【Java Web开发指南】ORM一些基础问题整理

深藏阁楼爱情的钟 2022-10-15 12:49 133阅读 0赞

### 文章目录 ###

*  1.什么是ORM?请说明两种常用的ORM框架的区别是什么。
 *  2.在进行实体关系映射时，如果遇到实体类中的属性名与表中的字段名不一致的情况，该如何解决。请给出至少两种解决方案。
 *  3.从线程安全及生命周期的角度说明SqlSessionFactory 与SqlSession的区别，并且给出至少两种能够获取SqlSessionFactory对象的方式。
 *  4.Mybatis 中一级缓存、二级缓存存放的位置分别是哪里?如果要在程序中实现二级缓存，需要进行的配置都有哪些?
 *  5 设计
 *   *  PO
     *  utils
     *  mapper

最强知识图谱！

学习Java或者全栈开发强烈推荐！我说的！

![在这里插入图片描述][20210525101927916.png_x-oss-process_image_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzgzODc4NQ_size_16_color_FFFFFF_t_70]

# 1.什么是ORM?请说明两种常用的ORM框架的区别是什么。 #

ORM即对象关系映射框架。使用ORM框架后，应用程序不再直接访问底层数据库，而是以面向对象的方式来操作持久化对象（PO），而ORM框架则会通过映射关系将这些面向对象的操作转换成底层的SQL操作。  
常见的ORM框架有Hibernate和MyBatis，区别如下：  
Hibernate是一个全表映射的框架。适合场景不太复杂且对性能要求不高的项目中使用。  
MyBatis是一个半自动映射的框架。对于一些复杂和需要优化性能的项目来说，显然使用MyBatis更合适。

# 2.在进行实体关系映射时，如果遇到实体类中的属性名与表中的字段名不一致的情况，该如何解决。请给出至少两种解决方案。 #

public class User
    { 
    	private Long id;
    	private String userName;
    	private String passWord;
    	/** * ... * get,set方法不再列出 * ... * */
    }

1.既然是因为名字不一致而导致Mybatis不能完成自动映射，那么就可以将两者的名字想办法保持一致，为查询出的数据库字段起个别名就可以，实例如下：

<select id="selectUserById" resultType="User">
            select 
            id,
            user_name as userName,
            user_password as password,
            from user
            where id = #{ id}
    </select>

2.另一种解决名字不一致的方式是事先指定映射关系，这样Mybatis也能自动完成映射。Mybatis提供了resultMap可以指定映射关系，示例如下：

<resultMap type="User" id="UserResultMap">
    		<id column="id" property="id"/>
    		<result column="user_name" property="userName"/>
    		<result column="user_password" property="userPassword"/>
    </resultMap>
    	
    	
    <select id="selectUserById" resultMap="UserResultMap">
            select 
            id,
            user_name,
            user_password,
            from user
            where id = #{ id}
    </select>

# 3.从线程安全及生命周期的角度说明SqlSessionFactory 与SqlSession的区别，并且给出至少两种能够获取SqlSessionFactory对象的方式。 #

SqlSessionFactory一旦被创建，SqlSessionFactory应该在你的项目执行期间都存在。没有理由来处理或重新创建它，最简单的就是使用单例模式。SqlSessionFactory对象是线程安全的。  
至于SqlSession，每个线程都应该有它自己的SqlSession实例。SqlSession的实例不能被共享，也是线程不安全的。因此最佳的范围是请求或方法范围。SqlSession是一个单线程对象。

SqlSessionFactory对象的实例可以通过SqlSessionFactoryBuilder对象来构建，而SqlSessionFactoryBuilder则可以通过XML配置文件或一个预先定义好的Configuration实例构建出SqlSessionFactory的实例。

//1.根据给定的Configuration对象创建一个SqlSessionFactory的实例
    SqlSessionFactory sqlSessionFactory = 
    new SqlSessionFactoryBuilder().build(config);
    //2.根据给定的inputStream字节流创建一个SqlSessionFactory的实例
    SqlSessionFactory sqlSessionFactory =
    new SqlSessionFactoryBuilder().build(inputStream);

InputStream inputStream= Resources.getResourceAsStream("mybatis-config.xml");

# 4.Mybatis 中一级缓存、二级缓存存放的位置分别是哪里?如果要在程序中实现二级缓存，需要进行的配置都有哪些? #

![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzgzODc4NQ_size_16_color_FFFFFF_t_70]  
清空之前，一条语句，一次查询，两个相同结果。  
清空之后，两条语句，两次查询，两个相同结果。  
二级缓存：  
在mybatis-config中：  
![在这里插入图片描述][20210525152718833.png]  
在mapper中：  
![在这里插入图片描述][20210525152746114.png]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzgzODc4NQ_size_16_color_FFFFFF_t_70 1]  
![在这里插入图片描述][watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzgzODc4NQ_size_16_color_FFFFFF_t_70 2]

CREATE TABLE t_account(
    	id INT(11) PRIMARY KEY,
    	money FLOAT(10),
    	userid INT(11) 
    	);
    alter TABLE t_account ADD CONSTRAINT 
    FK_ID FOREIGN KEY(id) REFERENCES t_user(id);

# 5 设计 #

## PO ##

Account.java

package com.test.po;
    
    public class Account {
        private int id;
    
        public int getId() {
            return id;
        }
    
        public void setId(int id) {
            this.id = id;
        }
    
        public float getMoney() {
            return money;
        }
    
        public void setMoney(float money) {
            this.money = money;
        }
    
        @Override
        public String toString() {
            return "Account{" +
                    "id=" + id +
                    ", money=" + money +
                    ", userid=" + userid +
                    '}';
        }
    
        public int getUserid() {
            return userid;
        }
    
        public void setUserid(int userid) {
            this.userid = userid;
        }
    
        private float money;
        private int userid;
    }

Role.java

package com.test.po;
    
    import java.util.List;
    
    public class Role {
        private Integer id;
        private String name;
    
        public Integer getId() {
            return id;
        }
    
        public void setId(Integer id) {
            this.id = id;
        }
    
        public String getName() {
            return name;
        }
    
        public void setName(String name) {
            this.name = name;
        }
    
        public String getDesc() {
            return desc;
        }
    
        @Override
        public String toString() {
            return "Role{" +
                    "id=" + id +
                    ", name='" + name + '\'' +
                    ", desc='" + desc + '\'' +
                    ", userList=" + userList +
                    '}';
        }
    
        public void setDesc(String desc) {
            this.desc = desc;
        }
    
        private String desc;
    
        public List<User> getUserList() {
            return userList;
        }
    
        public void setUserList(List<User> userList) {
            this.userList = userList;
        }
    
        private List<User> userList;
    
    }

User.java

package com.test.po;
    
    import java.util.List;
    
    public class User {
        private Integer id;
        private String username;
    
        public Integer getId() {
            return id;
        }
    
        public void setId(Integer id) {
            this.id = id;
        }
    
    
        @Override
        public String toString() {
            return "User{" +
                    "id=" + id +
                    ", username='" + username + '\'' +
                    ", sex='" + sex + '\'' +
                    ", address='" + address + '\'' +
                    ", roleList=" + roleList +
                    '}';
        }
    
        public String getUsername() {
            return username;
        }
    
        public void setUsername(String username) {
            this.username = username;
        }
    
        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;
        }
    
        private String sex;
        private String address;
    
    //    public List<Account> getAccountList() {
    //        return accountList;
    //    }
    //
    //    public void setAccountList(List<Account> accountList) {
    //        this.accountList = accountList;
    //    }
    //
    //    private List<Account> accountList;
        private List<Role> roleList;
    
    }

## utils ##

MyBatisUtils.java

package com.test.utils;
    
    import org.apache.ibatis.io.Resources;
    import org.apache.ibatis.session.SqlSession;
    import org.apache.ibatis.session.SqlSessionFactory;
    import org.apache.ibatis.session.SqlSessionFactoryBuilder;
    
    import java.io.InputStream;
    import java.io.Reader;
    
    public class MyBatisUtils {
        private static SqlSessionFactory sqlSessionFactory=null;
        static {
            try{
                Reader reader= Resources.getResourceAsReader("mybatis-config.xml");
                sqlSessionFactory= new SqlSessionFactoryBuilder().build(reader);
            }catch (Exception e){
                e.printStackTrace();
            }
        }
        public static SqlSession getSession(){
            return sqlSessionFactory.openSession();
        }
    }

## mapper ##

UserMapper.java

package com.test.mapper;
    
    import com.test.po.User;
    
    import java.util.List;
    
    public interface UserMapper {
        public List<User> findUserByName(String name);
        public List<User> findUserByDynamicQuery(User user);
        public User findAccountWithUser(Integer id);
        public List<User> findRoleWithUser(Integer id);
    }

UserMapper.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.test.mapper.UserMapper">
        
        <select id="findUserByName" parameterType="String" resultType="user">
    		select * from t_user where username like concat('%',#{value},'%')
    	</select>
        <select id="findUserByDynamicQuery" parameterType="user" resultType="user">
        select * from t_user where 1=1
            <choose>
            <when test="username!=null and username!=''">
                and username=#{username}
            </when>
            <when test="sex!=null and sex!=''">
                and sex=#{sex}
            </when>
            <otherwise>
                and username=#{username} and sex=#{sex}
            </otherwise>
            </choose>
        </select>
        <select id="findAccountWithUser" parameterType="Integer" resultMap="resultMap1">
            select u.*,a.id as account_id,a.money
                from t_user u,t_account a
                where u.id=a.userid
                and u.id=#{id}
        </select>
        <resultMap id="resultMap1" type="user">
            <id property="id" column="id"/>
            <result property="username" column="username"/>
            <result property="sex" column="sex"/>
            <result property="address" column="address"/>
            <collection property="accountList" ofType="account">
                <id property="id" column="account_id"/>
                <result property="money" column="money"/>
            </collection>
        </resultMap>
        <select id="findRoleWithUser" parameterType="Integer" resultMap="resultMap2">
            select * from t_user where id=#{id}
        </select>
        <resultMap id="resultMap2" type="user">
            <id property="id" column="id"/>
            <result property="username" column="username"/>
            <result property="sex" column="sex"/>
            <result property="address" column="address"/>
            <collection property="roleList" column="id" ofType="role" select="com.test.mapper.RoleMapper.findRoleById">
            </collection>
        </resultMap>
    </mapper>

RoleMapper.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.test.mapper.RoleMapper">
        <select id="findRoleById" parameterType="Integer" resultType="role">
            select * from t_role where id in(
                select role_id from user_role where user_id=#{id}
            )
        </select>
    </mapper>

applicationContext.xml

<?xml version="1.0" encoding="UTF-8"?>
    <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx" xmlns:context="http://www.springframework.org/schema/context" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.3.xsd http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-4.3.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-4.3.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-4.3.xsd">
        
        <context:property-placeholder location="classpath:db.properties"/>
        
    	<bean id="dataSource" class="org.apache.commons.dbcp2.BasicDataSource">
            
            <property name="driverClassName" value="${jdbc.driver}" />
            
            <property name="url" value="${jdbc.url}" />
            
            <property name="username" value="${jdbc.username}" />
            
            <property name="password" value="${jdbc.password}" />
            
            <property name="maxTotal" value="${jdbc.maxTotal}" />
            
            <property name="maxIdle" value="${jdbc.maxIdle}" />
            
            <property name="initialSize" value="${jdbc.initialSize}" />
    	</bean>
    
        
        <bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
             
             <property name="dataSource" ref="dataSource" />
             
       		 <property name="configLocation" value="classpath:mybatis-config.xml"/>
       </bean>
    
    	
    	<bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
    	     <property name="basePackage" value="com.test.mapper" />
    	</bean>
    
    </beans>

db.properties

jdbc.driver=com.mysql.jdbc.Driver
    jdbc.url=jdbc:mysql://localhost:3306/test
    jdbc.username=root
    jdbc.password=lixiang666
    jdbc.maxTotal=30
    jdbc.maxIdle=10
    jdbc.initialSize=5

log4j.properties

# Global logging configuration
    log4j.rootLogger=ERROR, stdout
    # MyBatis logging configuration...
    log4j.logger.com.test=DEBUG
    # Console output...
    log4j.appender.stdout=org.apache.log4j.ConsoleAppender
    log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
    log4j.appender.stdout.layout.ConversionPattern=%5p [%t] - %m%n

mybatis-config.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>
    	<properties resource="db.properties" />
    	
    	<typeAliases>
    		<package name="com.test.po" />
    	</typeAliases>
    
    	
    	<mappers>
    		<mapper resource="com/test/mapper/UserMapper.xml" />
    		<mapper resource="com/test/mapper/RoleMapper.xml" />
    	</mappers>
    </configuration>

test.java

import com.test.mapper.UserMapper;
    import com.test.po.Role;
    import com.test.po.User;
    import com.test.utils.MyBatisUtils;
    import org.apache.ibatis.session.SqlSession;
    import org.junit.Test;
    import org.springframework.context.ApplicationContext;
    import org.springframework.context.support.ClassPathXmlApplicationContext;
    
    import java.util.List;
    
    public class test {
        @Test
        public void findUserByLikeNameTest(){
            ApplicationContext act=new ClassPathXmlApplicationContext("applicationContext.xml");
            UserMapper userMapper=act.getBean(UserMapper.class);
            List<User> userList=userMapper.findUserByName("zhang");
            for(User user:userList){
                System.out.println(user.toString());
            }
        }
        @Test
        public void findUserByDynamicQueryTest(){
            ApplicationContext act=new ClassPathXmlApplicationContext("applicationContext.xml");
            UserMapper userMapper=act.getBean(UserMapper.class);
            User user=new User();
            user.setSex("female");
            List<User> userList=userMapper.findUserByDynamicQuery(user);
            for(User u:userList){
                System.out.println(u.toString());
            }
        }
        @Test
        public void findAccountWithUserTest(){
            ApplicationContext act=new ClassPathXmlApplicationContext("applicationContext.xml");
            UserMapper userMapper=act.getBean(UserMapper.class);
            User user=userMapper.findAccountWithUser(1);
            System.out.println(user.toString());
        }
        @Test
        public void findRoleWithUserTest(){
            ApplicationContext act=new ClassPathXmlApplicationContext("applicationContext.xml");
            UserMapper userMapper=act.getBean(UserMapper.class);
            List<User> userList=userMapper.findRoleWithUser(1);
            for(User u:userList) {
                System.out.println(u.toString());
            }
        }
    }

[20210525101927916.png_x-oss-process_image_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzgzODc4NQ_size_16_color_FFFFFF_t_70]: /images/20221014/ba0c456b569c49dcbd8c518c82731785.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzgzODc4NQ_size_16_color_FFFFFF_t_70]: /images/20221014/d740a95fdd064a12b49cbabb773203bb.png
[20210525152718833.png]: https://img-blog.csdnimg.cn/20210525152718833.png
[20210525152746114.png]: https://img-blog.csdnimg.cn/20210525152746114.png
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzgzODc4NQ_size_16_color_FFFFFF_t_70 1]: https://img-blog.csdnimg.cn/20210525153107674.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzgzODc4NQ==,size_16,color_FFFFFF,t_70
[watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzgzODc4NQ_size_16_color_FFFFFF_t_70 2]: https://img-blog.csdnimg.cn/20210525153145323.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3dlaXhpbl80MzgzODc4NQ==,size_16,color_FFFFFF,t_70