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);
} |
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 > |
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 > |
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 > |
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测试数据mit(); 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
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