一、前言
这一节，我们将学习一下如何通过MyBatis提供的association关键字来实现关联查询，主要是实现一对一关联查询。有两种实现方式，分别是嵌套查询和分段查询。
二、案例
完整的工程目录如下：
在MySQL数据库新建两张数据库表t_user和t_role，并插入若干条数据
CREATE TABLE t_user (
id int(10) NOT NULL AUTO_INCREMENT,
loginId varchar(20) DEFAULT NULL,
userName varchar(100) DEFAULT NULL,
roleId int(10),
note varchar(255) DEFAULT NULL,
PRIMARY KEY (id)
) ENGINE=InnoDB AUTO_INCREMENT=0 DEFAULT CHARSET=utf8;
INSERT INTO t_user(loginId,userName,roleId,note) VALUES ('queen', '奎恩', 1, '专门负责提鞋的。。。');
INSERT INTO t_user(loginId,userName,roleId,note) VALUES ('king', '金狮子', 2, '磁性果实能力');
INSERT INTO t_user(loginId,userName,roleId,note) VALUES ('Lucy', '路西', 3, '打败多弗朗明哥。。。');
=========================================================
CREATE TABLE t_role (
id int(10) NOT NULL AUTO_INCREMENT,
roleName varchar(20) DEFAULT NULL,
PRIMARY KEY (id)
) ENGINE=InnoDB AUTO_INCREMENT=0 DEFAULT CHARSET=utf8;
INSERT INTO t_role(roleName) VALUES ('自定义角色');
INSERT INTO t_role(roleName) VALUES ('前海贼');
INSERT INTO t_role(roleName) VALUES ('未来海贼王');
User实体类
public class User {
// ID，唯一性
private int id;
// 登录ID
private String loginId;
// 用户名
private String userName;
// 角色
private Role role;
// 备注
private String note;
public User(){
}
public User(int id, String loginId, String userName, Role role, String note) {
this.id = id;
this.loginId = loginId;
this.userName = userName;
this.role = role;
this.note = note;
}
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getLoginId() {
return loginId;
}
public void setLoginId(String loginId) {
this.loginId = loginId;
}
public String getUserName() {
return userName;
}
public void setUserName(String userName) {
this.userName = userName;
}
public Role getRole() {
return role;
}
public void setRole(Role role) {
this.role = role;
}
public String getNote() {
return note;
}
public void setNote(String note) {
this.note = note;
}
@Override
public String toString() {
return "User [id=" + id + ", loginId=" + loginId + ", userName="
+ userName + ", role=" + role + ", note=" + note + "]";
}
}
Role实体类
public class Role {
private int id;
private String roleName;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getRoleName() {
return roleName;
}
public void setRoleName(String roleName) {
this.roleName = roleName;
}
public Role(){
}
public Role(int id, String roleName) {
this.id = id;
this.roleName = roleName;
}
@Override
public String toString() {
return "Role [id=" + id + ", roleName=" + roleName + "]";
}
}
配置文件log4j.properties
log4j.rootLogger=DEBUG,A1
log4j.logger.com.queen = DEBUG
log4j.logger.org.mybatis = DEBUG
log4j.appender.A1=org.apache.log4j.ConsoleAppender
log4j.appender.A1.layout=org.apache.log4j.PatternLayout
log4j.appender.A1.layout.ConversionPattern=%-d{yyyy-MM-dd HH:mm:ss,SSS} [%t] [%c]-[%p] %m%n
配置文件db.properties
jdbc.driver=com.mysql.jdbc.Driver jdbc.url=jdbc:mysql://localhost:3306/mybatis jdbc.username=root jdbc.password=root
配置文件mybatis-config.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> <properties resource="db.properties"></properties> <environments default="development"> <environment id="development"> <transactionManager type="JDBC" /> <dataSource type="POOLED"> <property name="driver" value="com.mysql.jdbc.Driver" /> <property name="url" value="jdbc:mysql://localhost:3306/mybatis" /> <property name="username" value="root" /> <property name="password" value="root" /> </dataSource> </environment> </environments> <mappers> <mapper resource="UserMapper.xml" /> </mappers> </configuration>
新建接口类UserMapper.java，增加接口方法getUserById
public interface UserMapper {
/**
* 根据ID查询User信息，返回一条记录User
* @param id
* @return
*/
public User getUserById(int id);
}
UserMapper.xml文件配置，嵌套查询
<?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.queen.mybatis.mapper.UserMapper">
<resultMap type="com.queen.mybatis.bean.User" id="userResultMap">
<id property="id" column="id"/>
<result property="loginId" column="loginId" />
<result property="userName" column="userName"/>
<result property="note" column="note"/>
<!--assocication可以指定联合的JavaBean对象
property="role"指定哪个属性是联合的对象
javaType:指定这个属性对象的类型
-->
<association property="role" javaType="com.queen.mybatis.bean.Role">
<id column="role_id" property="id"/>
<result column="roleName" property="roleName"/>
</association>
</resultMap>
<select id="getUserById" resultMap="userResultMap">
select m.id id, m.loginId loginId, m.userName userName, m.roleId roleId,m.note note, n.id role_id, n.roleName roleName
from t_user m left join t_role n on m.roleId=n.id
where m.id=#{id}
</select>
</mapper>
新增测试类MyBatisTest，添加测试方法testGetUserByAssocication
@Test
public void testGetUserByAssocication() throws IOException {
SqlSessionFactory sqlSessionFactory = getSqlSessionFactory();
SqlSession openSession = sqlSessionFactory.openSession();
try {
UserMapper mapper = openSession.getMapper(UserMapper.class);
User user = mapper.getUserById(1);
System.out.println(user);
System.out.println(user.getRole());
} finally {
openSession.close();
}
}
控制台打印结果如下：
2017-08-08 22:46:52,470 [main] [com.queen.mybatis.mapper.UserMapper.getUserById]-[DEBUG] ==> Preparing: select m.id id, m.loginId loginId, m.userName userName, m.roleId roleId,m.note note, n.id role_id, n.roleName roleName from t_user m left join t_role n on m.roleId=n.id where m.id=? 2017-08-08 22:46:52,558 [main] [com.queen.mybatis.mapper.UserMapper.getUserById]-[DEBUG] ==> Parameters: 1(Integer) 2017-08-08 22:46:52,597 [main] [com.queen.mybatis.mapper.UserMapper.getUserById]-[DEBUG] <== Total: 1 User [id=1, loginId=queen, userName=奎恩, role=Role [id=1, roleName=自定义角色], note=专门负责提鞋的。。。] Role [id=1, roleName=自定义角色]
这样我们可以通过association也能够实现关联属性的查询，使用association定义关联的单个对象的封装结果。
接下来我们介绍第二种：使用association进行分步查询，那么分步查询是什么意思呢？用我们上述例子，即我们查完User表数据后，我们在查询Role表数据。分多条SQL执行。
新建一个接口类RoleMapper，新增接口方法getRoleById
public interface RoleMapper {
/**
* 根据ID查询Role信息，返回一条记录Role
* @param id
* @return
*/
public Role getRoleById(int id);
}
新建一个RoleMapper.xml文件
<?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.queen.mybatis.mapper.RoleMapper">
<select id="getRoleById" resultType="com.queen.mybatis.bean.Role">
select id, roleName from t_role where id=#{id}
</select>
</mapper>
修改UserMapper文件，新增一个方法getUserByIdStep
/** * 根据ID查询User信息，返回一条记录User，分步查询 * @param id * @return */ public User getUserByIdStep(int id);
修改UserMapper.xml文件，增加如下配置
<resultMap type="com.queen.mybatis.bean.User" id="userResultMapStep">
<id property="id" column="id"/>
<result property="loginId" column="loginId" />
<result property="userName" column="userName"/>
<result property="note" column="note"/>
<!-- association定义关联对象的封装规则
select:表明当前属性是调用select指定的方法查询出结果
column:指定将那一列的值传递给这个方法
整个流程：使用Select指定的方法(传入column指定的这列参数的值)查出对象，并封装给property指定的属性
-->
<association property="role" select="com.queen.mybatis.mapper.RoleMapper.getRoleById"
column="roleId">
</association>
</resultMap>
<select id="getUserByIdStep" resultMap="userResultMapStep">
select * from t_user where id=#{id}
</select>
修改测试类MyBatisTest，添加测试方法testGetUserByAssocicationStep
@Test
public void testGetUserByAssocicationStep() throws IOException {
SqlSessionFactory sqlSessionFactory = getSqlSessionFactory();
SqlSession openSession = sqlSessionFactory.openSession();
try {
UserMapper mapper = openSession.getMapper(UserMapper.class);
User user = mapper.getUserByIdStep(1);
System.out.println(user);
System.out.println(user.getRole());
} finally {
openSession.close();
}
}
控制台打印结果如下：
org.apache.ibatis.exceptions.PersistenceException: ### Error querying database. Cause: java.lang.IllegalArgumentException: Mapped Statements collection does not contain value for com.queen.mybatis.mapper.RoleMapper.getRoleById ### The error may exist in UserMapper.xml ### The error may involve com.queen.mybatis.mapper.UserMapper.getUserByIdStep ### The error occurred while handling results ### SQL: select * from t_user where id=? ### Cause: java.lang.IllegalArgumentException: Mapped Statements collection does not contain value for com.queen.mybatis.mapper.RoleMapper.getRoleById at org.apache.ibatis.exceptions.ExceptionFactory.wrapException(ExceptionFactory.java:26) at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:111) at org.apache.ibatis.session.defaults.DefaultSqlSession.selectList(DefaultSqlSession.java:102) at org.apache.ibatis.session.defaults.DefaultSqlSession.selectOne(DefaultSqlSession.java:66)
结果事与愿违，报错了，这是为什么呢？注意我们一定要将新增的RoleMapper.xml映射文件配置到mybatis-config.xml文件中，配置如下：
<mappers> <mapper resource="UserMapper.xml" /> <mapper resource="RoleMapper.xml" /> </mappers>
重新测试一下
2017-08-08 23:26:04,799 [main] [com.queen.mybatis.mapper.UserMapper.getUserByIdStep]-[DEBUG] ==> Preparing: select * from t_user where id=? 2017-08-08 23:26:04,853 [main] [com.queen.mybatis.mapper.UserMapper.getUserByIdStep]-[DEBUG] ==> Parameters: 1(Integer) 2017-08-08 23:26:04,909 [main] [com.queen.mybatis.mapper.RoleMapper.getRoleById]-[DEBUG] ====> Preparing: select id, roleName from t_role where id=? 2017-08-08 23:26:04,909 [main] [com.queen.mybatis.mapper.RoleMapper.getRoleById]-[DEBUG] ====> Parameters: 1(Integer) 2017-08-08 23:26:04,911 [main] [com.queen.mybatis.mapper.RoleMapper.getRoleById]-[DEBUG] <==== Total: 1 2017-08-08 23:26:04,911 [main] [com.queen.mybatis.mapper.UserMapper.getUserByIdStep]-[DEBUG] <== Total: 1 User [id=1, loginId=queen, userName=奎恩, role=Role [id=1, roleName=自定义角色], note=专门负责提鞋的。。。] Role [id=1, roleName=自定义角色]
从上述打印结果，可以看到控制台打印了两段SQL语句，一句用来查询User信息，另外一句用来查询Role信息，实现了分步查询的效果，且控制台打印结果正常。
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