Mybatis基于注解实现多表查询
对应的四种数据库表关系中存在四种关系：⼀对多，多对应，⼀对⼀，多对多。在前⽂中已经实现了xml配置⽅式实现表关系的查询，本⽂记录⼀下Mybatis怎么通过注解实现多表的查询，算是⼀个知识的补充。
同样的先介绍⼀下Demo的情况：存在两个实体类⽤户类和账户类，⽤户类可能存在多个账户，即⼀对多的表关系。每个账户只能属于⼀个⽤户，即⼀对⼀或者多对⼀关系。我们最后实现两个⽅法，第⼀个实现查询所有⽤户信息并同时查询出每个⽤户的账户信息，第⼆个实现查询所有的账户信息并且同时查询出其所属的⽤户信息。
1.项⽬结构
2.领域类
public class Account implements Serializable{
private Integer id;
private Integer uid;
private double money;
private User user; //加⼊所属⽤户的属性
省略get 和set ⽅法.............................
}
public class User implements Serializable{
private Integer userId;
private String userName;
private Date userBirthday;
private String userSex;
private String userAddress;
private List<Account> accounts;
省略get 和set ⽅法.............................
}
在User中因为⼀个⽤户有多个账户所以添加Account的列表，在Account中因为⼀个账户只能属于⼀个User，所以添加User的对象。
3.Dao层
1public interface AccountDao {
2/**
3 *查询所有账户并同时查询出所属账户信息
4*/
5 @Select("select * from account")
6 @Results(id = "accountMap",value = {
7 @Result(id = true,property = "id",column = "id"),
8 @Result(property = "uid",column = "uid"),
9 @Result(property = "money",column = "money"),
10//配置⽤户查询的⽅式 column代表的传⼊的字段，⼀对⼀查询⽤one select 代表使⽤的⽅法的全限定名， fetchType表⽰查询的⽅式为⽴即加载还是懒加载
11 @Result(property = "user",column = "uid",one = @One(select = "ample.dao.UserDao.findById",fetchType = FetchType.EAGER))
12 })
13 List<Account> findAll();
14
15/**
16 * 根据⽤户ID查询所有账户
17 * @param id
18 * @return
19*/
20 @Select("select * from account where uid = #{id}")
21 List<Account> findAccountByUid(Integer id);
22 }
23
24
25
26public interface UserDao {
27/**
28 * 查所有⽤户
29 * @return
30*/
31 @Select("select * from User")
32 @Results(id = "userMap",value = {@Result(id = true,column = "id",property = "userId"),
33 @Result(column = "username",property = "userName"),
34 @Result(column = "birthday",property = "userBirthday"),
35 @Result(column = "sex",property = "userSex"),
36 @Result(column = "address",property = "userAddress"),
37 @Result(column = "id",property = "accounts",many = @Many(select = "ample.dao.AccountDao.findAccountByUid",fetchType = FetchType.LAZY))
38 })
39 List<User> findAll();
40
41/**
42 * 保存⽤户
43 * @param user
44*/
45 @Insert("insert into user(username,birthday,sex,address) values(#{username},#{birthday},#{sex},#{address})")
46void saveUser(User user);
47
48/**
49 * 更新⽤户
50 * @param user
51*/
52 @Update("update user set username=#{username},birthday=#{birthday},sex=#{sex},address=#{address} where id=#{id}")
53void updateUser(User user);
54
55/**heap和stack区别
56 * 删除⽤户
57 * @param id
58*/
59 @Delete("delete from user where id=#{id}")
60void deleteUser(Integer id);
61
62/**
63 * 查询⽤户根据ID
64 * @param id
65 * @return
66*/
67 @Select("select * from user where id=#{id}")
68 @ResultMap(value = {"userMap"})
69 User findById(Integer id);
70
71/**
72 * 根据⽤户名称查询⽤户
73 * @param name
74 * @return
75*/
76// @Select("select * from user where username like #{name}")
77 @Select("select * from user where username like '%${value}%'")
78 List<User> findByUserName(String name);
79
80/**
81 * 查询⽤户数量
82 * @return
83*/
84 @Select("select count(*) from user")
85int findTotalUser();
span的用法View Code
在findAll()⽅法中配置@Results的返回值的注解，在@Results注解中使⽤@Result配置根据⽤户和账户的关系⽽添加的属性，User中的属性List<Account>⼀个⽤户有多个账户的关系的映射配置：@Result(column = "id",property = "accounts",many = @Many(select =
"ample.dao.AccountDao.findAccountByUid",fetchType = FetchType.LAZY))，使⽤@Many来向Mybatis表明其⼀对多的关系，@Many中的select属性对应的AccountDao中的findAccountByUid⽅法的全限定名，fetchType代表使⽤⽴即加载或者延迟加载，因为这⾥为⼀对多根据前⾯的讲解，懒加载的使⽤⽅式介绍⼀对多关系⼀般使⽤延迟加载，所以这⾥配置为LAZY⽅式。在Account中存在多对⼀或者⼀对⼀关系，所以配置返回值属性时使⽤：@Result(property = "user",column = "uid",one = @One(select = "ample.dao.UserDao.findById",fetchType = FetchType.EAGER))，property代表领域类中声明的属性，column代表传⼊后⾯select语句中的参数，因为这⾥为⼀对⼀或者说为多对
⼀，所以使⽤@One注解来描述其关系，EAGER表⽰使⽤⽴即加载的⽅式，select代表查询本条数据时所⽤的⽅法的全限定名，fetchType代表使⽤⽴即加载还是延迟加载。
4.Demo中Mybatis的配置
<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration
PUBLIC "-////DTD Config 3.0//EN"linux操作系统镜像下载
"/dtd/mybatis-3-config.dtd">
<configuration>
<settings>
<!--<!&ndash；开启全局的懒加载–>-->
<!--<setting name="lazyLoadingEnabled" value="true"/>-->
1到10随机数生成器不重复<!--<!&ndash；关闭⽴即加载，其实不⽤配置，默认为false–>-->
<!--<setting name="aggressiveLazyLoading" value="false"/>-->
<!--开启Mybatis的sql执⾏相关信息打印-->
<setting name="logImpl" value="STDOUT_LOGGING"/>
<!--<setting name="cacheEnabled" value="true"/>-->
</settings>
<typeAliases>
<typeAlias type="ample.domain.User" alias="user"/>
<package name="ample.domain"/>
</typeAliases>
<environments default="test">
<environment id="test">
<!--配置事务-->
<transactionManager type="jdbc"></transactionManager>
<!--配置连接池-->
<dataSource type="POOLED">
<property name="driver" value="sql.jdbc.Driver"/>
<property name="url" value="jdbc:mysql://localhost:3306/test1"/>
<property name="username" value="root"/>
<property name="password" value="123456"/>
</dataSource>
</environment>
</environments>
<mappers>
<package name="ample.dao"/>
</mappers>
</configuration>
主要是记得开启mybatis中sql执⾏情况的打印，⽅便我们查看执⾏情况。
5.测试
（1）测试查询⽤户同时查询出其账户的信息
测试代码：
public class UserTest {
private InputStream in;
private SqlSessionFactory sqlSessionFactory;
private SqlSession sqlSession;
private UserDao userDao;
@Before
public void init()throws Exception{
in = ResourceAsStream("l");
sqlSessionFactory = new SqlSessionFactoryBuilder().build(in);
sqlSession = sqlSessionFactory.openSession();
userDao = Mapper(UserDao.class);
}
@After
public void destory()throws Exception{
sqlSessionmit();
sqlSession.close();
in.close();
}
@Test
public void testFindAll(){
List<User> userList = userDao.findAll();
for (User user: userList){
System.out.println("每个⽤户信息");
System.out.println(user);
System.out.Accounts());
}
}
测试结果：
（2）查询所有账户信息同时查询出其所属的⽤户信息 测试代码：
public class AccountTest {
private InputStream in;
private SqlSessionFactory sqlSessionFactory;
private SqlSession sqlSession;
private AccountDao accountDao;
@Before
public void init()throws Exception{
in = ResourceAsStream("l"); sqlSessionFactory = new SqlSessionFactoryBuilder().build(in); sqlSession = sqlSessionFactory.openSession();
accountDao = Mapper(AccountDao.class);
}
@After
public void destory()throws Exception{
sqlSessionmit();
source代码sqlSession.close();
in.close();
}
@Test
public void testFindAll(){
List<Account> accountList = accountDao.findAll();
for (Account account: accountList){
System.out.println("查询的每个账户");
mysql语句多表查询System.out.println(account);
System.out.User());
}
}
}
测试结果：
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