mybatis 关联查询延迟加载

野性酷女 2023-01-07 07:12 140阅读 0赞

就是在需要用到数据时才进行加载，不需要用到数据时就不加载数据。延迟加载也称懒加载.  
好处：先从单表查询，需要时再从关联表去关联查询，大大提高数据库性能  
坏处：因为只有当需要用到数据时，才会进行数据库查询，所以可能造成用户等待时间变长，造成用户体验下降。

# model #

public class User implements Serializable { 
    
        private Integer id;
        private String username;
        private String address;
        private String sex;
        private Date birthday;
    
        //一对多关系映射：主表实体应该包含从表实体的集合引用
        private List<Account> accounts;
    }
    
    
    public class Account implements Serializable { 
    
        private Integer id;
        private Integer uid;
        private Double money;
    
        //从表实体应该包含一个主表实体的对象引用
        private User user;
    }

# interface #

public interface IUserDao { 
        List<User> findAll();
    }
    public interface IAccountDao { 
    List<Account> findAccountByUid(Integer uid);
    }

# Dao.xml #

IUserDao.xml

<mapper namespace="com.itheima.dao.IUserDao">
    
        
        <resultMap id="userAccountMap" type="user">
            <id property="id" column="id"></id>
            <result property="username" column="username"></result>
            <result property="address" column="address"></result>
            <result property="sex" column="sex"></result>
            <result property="birthday" column="birthday"></result>
            
            <collection property="accounts" ofType="account" select="com.itheima.dao.IAccountDao.findAccountByUid" column="id"></collection>
        </resultMap>
    
        
        <select id="findAll" resultMap="userAccountMap">
            select * from user
        </select>
    </mapper>

IAccountDao.xml

<select id="findAccountByUid" resultType="account">
        select * from account where uid = #{uid}
    </select>

# SqlMapConfig.xml #