Mapper接口与映射方式

// JDBC 的噩梦：SQL 字符串散落在业务代码中
String sql = "SELECT * FROM student WHERE major = '" + major + "'";
// 修改 SQL 需要改 Java 代码 → 重新编译 → 重新部署

package com.fly.entity;

import java.math.BigDecimal;

public class Student {
    private Integer id;
    private String name;
    private Integer age;
    private String major;
    private BigDecimal score;

    public Student() {}
    public Student(String name, Integer age, String major, BigDecimal score) {
        this.name = name; this.age = age; this.major = major; this.score = score;
    }
    // Getter / Setter 省略...
    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 Integer getAge() { return age; }
    public void setAge(Integer age) { this.age = age; }
    public String getMajor() { return major; }
    public void setMajor(String major) { this.major = major; }
    public BigDecimal getScore() { return score; }
    public void setScore(BigDecimal score) { this.score = score; }
    @Override
    public String toString() {
        return "Student{id=" + id + ", name='" + name + "', age=" + age 
            + ", major='" + major + "', score=" + score + "}";
    }
}

package com.fly.util;

import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;
import java.io.InputStream;

public class MyBatisUtil {
    private static final SqlSessionFactory factory;
    static {
        try (InputStream is = Resources.getResourceAsStream("mybatis-config.xml")) {
            factory = new SqlSessionFactoryBuilder().build(is);
        } catch (Exception e) { throw new ExceptionInInitializerError(e); }
    }
    public static SqlSessionFactory getFactory() { return factory; }
}

package com.fly.mapper;

import com.fly.entity.Student;
import java.util.List;

public interface StudentMapper {
    Student findById(Integer id);
    List<Student> findAll();
    int insert(Student student);
    int update(Student student);
    int deleteById(Integer id);
}

package com.fly.mapper;

import com.fly.entity.Student;
import org.apache.ibatis.annotations.*;
import java.util.List;

public interface StudentMapper {

    @Select("SELECT id, name, age, major, score FROM student WHERE id = #{id}")
    @Results(id = "studentMap", value = {
        @Result(property = "id", column = "id"),
        @Result(property = "name", column = "name"),
        @Result(property = "age", column = "age"),
        @Result(property = "major", column = "major"),
        @Result(property = "score", column = "score")
    })
    Student findById(Integer id);

    @Select("SELECT id, name, age, major, score FROM student")
    @ResultMap("studentMap")
    List<Student> findAll();

    @Insert("INSERT INTO student (name, age, major, score) VALUES (#{name}, #{age}, #{major}, #{score})")
    @Options(useGeneratedKeys = true, keyProperty = "id")
    int insert(Student student);

    @Update("UPDATE student SET name = #{name}, age = #{age}, major = #{major}, score = #{score} WHERE id = #{id}")
    int update(Student student);

    @Delete("DELETE FROM student WHERE id = #{id}")
    int deleteById(Integer id);
}

package com.fly.test;

import com.fly.entity.Student;
import com.fly.mapper.StudentMapper;
import com.fly.util.MyBatisUtil;
import org.apache.ibatis.session.SqlSession;

import java.math.BigDecimal;
import java.util.List;

public class MapperTest {
    public static void main(String[] args) {
        // 1. 查询所有
        try (SqlSession session = MyBatisUtil.getFactory().openSession()) {
            StudentMapper mapper = session.getMapper(StudentMapper.class);
            List<Student> all = mapper.findAll();
            System.out.println("【查询全部】共 " + all.size() + " 条记录:");
            all.forEach(s -> System.out.println("  " + s));
        }

        // 2. 按 ID 查询
        try (SqlSession session = MyBatisUtil.getFactory().openSession()) {
            StudentMapper mapper = session.getMapper(StudentMapper.class);
            Student s = mapper.findById(3);
            System.out.println("\n【按 ID 查询】id=3 => " + s);
        }

        // 3. 插入
        try (SqlSession session = MyBatisUtil.getFactory().openSession()) {
            StudentMapper mapper = session.getMapper(StudentMapper.class);
            Student newbie = new Student("赵六", 23, "人工智能", new BigDecimal("91.00"));
            mapper.insert(newbie);
            session.commit();
            System.out.println("\n【插入】生成主键: " + newbie.getId());
        }

        // 4. 更新
        try (SqlSession session = MyBatisUtil.getFactory().openSession()) {
            StudentMapper mapper = session.getMapper(StudentMapper.class);
            Student update = mapper.findById(1);
            update.setScore(new BigDecimal("99.00"));
            mapper.update(update);
            session.commit();
            System.out.println("\n【更新】大翔分数更新为: " + update.getScore());
        }

        // 5. 删除
        try (SqlSession session = MyBatisUtil.getFactory().openSession()) {
            StudentMapper mapper = session.getMapper(StudentMapper.class);
            int rows = mapper.deleteById(6); // 删除赵六
            session.commit();
            System.out.println("\n【删除】影响行数: " + rows);
        }

        // 6. 验证最终状态
        try (SqlSession session = MyBatisUtil.getFactory().openSession()) {
            StudentMapper mapper = session.getMapper(StudentMapper.class);
            List<Student> all = mapper.findAll();
            System.out.println("\n【最终数据】共 " + all.size() + " 条记录:");
            all.forEach(s -> System.out.println("  " + s));
        }
    }
}