Mybatis延迟加载的实现示例

<settings>
    <!-- 开启延迟加载 -->
    <setting name="lazyLoadingEnabled" value="true"/>
    <!-- 将积极加载改为消极加载（按需加载） -->
    <setting name="aggressiveLazyLoading" value="false"/>
</settings>

public class Account implements Serializable {
    private Integer id;
    private Integer uid;
    private Double money;
    // 关联的用户对象
    private User user;
    // getter/setter/toString省略
}

public class User implements Serializable {
    private Integer id;
    private String name;
    private Date birthday;
    private String sex;
    private String address;
    // getter/setter/toString省略
}

public interface AccountMapper {
    // 查询所有账户
    List<Account> findAll();
}

public interface UserMapper {
    // 根据ID查询用户
    User findById(Integer id);
}

<mapper namespace="com.qcby.mapper.AccountMapper">
    <!-- 延迟加载配置 -->
    <select id="findAll" resultMap="accountMap">
        select * from account
    </select>
    <resultMap id="accountMap" type="Account">
        <id column="id" property="id"/>
        <result column="uid" property="uid"/>
        <result column="money" property="money"/>
        <!-- 配置延迟加载 -->
        <association 
            property="user"        <!-- 关联对象在Account中的属性名 -->
            javaType="User"        <!-- 关联对象的类型 -->
            select="com.qcby.mapper.UserMapper.findById"  <!-- 延迟加载时执行的查询 -->
            column="uid"/>         <!-- 传递给select查询的参数（账户表中的uid字段） -->
    </resultMap>
</mapper>

<mapper namespace="com.qcby.mapper.UserMapper">
    <select id="findById" parameterType="int" resultType="User">
        select * from user where id = #{id}
    </select>
</mapper>

public class Test02 {
    private InputStream in;
    private SqlSession session;
    private AccountMapper mapper;
    @Before
    public void init() throws Exception {
        // 加载配置文件
        in = Resources.getResourceAsStream("SqlMapConfig.xml");
        // 创建工厂对象
        SqlSessionFactory factory = new SqlSessionFactoryBuilder().build(in);
        // 创建session对象
        session = factory.openSession();
        // 获取代理对象
        mapper = session.getMapper(AccountMapper.class);
    }
    @After
    public void destroy() throws Exception {
        session.close();
        in.close();
    }
    @Test
    public void testLazyLoading() {
        // 1. 查询所有账户（此时只会执行account表的查询）
        List<Account> accounts = mapper.findAll();
        // 2. 遍历账户列表
        for (Account account : accounts) {
            System.out.println("账户信息：" + account.getMoney());
            // 3. 当需要使用user属性时，才会执行user表的查询（延迟加载触发点）
            System.out.println("用户信息：" + account.getUser().getName());
        }
    }
}