一个Classes可以对应多个Student。
<resultMap type="classes" id="classesMap"> <id property="id" column="c_id"/> <result property="name" column="c_name"/> <!-- 映射students属性 --> <collection property="students" ofType="student"> <id property="id" column="id"/> <result property="name" column="name"/> <result property="address" column="address"/> <result property="tel" column="tel"/> <result property="score" column="score"/> </collection> </resultMap> <select id="getById" parameterType="int" resultMap="classesMap"> select * from classes as c,student as s where c.c_id = s.cid and c.c_id = #{id}; </select>需要注意的是:association标签,通过设置javaType属性,映射实体类,
collection标签,通过设置ofType属性映射实体类。
多对多
一个Customer可以对应多个Goods,一个Goods也可以对应多个Customer,所以双方都是用collection标签设置级联。
Customer:
package com.xf.pojo; import java.util.List; public class Customer { private int id; private String name; private List<Goods> goods; 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 List<Goods> getGoods() { return goods; } public void setGoods(List<Goods> goods) { this.goods = goods; } }
Goods:
package com.xf.pojo; import java.util.List; public class Goods { private int id; private String name; private List<Customer> customers; 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 List<Customer> getCustomers() { return customers; } public void setCustomers(List<Customer> customers) { this.customers = customers; } }
CustomerDao:
//根据id查询Customer public Customer getById(int id);
CustomerDao.xml:
<resultMap type="customer" id="customerMap"> <id property="id" column="c_id"/> <result property="name" column="c_name"/> <!-- 映射goods属性 --> <collection property="goods" ofType="goods"> <id property="id" column="g_id"/> <result property="name" column="g_name"/> </collection> </resultMap> <select id="getById" parameterType="int" resultMap="customerMap"> select * from customer as c,goods as g, customer_goods as cg where c.c_id = cg.c_id and g.g_id =c g.g_id and c.c_id = #{id}; </select>
GoodsDao:
//根据id查询Goods public Goods getById(int id);
GoodsDao.xml:
<resultMap type="goods" id="goodsMap"> <id property="id" column="g_id"/> <result property="name" column="g_name"/> <!-- 映射customers属性 --> <collection property="customers" ofType="customer"> <id property="id" column="c_id"/> <result property="name" column="c_name"/> </collection> </resultMap> <select id="getById" parameterType="int" resultMap="goodsMap"> select * from customer as c, goods as g,customer_goods as cg where c.c_id = cg.c_id and g.g_id = cg.g_id and g.g_id = #{id}; </select>