MyBatis入门-延迟加载与立即加载

灰太狼 2021-09-25 06:36 442阅读 0赞

**MyBatis入门-延迟加载与立即加载**

加载策略

延迟加载

立即加载

基础数据

实体类

dao层的两个接口

resource下的主映射文件与子映射文件

SqlMapConfig\_anno.xml

IAccountDao.xml

IUserDao.xml

测试类

--------------------

# 加载策略 #

## 延迟加载 ##

延迟加载(lazy load)是(也称为[懒加载][Link 1])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

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3Byb21zaW5n_size_16_color_FFFFFF_t_70][]

## **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 {
    
        /**
         * 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 ##

/**
     * @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 ##

<?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">
    
    <configuration>
        <settings>
            
            <setting name="lazyLoadingEnabled" value="true"/>
            <setting name="aggressiveLazyLoading" value="false"/>
        </settings>
        
        <environments default="mysql">
            
            <environment id="mysql">
                
                <transactionManager type="JDBC"></transactionManager>
                
                <dataSource type="POOLED">
                    
                    <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>
        
        <mappers>
            <mapper resource="com/dynamic_annotation/dao/IAccountDao.xml"></mapper>
            <mapper resource="com/dynamic_annotation/dao/IUserDao.xml"></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">
    
        
        <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>
           
    
            
            
         <association property="user" column="uid" javaType="com.dynamic_annotation.domain.User" select="com.dynamic_annotation.dao.IUserDao.findById">
    
         </association>
           </resultMap>
    
           
        <select id="findAll" resultMap="accountUserMap">
            select * from Account
        </select>
    
        
        <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">
    
        
        <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 property="accounts" ofType="com.dynamic_annotation.domain.Account" select="com.dynamic_annotation.dao.IAccountDao.findById" column="id"></collection>
        </resultMap>
    
        
        <select id="findAll" resultMap="userAccountMap">
            select * from User
        </select>
    
        
        <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);
        }
    }

其他Mybatis信息请查看相关文章

### [Mybatis入门-基于xml配置实现单表的增删改查][Mybatis_-_xml] ###

### [Mybatis入门-基于注解配置实现单表、多表的增删改查][Mybatis_-] ###

[Link 1]: https://baike.baidu.com/item/%E6%87%92%E5%8A%A0%E8%BD%BD/2838950
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3Byb21zaW5n_size_16_color_FFFFFF_t_70]: /images/20210923/67bf6dff45b34725981135317e1a659f.png
[Mybatis_-_xml]: https://blog.csdn.net/promsing/article/details/115380863
[Mybatis_-]: https://blog.csdn.net/promsing/article/details/115440344