这个问题通常是由于使用JPA中的@JoinTable注释创建了一个中间表，以便处理多对多关系并增加了额外的列。在保存实体时，添加的额外列被正确设置，但当更新实体时，这些列没有被更新。这是因为JPA不会自动更新中间表的列。

为了解决这个问题，可以使用@PreUpdate注释在更新前手动更新中间表的列。下面是一个具有额外列的Many to Many关系并使用Jointable的代码示例：

@Entity public class Student { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id;

private String name;

@ManyToMany(cascade = CascadeType.ALL)
@JoinTable(name = "student_course",
    joinColumns = @JoinColumn(name = "student_id"),
    inverseJoinColumns = @JoinColumn(name = "course_id"))
private Set courses = new HashSet<>();

@OneToMany(mappedBy = "student", cascade = CascadeType.ALL)
private Set studentCourses = new HashSet<>();

// Constructors, getters, and setters

}

@Entity public class Course { @Id @GeneratedValue(strategy = GenerationType.IDENTITY) private Long id;

private String name;

@ManyToMany(mappedBy = "courses")
private Set students = new HashSet<>();

// Constructors, getters, and setters

}

@Entity public class StudentCourse { @EmbeddedId private StudentCourseId id;

@ManyToOne
@MapsId("studentId")
private Student student;

@ManyToOne
@MapsId("courseId")
private Course course;

private int grade;

// Constructors, getters, and setters

}

@Embeddable public class StudentCourseId implements Serializable {

@Column(name = "student_id")
private Long studentId;

@Column(name = "course_id")
private Long courseId;

// Constructors, getters, and setters

}

在StudentCourse实体中，我们添加了一个额外的列名为'grade”。为了确保在更新StudentCourse实体时额外列的值也得到更新，我们在StudentCourse实体中添加了一个@PreUpdate方法。

@Entity public class StudentCourse { @EmbeddedId private StudentCourseId id;

@ManyToOne
@MapsId("studentId")
private Student student;

@ManyToOne
@MapsId("courseId")
private Course course;

private int grade;

// Constructors, getters, and