MyBatis入门-延迟加载与立即加载
加载策略
延迟加载
延迟加载(lazy load)是(也称为懒加载)Hibernate3关联关系对象默认的加载方式，延迟加载机制是为了避免一些无谓的性能开销而提出来的，所谓延迟加载就是当在真正需要数据的时候，才真正执行数据加载操作。延迟加载，可以简单理解为，只有在使用的时候，才会发出sql语句进行查询。
需要在主配置文件开启加载策略，子配置文件使用collection属性
立即加载
所谓立即加载就是查询时，所有的相关数据一次被读取出来，而不是分N次。
一对一实现理解加载——查询账户可查出用户信息（在查询账户是绑定查询用户信息的方法）
一对多实现延迟加载——查询用户可查出所以账户信息（在查询用户是绑定查询账户信息的方法）
基础数据
实体类
public class User implements Serializable {
private Integer id;
private String username;
private Date birthday;
private String sex;
private String address;
//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;
@Override
public String toString() {
return "User{" +
"id=" + id +
", username='" + username + '\'' +
", birthday=" + birthday +
", sex='" + sex + '\'' +
", address='" + address + '\'' +
'}';
}
public List<Account> getAccounts() {
return accounts;
}
public void setAccounts(List<Account> accounts) {
this.accounts = accounts;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public String getUsername() {
return username;
}
public void setUsername(String username) {
this.username = username;
}
public Date getBirthday() {
return birthday;
}
public void setBirthday(Date birthday) {
this.birthday = birthday;
}
public String getSex() {
return sex;
}
public void setSex(String sex) {
this.sex = sex;
}
public String getAddress() {
return address;
}
public void setAddress(String address) {
this.address = address;
}
}
public class Account implements Serializable {
private Integer id;
private Integer uid;
private Double money;
//一对一的关系中
//从表实体应该包含一个主表实体的对象引用
private User user;
public User getUser() {
return user;
}
public void setUser(User user) {
this.user = user;
}
public Integer getId() {
return id;
}
public void setId(Integer id) {
this.id = id;
}
public Integer getUid() {
return uid;
}
public void setUid(Integer uid) {
this.uid = uid;
}
public Double getMoney() {
return money;
}
public void setMoney(Double money) {
this.money = money;
}
@Override
public String toString() {
return "Account{" +
"id=" + id +
", uid=" + uid +
", money=" + money +
'}';
}
}dao层的两个接口
public interface IUserDao {
List<User> findAll();
User findById(Integer i);
}
public interface IAccountDao {
List<Account> findAll();
Account findById(Integer i);
}resource下的主映射文件与子映射文件
主：SqlMapConfig_anno.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">
<!--myBatis的主配置文件 -->
<configuration>
<settings>
<!--开启mybatis支持延迟加载-->
<setting name="lazyLoadingEnabled" value="true"/>
<setting name="aggressiveLazyLoading" value="false"/>
</settings>
<!--配置环境-->
<environments default="mysql">
<!--配置mysql环境-->
<environment id="mysql">
<!--配置事务的类型-->
<transactionManager type="JDBC"></transactionManager>
<!--配置数据源（连接池）-->
<dataSource type="POOLED">
<!--配置连接数据库的基本信息-->
<property name="driver" value="com.mysql.jdbc.Driver"/>
<property name="url" value="jdbc:mysql://localhost:3306/eesy"/>
<property name="username" value="root"/>
<property name="password" value="root"/>
</dataSource>
</environment>
</environments>
<!--映射文件 配置文件方式-->
<mappers>
<mapper resource="com/dynamic_annotation/dao/IAccountDao.xml"></mapper>
<mapper resource="com/dynamic_annotation/dao/IUserDao.xml"></mapper>
</mappers>
<!--映射文件 注解方式(使用注解就要删除源配置文件)-->
<!-- <mappers>
<mapper class="com.dynamic_basics.dao.IUserDao"></mapper>
</mappers>-->
</configuration>子：IAccountDao.xml
<?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.dynamic_annotation.dao.IAccountDao">
<!--定义封装account和user的resultMap-->
<resultMap id="accountUserMap" type="com.dynamic_annotation.domain.Account">
<id property="id" column="id"></id>
<result property="uid" column="uid"></result>
<result property="money" column="money"></result>
<!--立即加载-->
<!--使用select属性，使用其他DAO层的方法-->
<association property="user" column="uid" javaType="com.dynamic_annotation.domain.User" select="com.dynamic_annotation.dao.IUserDao.findById">
</association>
</resultMap>
<!--查询所有 id使用方法名-->
<select id="findAll" resultMap="accountUserMap">
select * from Account
</select>
<!--单个查询-->
<select id="findById" parameterType="int" resultType="com.dynamic_annotation.domain.Account">
select * from Account where uid=#{id}
</select>
</mapper>子：IUserDao.xml
<?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.dynamic_annotation.dao.IUserDao">
<!--定义封装account和user的resultMap-->
<resultMap id="userAccountMap" type="com.dynamic_annotation.domain.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 是用于建立一对多中集合属性的对应关系
ofType 用于指定集合元素的数据类型
select 是用于指定查询账户的唯一标识（账户的 dao 全限定类名加上方法名称）
column 是用于指定使用哪个字段的值作为条件查询
-->
<!--延迟加载-->
<collection property="accounts" ofType="com.dynamic_annotation.domain.Account" select="com.dynamic_annotation.dao.IAccountDao.findById" column="id"></collection>
</resultMap>
<!--查询所有 id使用方法名-->
<select id="findAll" resultMap="userAccountMap">
select * from User
</select>
<!--根据id查询-->
<select id="findById" parameterType="int" resultType="com.dynamic_annotation.domain.User">
select * from user where id=#{id}
</select>
</mapper>测试类
public class AnnotationTest {
private InputStream in;
private SqlSession sqlSession;
private IAccountDao accountDao;
private IUserDao userDao;
@Before
public void init()throws Exception{
//1.读取配置文件 Resources是myBatis封装的类
in= Resources.getResourceAsStream("SqlMapConfig_anno.xml");
//2.创建SQLSessionFactory工厂
// SqlSessionFactory factory=new SqlSessionFactoryBuilder().build(is);
SqlSessionFactoryBuilder builder=new SqlSessionFactoryBuilder();
SqlSessionFactory factory=builder.build(in);
//3.使用工厂生产SQLSession对象
sqlSession = factory.openSession();
//4.使用SQLSession创建DAO接口的代理对象
accountDao = sqlSession.getMapper(IAccountDao.class);
userDao=sqlSession.getMapper(IUserDao.class);
}
@After
public void destroy()throws Exception{
//6.释放资源
//提交事务
sqlSession.commit();
sqlSession.close();
in.close();
}
//入门案例
@Test
public void testFindAllAccount(){
List<Account> accounts = accountDao.findAll();
System.out.println("------每个account的信息------");
for (Account account : accounts) {
System.out.println(account);
System.out.println(account.getUser());
}
}
@Test
public void testFindAccount(){
Account account = accountDao.findById(46);
System.out.println(account);
}
@Test
public void testFindAllUser(){
List<User> users = userDao.findAll();
for (User user : users) {
System.out.println(user);
System.out.println(user.getAccounts());
}
}
@Test
public void testFindUser(){
User user = userDao.findById(49);
System.out.println(user);
}
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