self referencing relationship with spring data jpa

Question

I am building an application with spring boot and have to setup a self-referencing relationship.

In order to make the scenario simple and easy to understand, I have tried to work on a similar scenario with a department entity example.

Following is the scenario for which I need to setup a self-referencing relationship

1. A department is created with the following attributes - Name, CostCenter, Location.
2. A department can be related to parent department which is also of Department type
3. A department can have sub-departments (child) which are also of Department type.

To setup such a scenario, I have defined the entity Department in the following way.

Department.java

public class Department {

    @Id
    @GenericGenerator(name = "sequence_department_id", strategy = "com.app.mycompany.AgileCenterServices.util.DepartmentIdGenerator")
    @GeneratedValue(generator = "sequence_department_id")
    @Column(unique = true)
    private String id;

    private String name;

    private String location;

    private String costCenter;

    @ManyToOne(cascade=CascadeType.ALL)
    @JoinColumn(name="parentDepartment")
    private Department parentDepartment;

    @OneToMany(mappedBy="parentDepartment")
    private Set<Department> linkedDepartments = new HashSet<Department>();

    /* getters & setters */

}

DepartmentController.java

@CrossOrigin
    @RequestMapping(value = "/", method = RequestMethod.POST)
    public Department createDepartment(@RequestBody String trial) throws Exception {

        logger.info("Inside createDepartment() API ");

        ObjectMapper objmapper = new ObjectMapper();

        ObjectNode node = objmapper.readValue(trial, ObjectNode.class);

        Department deptInput = objmapper.convertValue(node, Department.class);

        Department deptRec = null;

        /* check if parent department information was passed */
        if(deptInput.getParentDepartment() != null) {
            Department parentDepartment = departmentRepository.findOne(deptInput.getParentDepartment().getId());
            deptInput.setParentDepartment(parentDepartment);

        }

        try {

            logger.info("createDepartment() :: Before save :: Input data ::: " + deptInput.toString());

            deptRec = departmentRepository.save(deptInput);

            logger.info("createDepartment() :: After save :: Saved successfully ::: " + deptRec.toString());
        } 
        catch (Exception ex) {
            ex.printStackTrace();
            throw ex;
        }

        logger.info("Leaving createDepartment() API");

        return deptRec;
    }

For now, I have just tried linking the department to another parentDepartment as shown in the above example and tried to create the department using spring boot-REST service

The departments are getting created appropriately.

Saved Department 1 with following input
{"name":"Sales", "costCenter": "SLS", "location":"Global"}
Output:
{
    "id": "1000",
    "name": "Sales",
    "location": "Global",
    "costCenter": "SLS",
    "parentDepartment": null,
    "linkedDepartments": []
}

    Saved department 2 with following input
    {"name":"Sales-IN", "costCenter": "SLS-IN", "location":"India", "parentDepartment":{"id":"1000"}}

  Output:   
    {
        "id": "1001",
        "name": "Sales-IN",
        "location": "India",
        "costCenter": "SLS-IN",
        "parentDepartment": {
            "id": "1000",
            "name": "Sales",
            "location": "Global",
            "costCenter": "SLS",
            "parentDepartment": null,
            "linkedDepartments": []
        },
        "linkedDepartments": []
    }

However, when I use postman to now query the data in departments, i notice the following exception

@CrossOrigin
    @RequestMapping(value="/", method = RequestMethod.GET)
    public Page<Department> listDepartments(Pageable pageable) {

        logger.info("Inside listDepartments() API");

        return departmentRepository.findAll(pageable);
    }

Exception

2019-03-06 20:04:12.190  WARN 19520 --- [io-8080-exec-10] .m.m.a.ExceptionHandlerExceptionResolver : Resolved exception caused by handler execution: org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: Infinite recursion (StackOverflowError); nested exception is com.fasterxml.jackson.databind.JsonMappingException: Infinite recursion (StackOverflowError) (through reference chain: com.app.mycompany.AgileCenterServices.entities.Department["linkedDepartments"]->org.hibernate.collection.internal.PersistentSet[0]->com.app.mycompany.AgileCenterServices.entities.Department["parentDepartment"]->com.app.mycompany.AgileCenterServices.entities.Department["linkedDepartments"]->org.hibernate.collection.internal.PersistentSet[0]->com.app.mycompany.AgileCenterServices.entities.Department["parentDepartment"]->com.app.mycompany.AgileCenterServices.entities.Department["linkedDepartments"]

To fix the above issue, I set @JsonBackReference on the "linkedDepartments" attribute after which the "GET" operation works properly. but the save operation now fails with

2019-03-06 20:19:03.176  WARN 19520 --- [nio-8080-exec-3] .m.m.a.ExceptionHandlerExceptionResolver : Resolved exception caused by handler execution: org.springframework.web.util.NestedServletException: Handler dispatch failed; nested exception is java.lang.StackOverflowError

What am i doing wrong here?

Answer 1

I tried your code and as long as you have @JsonBackReference on linkedDepartments or parentDepartment, it won't be serialized, which resolves infinite recursion. The save method also works fine. I can post code to GitHub for you reference. Let me know.