如何修复 spring 引导一对多双向无限循环？

Question

我正在尝试使用 spring 引导和 spring 数据 jpa 创建一对多双向映射请查看下面的实体

Employer Entity

@Entity  
public class Employer  
{  
private Long id;  
private String employerName;  
private List<Employee> employees;  

@Id  
@GeneratedValue(strategy=GenerationType.AUTO)  
public Long getId()  
{  
    return id;  
}  
public void setId(Long id)  
{  
    this.id = id;  
}  
public String getEmployerName()  
{  
    return employerName;  
}  
public void setEmployerName(String employerName)  
{  
    this.employerName = employerName;  
}  

@OneToMany(cascade=CascadeType.ALL, mappedBy="employer")  
public List<Employee> getEmployees()  
{  
    return employees;  
}  
public void setEmployees(List<Employee> employees)  
{  
    this.employees = employees;  
}  
}

Employee Entity

@Entity  
public class Employee  
{  
private Long id;  
private String employeeName;  
private Employer employer;  

@Id  
@GeneratedValue(strategy=GenerationType.AUTO)  
public Long getId()  
{  
    return id;  
}  
public void setId(Long id)  
{  
    this.id = id;  
}  
public String getEmployeeName()  
{  
    return employeeName;  
}  
public void setEmployeeName(String employeeName)  
{  
    this.employeeName = employeeName;  
}  
@ManyToOne(cascade=CascadeType.ALL, fetch = FetchType.LAZY)  
public Employer getEmployer()  
{  
    return employer;  
}  
public void setEmployer(Employer employer)  
{  
    this.employer = employer;  
}  
}

Employer Repo

public interface EmployerServices extends JpaRepository<Employer, Long> {
}

Employee Repo

public interface EmployeeServices extends JpaRepository<Employee, Long> {
}

REST Controller is

 @RestController
 public class Controller {
 @Autowired EmployeeServices employeeServices;
 @Autowired EmployerServices employerServices;
 @GetMapping("/getempr")
 public Object getempr(){
    return employerServices.findOne(1L);
 }
}

现在问题开始了，开始看我的输出

它看起来像一个内斗循环，并且我的服务器抛出错误 getOutputStream() 已经被调用用于此响应。

 I used @JsonBackReference & @JsonManagedReference

注释，但问题是它像一对多一样工作

 {
   "id":1,
   "employerName":"employer",
   "employees":[
     {"id":1,"employeeName":"emp1"},
     {"id":2,"employeeName":"emp2"}
   ]
}

如果我想像所有雇员和雇主一样引起多对一的关注。输出是

 [
  {
   "id":1,
   "employeeName":"emp1"
  },
  {
    "id":2,
    "employeeName":"emp2"}
 ]

它没有向我显示雇主详细信息。

请大家告诉我我做错了什么。提前致谢！！

Answer 1

而不是使用 @JsonBackReference 和 @JsonManagedReference 尝试使用注释 @JsonIgnoreProperties:

@JsonIgnoreProperties("employer")
private List<Employee> employees;  

@JsonIgnoreProperties("employees")
private Employer employer;

它阻止 Jackson 呈现关联对象的指定属性。

Answer 2

您可以通过注释修改两次来解决您的问题。
Employer.class

@Entity
public class Employer {
    private Long id;
    private String employerName;

    @OneToMany(cascade = CascadeType.ALL,
            mappedBy = "employer",
            orphanRemoval = true)
    private List<Employee> employees;

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getEmployerName() {
        return employerName;
    }

    public void setEmployerName(String employerName) {
        this.employerName = employerName;
    }

    public List<Employee> getEmployees() {
        return employees;
    }

    public void setEmployees(List<Employee> employees) {
        this.employees = employees;
    }
}

Employee.class

@Entity
public class Employee {
    private Long id;
    private String employeeName;


    @ManyToOne(fetch = FetchType.LAZY)
    @JoinColumn(name = "employer_id")
    private Employer employer;

    @Id
    @GeneratedValue(strategy = GenerationType.AUTO)
    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }

    public String getEmployeeName() {
        return employeeName;
    }

    public void setEmployeeName(String employeeName) {
        this.employeeName = employeeName;
    }

    public Employer getEmployer() {
        return employer;
    }

    public void setEmployer(Employer employer) {
        this.employer = employer;
    }
}

有关详细信息，请访问此 link。

Answer 3

JSON 是双向映射的问题。使用以下属性。

@JsonIgnoreProperties("employer")
@JsonIgnoreProperties("employees")

请保持抓取类型为 eager。

希望这会奏效。

Answer 4

像这样更改您的 getEmployer 方法：

@ManyToOne(cascade=CascadeType.ALL, fetch = FetchType.LAZY)
@JsonProperty(access = JsonProperty.Access.WRITE_ONLY)
public Employer getEmployer()  
{  
    return employer;  
}

Answer 5

使用

@JsonProperty(access = Access.WRITE_ONLY)
private List<Employee> employees;

这样它会在响应中打印到 JSON 时忽略员工（从而防止循环），但仍会考虑您在请求中传递的 JSON 数据（员工列表）正文，以便它可用于持久化。

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