JPA：在实体类中添加 toString 方法时出现 java.lang.StackOverflowError

Question

在我在实体类中添加 toSting() 之前一切正常。

之后我开始在运行时收到以下错误：

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(); }
                }

    }
}

---

编辑：添加了学生实体的代码

@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

根据添加的 Student 类更新

根据堆栈跟踪，您的问题与Student.toString() 相关，所以这是发生了什么：

在Teacher.toString() 中，您通过将students 成员放在String 连接语句中隐式调用Student.toString()：+ students +。在Student.toString() 中，代码通过在String 连接语句中包含teacher 成员来执行类似的操作。

这意味着调用Teacher.toString() 或Student.toString() 最终将导致一个永无止境的循环，其中：Teacher.toString() 隐式调用Student.toString()，而后者又隐式调用Teacher.toString()，而后者又调用@987654335 @，然后调用...

2 个.toString() 实现在一个永无止境的循环中不断地来回调用，来回调用，最终溢出堆栈并导致java.lang.StackOverflowError。

要纠正此问题，您应该删除对实体的.toString() 方法的隐式引用。作为替代，您可以让Teacher.toString() 简单地输出students 集合的length()，并可能包含Student 名称的列表。在Student.toString() 中，只需包含Teacher.name 成员。

Answer 2

您必须从 toString() 方法中排除多余的参数。

• 在 Java 中使用 Lombok 和 @ToString.Exclude
• 在 Kotlin 中使用 custom annotation 和 @ExcludeToString

GL

Answer 3

实体班老师有toString()问题：

@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
            + "]";
}

}