编写Student类、StudentDao类以及StudentService类，实现查询、修改、更新、删除学生信息，并配置成Spring bean StudentDao需要通过依赖注入StudentS

在Java Spring框架中，我们通常会设计出这样的层次结构：

1. **Student类** (表示实体):

public class Student {
    private String id;
    private String name;
    private String major;

    // getters and setters
    public String getId() { return id; }
    public void setId(String id) { this.id = id; }
    public String getName() { return name; }
    public void setName(String name) { this.name = name; }
    public String getMajor() { return major; }
    public void setMajor(String major) { this.major = major; }
}

2. **StudentDao接口** (数据访问层，用于数据库操作):

import org.springframework.stereotype.Repository;
@Repository
public interface StudentDao {
    Student findById(Long id);
    void update(Student student);
    void deleteById(Long id);
    void save(Student student); // 添加新学生
}

3. **StudentServiceImpl类** (业务逻辑层，实现数据访问接口):

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Service;
@Service
public class StudentServiceImpl implements StudentDao {
    @Autowired
    private StudentRepository repository;

    @Override
    public Student findById(Long id) {
        return repository.findById(id).orElse(null);
    }

    @Override
    public void update(Student student) {
        repository.save(student);
    }

    @Override
    public void deleteById(Long id) {
        repository.deleteById(id);
    }

    @Override
    public void save(Student student) {
        repository.save(student);
    }
}

4. **StudentRepository接口** (通常由Spring Data JPA提供，简化了CRUD操作):

import org.springframework.data.jpa.repository.JpaRepository;
public interface StudentRepository extends JpaRepository<Student, Long> {}

5. **配置Spring beans** (在Spring的XML配置文件或注解式配置中):

<!-- XML配置 -->
<bean id="studentDao" class="com.example.StudentServiceImpl">
    <property name="studentRepository" ref="studentRepository"/>
</bean>

// Annotation-based configuration
@Configuration
public class AppConfig {
    @Bean
    public StudentDao studentDao(StudentRepository repository) {
        return new StudentServiceImpl(repository);
    }
    @Bean
    public StudentRepository studentRepository() {
        return new JpaStudentRepository(); // 或者具体实现
    }
}

在这个架构中，`StudentDao`是数据访问接口，`StudentServiceImpl`实现了它并利用`StudentRepository`来执行实际的数据库操作。Spring的依赖注入保证了`StudentDao`实例能够自动获取到`StudentRepository`。