JPA: java.lang.StackOverflowError on adding toString method in entity classes

Question

Everything worked fine until I added toSting() in my entity classes.

After which I start getting the following error in runtime:

Exception in thread "main" java.lang.StackOverflowError
    at java.lang.AbstractStringBuilder.append(Unknown Source)
    at java.lang.StringBuilder.append(Unknown Source)
    at java.lang.StringBuilder.<init>(Unknown Source)
    at entity.Guide.toString(Guide.java:51)
    at java.lang.String.valueOf(Unknown Source)
    at java.lang.StringBuilder.append(Unknown Source)
    at entity.Student.toString(Student.java:45)
        ...

---

@Entity
public class Teacher {

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private Long id;    

    private String name;

    @OneToMany(mappedBy="teacher", cascade={CascadeType.PERSIST})
    private Set<Student> students = new HashSet<Student>(); 

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

    public Set<Student> getStudents() {
        return students;
    }       
    public void addStudent(Student student) {
        students.add(student);
        student.setTeacher(this);
    }
    @Override
    public String toString() {
        return "Teacher[id=" + id + ", name=" + name
                + ", students=" + students + "]";
    }

}

---

public class SnafuClient {
    public static void main(String[] args) {

        EntityManagerFactory emf = Persistence.createEntityManagerFactory("snafu");
        EntityManager em = emf.createEntityManager();
        EntityTransaction txn = em.getTransaction();

                try {
                    txn.begin();

                    Query query = em.createQuery("select teacher from Teacher teacher");
                    List<Teacher> teachers = query.getResultList();

                    for (Teacher teacher: teachers) {
                System.out.println(teacher);
            }


                    txn.commit();
                }   catch(Exception e) {
                    if(txn != null) { txn.rollback(); }
                    e.printStackTrace();
             }  finally {
                    if(em != null) { em.close(); }
                }

    }
}

---

EDIT: Code for Student entity added

@Entity
public class Student {

    @Id
    @GeneratedValue(strategy=GenerationType.AUTO)
    private Long id;

    private String name;

    @ManyToOne(cascade={CascadeType.PERSIST, CascadeType.REMOVE})
    @JoinColumn(name="teacher_id")
    private Teacher teacher;

    public Student() {}
    public Student(String name, Teacher teacher) {
        this.name = name;
        this.teacher = teacher;
    }

    public Teacher getTeacher() {
        return teacher;
    }
    public void setTeacher(Teacher teacher) {
        this.teacher = teacher;
    }
    @Override
    public String toString() {
        return "Student [id=" + id + 
                + ", name=" + name + ", teacher=" + teacher + "]";
    }   

}

Answer 1

Updated based on the addition of the Student class

According to the stack trace, your problem is associated with the Student.toString(), so here's what is happening:

In Teacher.toString(), you are implicitly calling the Student.toString() by placing the students member within a String concatenation statement: + students +. Within Student.toString() the code does something similar, by including the teacher member within a String concatenation statement.

This means that calling either Teacher.toString() or Student.toString() will end up causing a never-ending loop where: Teacher.toString() implicitly calls Student.toString(), which in turn implicitly calls Teacher.toString(), which in turn calls Student.toString(), which in turn calls...

The 2 .toString() implementations keep calling back and forth, back and forth, back and forth, in a never-ending loop, which eventually overflows the stack and results in a java.lang.StackOverflowError.

To correct the problem, you should remove the implicit references to the .toString() methods of the entities. As a replacement, you could have Teacher.toString() simply output the length() of the students collection and maybe include a list of the Student name(s). And in the Student.toString(), simply include the Teacher.name member.