JAVA高级面试进阶训练营视频教程

admin

pom的依赖配置：

 1 <dependencies>
 2     <dependency>
 3         <groupId>org.mybatis</groupId>
 4         <artifactId>mybatis</artifactId>
 5         <version>3.4.5</version>
 6     </dependency>
 7     <dependency>
 8         <groupId>junit</groupId>
 9         <artifactId>junit</artifactId>
10         <version>4.12</version>
11     </dependency>
12     <dependency>
13         <groupId>mysql</groupId>
14         <artifactId>mysql-connector-java</artifactId>
15         <version>8.0.17</version>
16     </dependency>
17 </dependencies>

 

MySQL数据库的建表语句：

1 CREATE TABLE `tb_user` (
2   `id` int(11) NOT NULL AUTO_INCREMENT,
3   `username` varchar(32) NOT NULL COMMENT '用户名',
4   `sex` char(1) DEFAULT NULL COMMENT '性别',
5   `birthday` date DEFAULT NULL COMMENT '生日',
6   `address` varchar(256) DEFAULT NULL COMMENT '地址',
7   PRIMARY KEY (`id`)
8 ) ;

 

Java实体类代码：

 1 public class User {
 2 
 3     private int id;
 4     private String name;
 5     private String sex;
 6     private Date birthday;
 7     private String address;
 8 
 9     @Override
10     public String toString() {
11         return "User{" +
12                 "id=" + id +
13                 ", name='" + name + '\'' +
14                 ", sex='" + sex + '\'' +
15                 ", birthday=" + birthday +
16                 ", address='" + address + '\'' +
17                 '}';
18     }
19 
20     public int getId() {
21         return id;
22     }
23 
24     public void setId(int id) {
25         this.id = id;
26     }
27 
28     public String getName() {
29         return name;
30     }
31 
32     public void setName(String name) {
33         this.name = name;
34     }
35 
36     public String getSex() {
37         return sex;
38     }
39 
40     public void setSex(String sex) {
41         this.sex = sex;
42     }
43 
44     public Date getBirthday() {
45         return birthday;
46     }
47 
48     public void setBirthday(Date birthday) {
49         this.birthday = birthday;
50     }
51 
52     public String getAddress() {
53         return address;
54     }
55 
56     public void setAddress(String address) {
57         this.address = address;
58     }
59 }

 

MyBatis测试方法代码：

 1     @Test
 2     public void test01() {
 3 
 4         InputStream inputStream = null;
 5         try {
 6             inputStream = Resources.getResourceAsStream("SqlMapConfig.xml");
 7         } catch (IOException e) {
 8             e.printStackTrace();
 9         }
10 
11         SqlSessionFactoryBuilder builder = new SqlSessionFactoryBuilder();
12 
13         SqlSessionFactory factory = builder.build(inputStream);
14 
15         SqlSession sqlSession = factory.openSession();
16 
17         List<User> list = sqlSession.selectList("test.queryList");
18 
19         list.forEach(System.out::println);
20 
21         sqlSession.close();
22 
23     }

 

SqlMapConfig的配置：

 1 <?xml version="1.0" encoding="UTF-8" ?>
 2 <!DOCTYPE configuration
 3         PUBLIC "-//mybatis.org//DTD Config 3.0//EN"
 4         "http://mybatis.org/dtd/mybatis-3-config.dtd">
 5 <configuration>
 6     <environments default="development">
 7         <environment id="development">
 8             <transactionManager type="JDBC" />
 9             <dataSource type="POOLED">
10                 <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
11                 <property name="url" value="jdbc:mysql://127.0.0.1:3306/mybatis01?serverTimezone=UTC" />
12                 <property name="username" value="root" />
13                 <property name="password" value="root" />
14             </dataSource>
15         </environment>
16     </environments>
17 
18     <mappers>
19         <mapper resource="UserMapper.xml" />
20     </mappers>
21 </configuration>    

 

UserMapper的配置：

 1 <?xml version="1.0" encoding="utf-8" ?>
 2 <!DOCTYPE mapper
 3         PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
 4         "http://mybatis.org/dtd/mybatis-3-mapper.dtd">
 5 <mapper namespace="test">
 6 
 7    <select id="queryList" resultType="com.saika.mybatis.pojo.User">
 8       SELECT * from user
 9    </select>
10 
11 </mapper>

 

实体类属性name与表字段username不一致

 

方案一

采用SQL语句AS起别名的形式，为查询语句的显示列定义别名，让这个别名与实体类的属性名相同以完成自动映射。

SQL语句：

1 SELECT id,username AS name ,sex,birthday,address FROM user

 

缺点：

1.如果不一致的字段过多会导致语句过长。

2.如果查询语句有多个，每次都要起别名会导致重复代码增多，多次定义显示列增加了工作量。

 

方案二

修改实体类set方法的方法名对应表字段名。

MyBatis映射的原理就是通过返回的字段名找实体类的setXxx方法进行封装。

Java代码：

1     public void setUsername(String name) {
2         this.name = name;
3     }

 

方案三

利用<sql>标签提取SQL语句片段统一配置别名，在原查询语句上用<include>标签引入，简化操作。

SQL语句：

1    <sql id="user">
2       id,username AS name,sex,birthday,address
3    </sql>
4 
5    <select id="queryList" resultType="com.saika.mybatis.pojo.User">
6       SELECT <include refid="user"></include> FROM user
7    </select>

 

方案四

使用<resultMap>标签配置手动映射。

id 是这个封装规则的唯一标识，用于被查询标签中的resultMap所引用。

type 用于指定将查询结果封装到哪个实体类中。

column是表的字段。

property是实体类的属性。

SQL语句：

1    <resultMap id="userResultMap" type="com.saika.mybatis.pojo.User">
2       <result column="username" property="name"/>
3    </resultMap>
4 
5    <select id="queryList" resultMap="userResultMap">
6       SELECT id,username,sex,birthday,address FROM user
7    </select>

 注解形式：

1 @Result(property ="name",column = "username")