springmvc用jackson返回json无限循环

springmvc使用jackson转json解决双向关联关系(例如一对多)循环调用。目前只是避开了循环调用，并不是真正的从根本上解决。就算用ObjectMapper Hibernate4Module也是解决懒加载转json的问题。

问题简说

实体类：老师Teacher.java

@OneToMany(fetch= FetchType.LAZY,cascade = CascadeType.ALL)
@JoinColumn(name="teacher_id")
public Set<Student> getStudents() {
    return students;
}

实体类：学生Student.java

@ManyToOne
@JoinColumn(name="teacher_id")
public Teacher getTeacher() {
    return teacher;
}

服务层：学生服务层StudentServiceImpl.java

@Override
@Transactional(readOnly = true)
public Student findById(Long id){
    Student student = studentRepository.findOne(id);
    //模拟用到了关联数据，或者使用了自动映射复制到vo、dto等等
    Teacher teacher = student.getTeacher();
    Set<Student> students = teacher.getStudents();
    System.out.println(students);
    return student;
}

控制层：返回json数据到前台

@RequestMapping(value = "/find", method = RequestMethod.GET)
@ResponseBody
public Student find(HttpServletRequest request, HttpServletResponse response){
    Student student = studentService.findById(1L);
    return student;
}

由于hibernate双向关联关系，查出来的实体对象返回前台输出json，会循环嵌套。如：

{"id":1,"name":"Student-1","teacher":{"id":1,"name":"Teacher","students":[{"id":2,"name":"Student-2","teacher":{"id":1,"name":"Teacher","students":[{"id":2,"name":"Student-2","teacher":{"id":1,"name":"Teacher","students":[{"id":2,"name":"Student-2","teacher":{"id":1,"name":"Teacher","students":[{"id":2,"name":"Student-2","teacher":{"id":1,"name":"Teacher","students":[{"id":2,"name":"Student-2","teacher":{"id":1,"name":"Teacher","students":[{"id":2,"name":"Student-2","teacher":{"id":1,"name":"Teacher","students":[{"id":2,"name":"Student-2","teacher":{"id":1,"name":"Teacher","students":[{"id":2,"name":"Student-2","teacher":{"id":1,"name":"Teacher","students":[{"id":2,"name":"Student-2","teacher":{"id":1,"name":"Teacher","students":[{"id":2,"name":"Student-2","teacher":{"id":1,"name":"Teacher","students":[{"id":2,"name":"Student-2","teacher":{"id":1,"name":"Teacher","students":[{"id":2,"name":"Student-2","teacher":{"id":1,"name":"Teacher","students":[{"id":2,"name":"Student-2","teacher":{"id":1,"name":"Teacher","students":[{"id":2,"name":"Student-2","teacher":{"id":1,"name":"Teacher","students":...
......
......

解决方法

1、最简单的办法

在返回的时候吧循环嵌套设置为空，或mapping转vo,dto的时候过滤该字段。

缺点：对象复杂时比较麻烦，用mapper复制对象又要考虑过滤字段操作。

Student student = studentRepository.findOne(id);
Teacher teacher = student.getTeacher();
teacher.setStudents(null);
return student;

输出：

{"id":1,"name":"Student-1","teacher":{"id":1,"name":"Teacher","students":null}}

2、springmvc返回jackson注解处理

@JsonBackReference：

标记在子类对父类的引用变量上，标注的属性在将对象转换为json数据时，会被忽略。

它的作用相当于@JsonIgnore，此时可以没有@JsonManagedReference。

@JsonManagedReference：

标记在父类对子类的引用变量上，标注的属性在被对象转换为json数据时，不会被忽略。

如果没有它在json数据转换为对象时，不会注入@JsonBackReference标记属性。

@JsonIgnore：

直接忽略某个属性，以断开无限递归，序列化或反序列化均忽略。

缺点：学生类找不到老师类的ID了，整个属性都被忽略掉了，如果需要，要另外起属性定义赋值如return teacher.getId();

用户类（子）多的一端

@JsonBackReference
private Teacher teacher;

老师类（父）一的一端

@JsonManagedReference
private Set<Student> students;

输出：

{"id":1,"name":"Student-1"}

后面如果发现有更好的办法再补上。

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