MyBatis的关联查询和延迟加载
MyBatis的关联查询
1.什么是关联查询
-
实体间的关系（拥有 has、属于 belong）
-
OneToOne：一对一关系（account ←→ user）
-
OneToMany：一对多关系（user ←→ account）
-
ManyToMany：多对多关系（user ←→ role）
-
-
什么是关联查询
当访问关系的一方时，如果需要查看与之关联的另一方数据，则必须使用表链接查询，将查询到的另一方数据，保存在本方的属性中
-
关联查询的语法
指定“一方”关系时（对象），使用
< association javaType="" >指定“多方”关系时（集合），使用
< collection ofType="" >
2.一对一查询
需求：查询账户信息，关联查询用户信息。
分析：因为一个账户信息只能供某个用户使用，所以从查询账户信息出发关联查询用户信息为一对一查询。
2.1.pojo
package com.jyx.pojo;
public class Account {
private Integer id;
private Integer uid;
private Integer money;
private User user;
public User getUser() {
return user;
}
public void setUser(User user) {
this.user = user;
}
public Account() {
}
@Override
public String toString() {
return "Account{" +
"id=" + id +
", uid=" + uid +
", money=" + money +
", user=" + user +
'}';
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public Integer getUid() {
return uid;
}
public void setUid(Integer uid) {
this.uid = uid;
}
public Integer getMoney() {
return money;
}
public void setMoney(Integer money) {
this.money = money;
}
public Account(Integer id, Integer uid, Integer money) {
this.id = id;
this.uid = uid;
this.money = money;
}
}
package com.jyx.pojo;
import java.io.Serializable;
import java.util.Date;
public class User implements Serializable {
private Integer id;
private String username;
private Date birthday;
private String sex;
private String address;
private String password;
@Override
public String toString() {
return "User{" +
"id=" + id +
", username='" + username + '\'' +
", birthday=" + birthday +
", sex='" + sex + '\'' +
", address='" + address + '\'' +
", password='" + password + '\'' +
'}';
}
public String getPassword() {
return password;
}
public void setPassword(String password) {
this.password = password;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public Date getBirthday() {
return birthday;
}
public void setBirthday(Date birthday) {
this.birthday = birthday;
}
public String getSex() {
return sex;
}
public void setSex(String sex) {
this.sex = sex;
}
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
}
2.2.mapper
package com.jyx.mapper;
import com.jyx.pojo.Account;
public interface AccountDao {
Account getAccountById(Integer id);
}
<?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="com.jyx.mapper.AccountDao">
<!--
id：和接口方法名保持一致
resultType：和接口返回类型保持一致
-->
<resultMap id="getAccountByIdResultMap" type="account">
<id column="aid" property="id"></id>
<result column="uid" property="uid"></result>
<result column="money" property="money"></result>
<!--
一对一查询使用association标签
property="user"：Account的属性名
javaType="user"：等价于resultType
-->
<association property="user" javaType="user">
<id column="id" property="id"></id>
<result column="username" property="username"></result>
<result column="password" property="password"></result>
<result column="birthday" property="birthday"></result>
<result column="sex" property="sex"></result>
<result column="address" property="address"></result>
</association>
</resultMap>
<select id="getAccountById" parameterType="int" resultMap="getAccountByIdResultMap">
SELECT a.id aid ,a.uid uid ,a.money money ,u.* FROM account a JOIN user u on a.uid=u.id where a.id=#{id}
</select>
</mapper>
3.一对多查询
需求：查询所有用户信息及用户关联的账户信息。
分析：用户信息和他的账户信息为一对多关系，并且查询过程中如果用户没有账户信息，此时也要将用户信息查询出来，此时左外连接查询比较合适。
3.1.pojo
package com.by.pojo;
import java.io.Serializable;
import java.util.Date;
import java.util.List;
//一方
public class User implements Serializable {
private Integer id;
private String username;
private Date birthday;
private String sex;
private String address;
//多方
private List<Account> accountList;
public Integer getId() {
return id;
}
public String getUsername() {
return username;
}
public List<Account> getAccountList() {
return accountList;
}
public void setAccountList(List<Account> accountList) {
this.accountList = accountList;
}
public void setId(Integer id) {
this.id = id;
}
public void setUsername(String username) {
this.username = username;
}
public Date getBirthday() {
return birthday;
}
public void setBirthday(Date birthday) {
this.birthday = birthday;
}
public String getSex() {
return sex;
}
public void setSex(String sex) {
this.sex = sex;
}
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
@Override
public String toString() {
return "User{" +
"id=" + id +
", username='" + username + '\'' +
", birthday=" + birthday +
", sex='" + sex + '\'' +
", address='" + address + '\'' +
'}';
}
}
3.2.mapper
package com.by.mapper;
import com.by.pojo.User;
import java.util.List;
public interface UserMapper {
User getUserById(Integer id);
}
<?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">
<!--
namespace：保持唯一，推荐写接口全类名
Map<namespace+id, MappedStatement(sql, resultType)>
<namespace+id, MappedStatement(sql, resultType)>
<namespace+id, MappedStatement(sql, resultType)>
<namespace+id, MappedStatement(sql, resultType)>
-->
<mapper namespace="com.by.mapper.UserMapper">
<resultMap id="getUserByIdResult" type="com.by.pojo.User">
<id column="id" property="id"></id>
<result column="username" property="username"/>
<result column="address" property="address"/>
<result column="sex" property="sex"/>
<result column="birthday" property="birthday"/>
<!--一对多使用collection标签指定数据的封装规则-->
<collection property="accountList" ofType="com.by.pojo.Account">
<id column="aid" property="id"></id>
<result column="uid" property="uid"></result>
<result column="money" property="money"></result>
</collection>
</resultMap>
<select id="getUserById" parameterType="int" resultMap="getUserByIdResult">
SELECT u.*, a.id aid, a.uid uid, a.money money FROM user u LEFT JOIN account a ON u.id=a.uid WHERE u.id=#{id}
</select>
</mapper>
3.3.测试
@Test
public void getUserById(){
UserDao userDao = sqlSession.getMapper(UserDao.class);
User user = userDao.getUserById(1);
System.out.println(user);
}
4.多对多查询
需求：查询角色及角色赋予的用户信息。
分析：一个用户可以拥有多个角色，一个角色也可以赋予多个用户，用户和角色为双向的一对多关系，多对多关系其实我们看成是双向的一对多关系。
4.1.pojo
package com.by.pojo;
import java.util.List;
//一方
public class Role {
private Integer id;
private String roleName;
private String roleDesc;
//多方
private List<User> userList;
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getRoleName() {
return roleName;
}
public void setRoleName(String roleName) {
this.roleName = roleName;
}
public String getRoleDesc() {
return roleDesc;
}
public void setRoleDesc(String roleDesc) {
this.roleDesc = roleDesc;
}
public List<User> getUserList() {
return userList;
}
public void setUserList(List<User> userList) {
this.userList = userList;
}
@Override
public String toString() {
return "Role{" +
"id=" + id +
", roleName='" + roleName + '\'' +
", roleDesc='" + roleDesc + '\'' +
", userList=" + userList +
'}';
}
}
4.2.mapper
package com.jyx.mapper;
import com.jyx.pojo.Role;
public interface RoleMapper {
Role getRoleById(Integer id);
}
<?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="com.jyx.mapper.RoleMapper">
<resultMap id="getRoleByIdResultMap" type="com.jyx.pojo.Role">
<id column="rid" property="id"></id>
<result column="role_name" property="roleName"></result>
<result column="role_desc" property="roleDesc"></result>
<!-- 结果映射 -->
<!-- 指定关系表中数据的封装规则 -->
<collection property="userList" ofType="com.jyx.pojo.User">
<id column="id" property="id"></id>
<result column="username" property="username"></result>
<result column="address" property="address"></result>
<result column="sex" property="sex"></result>
<result column="birthday" property="birthday"></result>
</collection>
</resultMap>
<select id="getRoleById" parameterType="int" resultMap="getRoleByIdResultMap">
select r.id rid , r.role_name ,r.role_desc,u.* from user_role ur JOIN role r ON ur.rid=r.id JOIN user u ON ur.uid=u.id where r.id=#{id}
</select>
</mapper>
4.3.测试
@Test
public void getRoleById(){
RoleMapper mapper = sqlSession.getMapper(RoleMapper.class);
Role roleList = mapper.getRoleById(1);
System.out.println(roleList);
}
MyBatis的延迟加载
1.什么是延迟加载？
开启延迟加载后，在真正使用数据的时候才发起级联查询，不用的时候不查询。
2.mapper
package com.jyx.mapper;
import com.jyx.pojo.User;
public interface UserDao {
User getUserById(Integer id);
User getUserById2(Integer id);
}
<?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="com.jyx.mapper.UserDao">
<resultMap id="getUserById2ResultMap" type="com.jyx.pojo.User">
<id column="id" property="id"></id>
<result column="username" property="username"></result>
<result column="address" property="address"></result>
<result column="sex" property="sex"></result>
<result column="birthday" property="birthday"></result>
<!--延迟加载的一对多-->
<!--
property="accountList"：属性名
ofType="com.by.pojo.Account"：集合的泛型，等价于resultType
select="com.by.mapper.AccountMapper.selectAccountByUid"：要调用的select标签的id
column="id"：传递给select查询的参数
fetchType="lazy"：局部开启懒加载
-->
<collection property="accountList"
ofType="com.jyx.pojo.Account"
select="com.jyx.mapper.AccountDao.selectAccountByUid"
column="id"
fetchType="lazy">
</collection>
</resultMap>
<select id="getUserById2" parameterType="int" resultMap="getUserById2ResultMap">
select * from user where id =#{id};
</select>
</mapper>
fetchType=“lazy”：局部开启懒加载
配置全局延迟加载
mybatis-config.xml文件中
<!– 打开延迟加载的开关 –>
<?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="com.jyx.mapper.AccountDao">
<select id="selectAccountByUid" parameterType="int" resultType="com.jyx.pojo.Account">
SELECT * FROM account WHERE uid=#{uid}
</select>
</mapper>
3.测试
@Test
public void testGetUserById2(){
//不执行延迟加载
UserDao userDao = sqlSession.getMapper(UserDao.class);
User user = userDao.getUserById2(41);
System.out.println(user.getUsername());
//执行延迟加载
List<Account> accountList = user.getAccountList();
for (Account account : accountList) {
System.out.println(account);
}