查询订单表的时候，可以关联查出订单的详情，也可以需要订单详情的时候，再去订单详情表查询。前者对应的就是立即加载，后者对应的就是懒加载。
延迟加载：在真正使用数据的时候才发起查询，不用的时候不查询关联的数据，延迟加载又叫按需查询（懒加载）
立即加载：不管用不用，只要一调用方法，马上发起查询。
举例前的数据准备
dao层：
IUserDao：
/**
* 用户的持久层接口
*/
public interface IUserDao {
//同时获取用户的所用账户
List<User> findAll();
User findUserById(Integer id);
}
IAccountDao：
public interface IAccountDao {
List<Account> findAll();
//查询所有账户，包含对应的用户名称和地址-通过写account的子类方式查询
List<AccountUser> findAllAccount();
}
domain层：
Account：
public class Account implements Serializable {
//windows下的mysql 和 实体对应不区分大小写
private Integer id;
private Integer uid;
private Double money;
//一对一
private User user;
//生成getter()和setter()、toString()
}
User :
public class User implements Serializable {
private Integer id;
private String username;
private Date birthday;
private String sex;
private String address;
//一对多
private List<Account> accounts;
//生成getter()和setter()、toString()
}
立即加载
查询用户以及该用户下的所用账户
IUserDao.xml
<mapper namespace="cn.ith.dao.IUserDao">
<resultMap id="userAccountMap" type="user">
<id property="id" column="id"></id>
<result property="username" column="username"></result>
<result property="birthday" column="birthday"></result>
<result property="sex" column="sex"></result>
<result property="address" column="address"></result>
<!--ofType：集合中元素的类型-->
<collection property="accounts" ofType="account">
<id property="id" column="aid"></id>
<result property="uid" column="uid"></result>
<result property="money" column="money"></result>
</collection>
</resultMap>
<select id="findAll" resultMap="userAccountMap">
select a.id as aid, a.uid, a.money, u.* from `user` u left join account a on u.id=a.uid
</select>
</mapper>
测试类：
public class IAccountDaoTest {
private InputStream in;
private SqlSession session;
private IAccountDao accountDao;
@Before
public void init() throws IOException {
//1读取配置文件
in = Resources.getResourceAsStream("SqlMapConfig.xml");
//2创建SqlSessionFactory工厂
SqlSessionFactoryBuilder builder = new SqlSessionFactoryBuilder();
SqlSessionFactory factory = builder.build(in);
//3使用工厂生产SqlSession对象
session = factory.openSession();
//4使用SqlSession创建Dao接口的代理对象
accountDao = session.getMapper(IAccountDao.class);
}
@After
public void destory() throws IOException {
session.commit();
//6释放资源
session.close();
in.close();
}
@Test
public void findAll(){
List<User> users = userDao.findAll();
for (User user : users) {
System.out.println(user);
List<Account> accounts = user.getAccounts();
for (Account account : accounts) {
System.out.println(account);
}
System.out.println("-----------------------");
}
}
}
结果：
延迟加载
在MyBatis的主配置文件中，一般是SqlMapConfig.xml文件，加入如下配置：
<configuration>
......
<settings>
<!--开启mybatis支持延迟加载-->
<setting name="lazyLoadingEnabled" value="true"/>
<setting name="aggressiveLazyLoading" value="false"/>
</settings>
......
</configuration>
IAccountDao.xml
<mapper namespace="cn.ith.dao.IAccountDao">
<!--定义封装account和user的resultMap-->
<resultMap id="accountUserMap" type="account">
<id property="id" column="id"></id>
<result property="uid" column="uid"></result>
<result property="money" column="money"></result>
<!--select:查询用户的唯一标识 column:findUserById的id参数-->
<association property="user" column="uid" javaType="user" select="cn.ith.dao.IUserDao.findUserById">
</association>
</resultMap>
<select id="findAll" resultMap="accountUserMap">
SELECT * FROM account
</select>
<select id="findAccountByUid" resultType="account">
select * from account where uid = #{uid}
</select>
</mapper>
IUserDao.xml
<mapper namespace="cn.ith.dao.IUserDao">
<resultMap id="userAccountMap" type="user">
<id property="id" column="id"></id>
<result property="username" column="username"></result>
<result property="birthday" column="birthday"></result>
<result property="sex" column="sex"></result>
<result property="address" column="address"></result>
<!--ofType：集合中元素的类型-->
<collection property="accounts" column="id" ofType="account" select="cn.ith.dao.IAccountDao.findAccountByUid">
</collection>
</resultMap>
<!--配置查询所有-->
<select id="findAll" resultMap="userAccountMap">
select * from user
</select>
<select id="findUserById" parameterType="Integer" resultType="user">
select * from user where id=#{id}
</select>
</mapper>
测试，当只需要用户信息的时候，只执行了select * from user
public class IAccountDaoTest {
private InputStream in;
private SqlSession session;
private IAccountDao accountDao;
@Before
public void init() throws IOException {
//1读取配置文件
in = Resources.getResourceAsStream("SqlMapConfig.xml");
//2创建SqlSessionFactory工厂
SqlSessionFactoryBuilder builder = new SqlSessionFactoryBuilder();
SqlSessionFactory factory = builder.build(in);
//3使用工厂生产SqlSession对象
session = factory.openSession();
//4使用SqlSession创建Dao接口的代理对象
accountDao = session.getMapper(IAccountDao.class);
}
@After
public void destory() throws IOException {
session.commit();
//6释放资源
session.close();
in.close();
}
@Test
public void findAll(){
List<User> users = userDao.findAll();
}
}
测试，当需要用户下的账户信息的时候，又去分别执行了select * from account where uid = ?
@Test
public void findAll(){
List<User> users = userDao.findAll();
for (User user : users) {
System.out.println(user);
List<Account> accounts = user.getAccounts();
for (Account account : accounts) {
System.out.println(account);
}
System.out.println("-----------------------");
}
}
