JPA collection field of an entity is modifiable

Question

I have a JPA Entity with definition like this:

@Entity
@Table(name = "JPA_TEACHER")
public class Teacher implements ITeacher{
    @Id
    private String id;

    @Column(name = "NAME")
    private String name;

    @OneToMany(cascade = CascadeType.ALL, fetch = FetchType.LAZY, orphanRemoval = true)
    @JoinColumn(name="TEACHER_ID", referencedColumnName="ID")
    private List<Student> students;

    public Teacher() {
        super();
    }

    public Teacher(String name) {
        super();
        this.name = name;
    }

    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 List<Student> getStudents() {
        return students;
    }

    public void setStudents(List<Student> students) {
        this.students = students;
    }

    public void addStudents(Student student) {
        if(this.students == null){
            this.students = new ArrayList<Student>();
        }
        this.students.add(student);
    }
}

I get a list of teacher with a named query with the Entity Manager within a EJB context.
Then I create a new ArrayList with the result list, since the result list returned by JPA is read-only.
I try to add students to the students field of some teachers whose students field is null. Then I get a NullPointException, no matter that I have tried to assign a new ArrayList to the field when it's null. It seems that the students field is modifiable. But other fields such as name is modifiable.

I have googled but found nothing. Hope somebody have an idea about this. Thanks a lot.

Every field there is modifiable, also the list of students. You get the NullPointerException because of other things. — V G, Dec 18 '13 at 09:02

score 1 · Answer 1 · answered Dec 18 '13 at 11:39

1

Remove the S at the end from one stundent to add him.

this.students.add(student);

answered Dec 18 '13 at 11:39

Big Bad Baerni

996
7
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Oh, I am very sorry. This is my fault because of my carelessness, but not the answer of this question. Thanks:-) BTW, I have corrected the spelling error. – BlueMice Dec 19 '13 at 01:41

score 1 · Answer 2 · edited May 23 '17 at 10:32

In addition to Big Bad Baerni response, ensure that your student have the teacher property specified as student is the owner of the relationship here.

public void addStudents(Student student) {
    if(this.students == null){
        this.students = new ArrayList<Student>();
    }
    student.setTeacher(this)
    this.students.add(student);
}

See In a bidirectional JPA OneToMany/ManyToOne association, what is meant by "the inverse side of the association"?

I don't know your domain but IMHO, there should be a manyToMany relationship here

JPA collection field of an entity is modifiable

2 Answers2