在实体类中添加toString方法的java.lang.StackOverflow错误

Question

在我在实体类中添加了toSting（）之前，一切都很顺利。

之后，我开始在运行时得到以下错误：

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

实体类教师有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
            + "]";
}

}

Answer 2

必须从toString（）方法中排除冗余参数。

• 在Java中，使用Lombok和@ToString。排除
• 在Kotlin中使用带有@ExcludeToString

GL

Answer 3

根据堆栈跟踪，您的问题与Student.toString（）相关联，因此发生了以下情况：

在<代码>老师。toString（），您正在隐式调用学生。toString（）方法是将students成员放在String连接语句中：students。在学生中。toString（）代码执行类似操作，将teacher成员包含在String连接语句中。

这意味着调用教师。toString（）或学生。toString（）将导致一个永无止境的循环，其中：Teacher。toString（）隐式调用Student。toString（），它反过来隐式调用教师。toString（），它依次调用Student。toString（），它依次调用。。。

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

要纠正这个问题，您应该删除对实体的. toString（）方法的隐式引用。作为替换，您可以有Teacher.toString（）简单地输出长度（）的学生集合，并可能包括一个列表的学生名称。在Student.toString（）中，只需包含Teacher.name成员。