双向@ManyToMany的JSON映射的无限递归

Question

I have following scenario: There are companies and employees. Each company has a set of employees. Each employee can work for several companies. So I implemented following relationships:

Company.class:

@JoinTable(name = "company_employee", joinColumns = @JoinColumn(name = "company_id") , inverseJoinColumns = @JoinColumn(name = "employee_id") )
@ManyToMany(fetch = FetchType.LAZY)
private Set<Employee> employees;

Employee.class:

@JoinTable(name = "company_employee", joinColumns = @JoinColumn(name = "employee_id") , inverseJoinColumns = @JoinColumn(name = "company_id") )
@ManyToMany(fetch = FetchType.EAGER)
private Set<Company> companies;

The goal is the ability to fetch List<Employee> , each Employee with Set<Company> , or another scenario, to fetch List<Company> , but without fetching related Set<Employee> . But if I fetch a Company by id, related Set<Employee> should also be fetched.

What I did in daos: If I fetch Lists in both cases, it work as supposed to work without issues: what is eager - is fetched, what is lazy - is not fetched. To fetch Employee by id I do following, and it fetches an Employee with not empty Set<Company> , as I expect it to:

public Employee find(Long id) {
        Employee employee = entityManager.find(Employee.class, id);
        return employee;

To fetch Company by id with not empty Set<Employee> I tried:

variant 1:

public Company find(Long id) {
        TypedQuery<Company> query = em.createQuery("select distinct e from Company e left outer join fetch e.employees where e.company_id=:company_id", Company.class);
        query.setParameter("company_id", id);
        Company company = query.getSingleResult();
        return company;
    }

variant2:

 public Company find(Long id) {
            Company company = entityManager.find(Company.class, id);
            Hibernate.initialize(company.getEmployees());
            return company;
        }

As I see both variant1 and variant2 give org.springframework.http.converter.HttpMessageNotWritableException due to the infinite recursion during JSON serialization. I tried to use @JsonManagedReference and @JsonBackReference , but then Set annotated with @JsonBackReference is never fetched regarless any join fetch or Hibernate.initialize I use. So what is the correct approach to implement what I need? Thank you.

Answer 1

It seems I found a solution: using @JsonIdentityInfo . More information here Now my entities look like that (removed all unrelated fields and methods), and they work exactly as I need them to:

Company.class:

@Entity
@JsonIdentityInfo(generator=ObjectIdGenerators.PropertyGenerator.class, property="company_id")
public class Company {
     @JoinTable(name = "company_employee", joinColumns = @JoinColumn(name = "company_id") , inverseJoinColumns = @JoinColumn(name = "employee_id") )
     @ManyToMany(fetch = FetchType.LAZY)
     private Set<Employee> employees;
}

Employee.class:

 @Entity
 @JsonIdentityInfo(generator=ObjectIdGenerators.PropertyGenerator.class, property="employee_id")
 public class Employee {
     @JoinTable(name = "company_employee", joinColumns = @JoinColumn(name = "employee_id") , inverseJoinColumns = @JoinColumn(name = "company_id") )
     @ManyToMany(fetch = FetchType.EAGER)
     private Set<Company> companies;
 }

I hope it will help to somebody.

UPDATE 1 Actually, as Neil Stockton noticed, the mapping is not quite correct and is not legal bidirectional. So I just changed one of entities like below, then no additional annotations needed to avoid infinite recursion

     @Entity
     public class Employee {
         @ManyToMany(fetch = FetchType.EAGER, mappedBy="employees")
         private Set<Company> companies;
     }

I think it's important update.