Spring Framework 概述

// 操作前：StudentDao.java —— 纯 JDBC，无框架支持
public class StudentDao {
    private DataSource dataSource;

    public StudentDao() {
        // 手动创建连接池，硬编码配置
        HikariConfig config = new HikariConfig();
        config.setJdbcUrl("jdbc:mysql://localhost:3306/student_db");
        config.setUsername("root");
        config.setPassword("secret");
        this.dataSource = new HikariDataSource(config);
    }

    public Student findById(int studentId) throws SQLException {
        Connection conn = dataSource.getConnection();
        PreparedStatement ps = conn.prepareStatement(
            "SELECT id, name, age, class_name FROM students WHERE id = ?"
        );
        ps.setInt(1, studentId);
        ResultSet rs = ps.executeQuery();

        Student student = null;
        if (rs.next()) {
            student = new Student();
            student.setId(rs.getInt("id"));
            student.setName(rs.getString("name"));
            student.setAge(rs.getShort("age"));
            student.setClassName(rs.getString("class_name"));
        }

        rs.close();
        ps.close();
        conn.close();  // 如果前面抛异常，这里可能执行不到，导致连接泄漏
        return student;
    }
}

// 操作前：StudentService.java —— 手动创建依赖，紧耦合
public class StudentService {
    private StudentDao studentDao = new StudentDao();  // 硬编码依赖
    private EmailSender emailSender = new EmailSender(); // 硬编码依赖

    public void enrollStudent(Student student) {
        try {
            studentDao.save(student);
            emailSender.send("admin@learnto.cn", "新学生入学：" + student.getName());
        } catch (SQLException e) {
            // 事务？不存在的，只能手动 rollback
            throw new RuntimeException("入学失败", e);
        }
    }
}

// 操作后：AppConfig.java —— Spring 配置类
@Configuration
@ComponentScan("com.feixiang.student")
@PropertySource("classpath:application.properties")
public class AppConfig {

    @Bean
    public DataSource dataSource(
            @Value("${db.url}") String url,
            @Value("${db.username}") String username,
            @Value("${db.password}") String password) {
        HikariConfig config = new HikariConfig();
        config.setJdbcUrl(url);
        config.setUsername(username);
        config.setPassword(password);
        return new HikariDataSource(config);
    }

    @Bean
    public JdbcTemplate jdbcTemplate(DataSource dataSource) {
        return new JdbcTemplate(dataSource);
    }
}

// 操作后：StudentDao.java —— 使用 Spring JDBC
@Repository
public class StudentDao {
    private final JdbcTemplate jdbcTemplate;

    public StudentDao(JdbcTemplate jdbcTemplate) {
        this.jdbcTemplate = jdbcTemplate;
    }

    public Student findById(int studentId) {
        return jdbcTemplate.queryForObject(
            "SELECT id, name, age, class_name FROM students WHERE id = ?",
            new BeanPropertyRowMapper<>(Student.class),
            studentId
        );
    }

    public void save(Student student) {
        jdbcTemplate.update(
            "INSERT INTO students(name, age, class_name) VALUES(?, ?, ?)",
            student.getName(), student.getAge(), student.getClassName()
        );
    }
}

// 操作后：StudentService.java —— 依赖注入 + 声明式事务
@Service
public class StudentService {
    private final StudentDao studentDao;
    private final EmailSender emailSender;

    public StudentService(StudentDao studentDao, EmailSender emailSender) {
        this.studentDao = studentDao;
        this.emailSender = emailSender;
    }

    @Transactional
    public void enrollStudent(Student student) {
        studentDao.save(student);
        emailSender.send("admin@learnto.cn", "新学生入学：" + student.getName());
    }
}

// ❌ 错误认知：以为 Spring Boot 是 Spring 的替代品
@SpringBootApplication  // 这是 Spring Boot 的注解，不是 Spring Framework 原生
public class StudentApplication {
    public static void main(String[] args) {
        SpringApplication.run(StudentApplication.class, args);
    }
}