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
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public class User implements Serializable {
/**
* Java实体类为什么要实现Serializable接口
* 1.用于序列化与反序列化--一个类只有实现了Serializable接口,它的对象才能被序列化。
* 2.Serializable接口就是Java提供用来进行高效率的异地共享实例对象的机制,实现这个接口即可。
*/
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
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/**
* @Author: Promsing
* @Date: 2021/4/4 - 16:21
* @Description: 描述 形容
* @version: 1.0
*/
public interface IUserDao {
/**
* 查询所有
* @return
*/
List<User> findAll();
/**
* 根据id查询
* @param i
* @return
*/
User findById(Integer i);
}
public interface IAccountDao {
/**
* 查询所有账户,同时还有获取当前账户所属的用户信息
* @return
*/
List<Account> findAll();
/**
* 根据用户id查询
* @param i
* @return
*/
Account findById(Integer i);
}
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resource下的主映射文件与子映射文件
主:SqlMapConfig_anno.xml
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<? 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 >
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子:IAccountDao.xml
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<? 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 >
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子:IUserDao.xml
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<? 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 >
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测试类
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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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原文链接:https://blog.csdn.net/promsing/article/details/115430797