Java mybatis 无参构造器

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wzw45233 2024-10-22 14:29:24 ©著作权

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在使用 MyBatis 进行持久层开发时，实体类通常需要提供一个无参构造器。这是因为 MyBatis 在映射结果集到实体对象时，会通过反射调用无参构造器来创建对象实例。如果实体类没有无参构造器，MyBatis 将无法创建对象，从而导致运行时错误。

以下是一个简单的示例，展示如何在实体类中定义无参构造器，并使用 MyBatis 进行基本的 CRUD 操作。

实体类

假设有一个 User 实体类，包含 id、name 和 email 属性：

public class User {
    private Integer id;
    private String name;
    private String email;

    // 无参构造器
    public User() {
    }

    // 全参构造器（可选）
    public User(Integer id, String name, String email) {
        this.id = id;
        this.name = name;
        this.email = email;
    }

    // Getter 和 Setter 方法
    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 getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    @Override
    public String toString() {
        return "User{" +
                "id=" + id +
                ", name='" + name + '\'' +
                ", email='" + email + '\'' +
                '}';
    }
}

MyBatis 配置

MyBatis 配置文件 `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>
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.cj.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/your_database?useSSL=false&serverTimezone=UTC"/>
                <property name="username" value="your_username"/>
                <property name="password" value="your_password"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="mapper/UserMapper.xml"/>
    </mappers>
</configuration>

Mapper 接口 `UserMapper.java`

import org.apache.ibatis.annotations.*;

import java.util.List;

@Mapper
public interface UserMapper {
    @Select("SELECT * FROM users")
    List<User> getAllUsers();

    @Select("SELECT * FROM users WHERE id = #{id}")
    User getUserById(@Param("id") Integer id);

    @Insert("INSERT INTO users(name, email) VALUES(#{name}, #{email})")
    @Options(useGeneratedKeys = true, keyProperty = "id")
    int insertUser(User user);

    @Update("UPDATE users SET name = #{name}, email = #{email} WHERE id = #{id}")
    int updateUser(User user);

    @Delete("DELETE FROM users WHERE id = #{id}")
    int deleteUser(@Param("id") Integer id);
}

Mapper XML 文件 `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.example.mapper.UserMapper">

    <resultMap id="UserResultMap" type="com.example.entity.User">
        <id property="id" column="id"/>
        <result property="name" column="name"/>
        <result property="email" column="email"/>
    </resultMap>

    <select id="getAllUsers" resultMap="UserResultMap">
        SELECT * FROM users
    </select>

    <select id="getUserById" parameterType="int" resultMap="UserResultMap">
        SELECT * FROM users WHERE id = #{id}
    </select>

    <insert id="insertUser" parameterType="com.example.entity.User" useGeneratedKeys="true" keyProperty="id">
        INSERT INTO users(name, email) VALUES(#{name}, #{email})
    </insert>

    <update id="updateUser" parameterType="com.example.entity.User">
        UPDATE users SET name = #{name}, email = #{email} WHERE id = #{id}
    </update>

    <delete id="deleteUser" parameterType="int">
        DELETE FROM users WHERE id = #{id}
    </delete>

</mapper>

使用示例

import com.example.entity.User;
import com.example.mapper.UserMapper;
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.IOException;
import java.io.InputStream;
import java.util.List;

public class Main {
    public static void main(String[] args) {
        String resource = "mybatis-config.xml";
        InputStream inputStream;
        try {
            inputStream = Resources.getResourceAsStream(resource);
            SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
            SqlSession session = sqlSessionFactory.openSession();

            UserMapper userMapper = session.getMapper(UserMapper.class);

            // 插入新用户
            User newUser = new User(null, "Alice", "alice@example.com");
            userMapper.insertUser(newUser);
            session.commit();

            // 查询所有用户
            List<User> users = userMapper.getAllUsers();
            for (User user : users) {
                System.out.println(user);
            }

            // 更新用户
            newUser.setName("Alicia");
            userMapper.updateUser(newUser);
            session.commit();

            // 删除用户
            userMapper.deleteUser(newUser.getId());
            session.commit();

            session.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}