@Entity
public class School {
 @Id
 @GeneratedValue(strategy = GenerationType.IDENTITY)
    private Integer schoolId;
...
 @ElementCollection
 @CollectionTable(
            name = "grades",
            joinColumns = @JoinColumn(name = "schoolId")
    )
 private Set<Grade> grades;

}
----------------------------------

@Embeddable
public class Grade {

private int gradeId

@ElementCollection
@CollectionTable(
            name = "Class",
            joinColumns = @JoinColumn(name = "gradeId")
    )
    private Set<Class> classes;

}


------------------------------------

@Embeddable
public class Class {

private int numOfStudent
private int female;
private int mamle;

}

I want to get mapping tablels. But, get this error.

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'entityManagerFactory' defined in class path resource [org/springframework/boot/autoconfigure/orm/jpa/HibernateJpaConfiguration.class]: Property 'com.School.grades.collection&&element.clases' belongs to an '@Embeddable' class that is contained in an '@ElementCollection' and may not be a '@ElementCollection'

I’m not sure above code is correct. What can I do to fix this error?

try to use @secondrytable and @AttributeOverride.
But.. couldn’t find solution.