SqlSession核心方法

// JDBC 原始写法：查询单条学生记录
public Student findById_JDBC(int id) {
    String sql = "SELECT id, name, age, major, score FROM student WHERE id = ?";
    try (Connection conn = DriverManager.getConnection(url, user, password);
         PreparedStatement ps = conn.prepareStatement(sql)) {
        ps.setInt(1, id);
        try (ResultSet rs = ps.executeQuery()) {
            if (rs.next()) {
                Student stu = new Student();
                stu.setId(rs.getInt("id"));
                stu.setName(rs.getString("name"));
                stu.setAge(rs.getInt("age"));
                stu.setMajor(rs.getString("major"));
                stu.setScore(rs.getBigDecimal("score"));
                return stu;
            }
        }
    } catch (SQLException e) {
        throw new RuntimeException(e);
    }
    return null;
}

Student student = session.selectOne("com.fly.mapper.StudentMapper.findById", 1);

package com.fly.test;

import com.fly.entity.Student;
import com.fly.util.MyBatisUtil;
import org.apache.ibatis.exceptions.TooManyResultsException;
import org.apache.ibatis.session.SqlSession;

public class SelectOneDemo {

    public static void main(String[] args) {
        // 1. 正确使用 selectOne：查询 ID=1 的大翔
        System.out.println("========== 1. selectOne 正确查询单条 ==========");
        try (SqlSession session = MyBatisUtil.getSqlSessionFactory().openSession()) {
            String statement = "com.fly.mapper.StudentMapper.findById";
            Student student = session.selectOne(statement, 1);
            System.out.println("查询结果: " + student);
        }

        // 2. 错误使用 selectOne：查询专业为"计算机科学"的学生（实际有2条）
        System.out.println("\n========== 2. selectOne 结果不唯一，抛出异常 ==========");
        try (SqlSession session = MyBatisUtil.getSqlSessionFactory().openSession()) {
            String statement = "com.fly.mapper.StudentMapper.findByMajor";
            try {
                // 计算机科学专业有大翔(id=1)和小崔(id=3)，共2条记录
                Student student = session.selectOne(statement, "计算机科学");
                System.out.println("查询结果: " + student);
            } catch (TooManyResultsException e) {
                System.err.println("【异常捕获】TooManyResultsException: " + e.getMessage());
                System.err.println("【原因】selectOne 要求结果必须为 0 或 1 条，实际返回了多条");
                System.err.println("【正解】预期可能返回多条时，应使用 selectList");
            }
        }
    }
}

package com.fly.test;

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

import java.util.List;

public class RowBoundsDemo {

    public static void main(String[] args) {
        System.out.println("========== 1. 不带 RowBounds：查询全部 5 条 ==========");
        try (SqlSession session = MyBatisUtil.getSqlSessionFactory().openSession()) {
            String statement = "com.fly.mapper.StudentMapper.findAll";
            List<Student> allList = session.selectList(statement, null);
            System.out.println("总记录数: " + allList.size());
            allList.forEach(s -> System.out.println("  " + s));
        }

        System.out.println("\n========== 2. 带 RowBounds：offset=2, limit=2 ==========");
        System.out.println("【注意】SQL 中不会出现 LIMIT，全量数据仍会从 MySQL 返回");
        try (SqlSession session = MyBatisUtil.getSqlSessionFactory().openSession()) {
            String statement = "com.fly.mapper.StudentMapper.findAll";
            // offset=2 表示跳过前 2 条，limit=2 表示最多取 2 条
            RowBounds rowBounds = new RowBounds(2, 2);
            List<Student> pageList = session.selectList(statement, null, rowBounds);
            System.out.println("分页后记录数: " + pageList.size());
            pageList.forEach(s -> System.out.println("  " + s));
        }

        System.out.println("\n========== 3. 带 RowBounds：offset=0, limit=3 ==========");
        try (SqlSession session = MyBatisUtil.getSqlSessionFactory().openSession()) {
            String statement = "com.fly.mapper.StudentMapper.findAll";
            RowBounds rowBounds = new RowBounds(0, 3);
            List<Student> pageList = session.selectList(statement, null, rowBounds);
            System.out.println("分页后记录数: " + pageList.size());
            pageList.forEach(s -> System.out.println("  " + s));
        }
    }
}

package com.fly.test;

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

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

public class TransactionDemo {

    /**
     * 查询并打印当前所有学生
     */
    private static void printAllStudents(SqlSession session) {
        String statement = "com.fly.mapper.StudentMapper.findAll";
        List<Student> list = session.selectList(statement);
        System.out.println("当前共 " + list.size() + " 条记录:");
        list.forEach(s -> System.out.println("  " + s));
    }

    public static void main(String[] args) {
        // ========== 事务 1：正常提交 ==========
        System.out.println("========== 事务 1：插入新学生并提交 ==========");
        try (SqlSession session = MyBatisUtil.getSqlSessionFactory().openSession()) {
            String statement = "com.fly.mapper.StudentMapper.insertStudent";

            // 插入学生 "赵六"
            Student zhaoliu = new Student("赵六", 23, "人工智能", new BigDecimal("91.00"));
            int rows1 = session.insert(statement, zhaoliu);
            System.out.println("【插入赵六】影响行数: " + rows1 + ", 生成主键: " + zhaoliu.getId());

            // 插入学生 "孙七"
            Student sunqi = new Student("孙七", 24, "数据科学", new BigDecimal("89.50"));
            int rows2 = session.insert(statement, sunqi);
            System.out.println("【插入孙七】影响行数: " + rows2 + ", 生成主键: " + sunqi.getId());

            // 提交事务
            session.commit();
            System.out.println("【事务提交】数据已持久化到数据库");

            // 在同一个 Session 中查询验证
            printAllStudents(session);
        }

        // ========== 事务 2：故意异常后回滚 ==========
        System.out.println("\n========== 事务 2：插入后回滚 ==========");
        SqlSession session = null;
        try {
            session = MyBatisUtil.getSqlSessionFactory().openSession();
            String statement = "com.fly.mapper.StudentMapper.insertStudent";

            // 插入学生 "周八"
            Student zhouba = new Student("周八", 25, "网络安全", new BigDecimal("93.00"));
            int rows3 = session.insert(statement, zhouba);
            System.out.println("【插入周八】影响行数: " + rows3 + ", 生成主键: " + zhouba.getId());

            // 模拟业务异常：比如后续校验发现该学生不符合入学条件
            if (zhouba.getAge() > 24) {
                throw new RuntimeException("【业务异常】年龄超过 24 岁，不符合入学条件");
            }

            session.commit();
        } catch (Exception e) {
            System.err.println(e.getMessage());
            if (session != null) {
                session.rollback();
                System.err.println("【事务回滚】周八的插入操作已被撤销");
            }
        } finally {
            if (session != null) {
                session.close();
            }
        }

        // ========== 最终验证：开启新 Session 查询数据库真实状态 ==========
        System.out.println("\n========== 最终验证：数据库真实状态 ==========");
        try (SqlSession session = MyBatisUtil.getSqlSessionFactory().openSession()) {
            printAllStudents(session);
        }
    }
}