Spring boot (Java) ManyToMany 415 error when sending POST

Question

I have the most basic ManyToMany example, Project and Employee.

With a PUT request, I will project to employee and within the same function employee to project.

public Employee assignProjectToEmployee(Long empId, Long projectId) {
...
projectSet.add(project);
employee.setAssignedProjects(projectSet);

part of code Employee Entity:

    @ManyToMany
    @JoinTable(name = "youtube-dev-2-prod-coding-employee_project",
            joinColumns = @JoinColumn(name = "employee_id"),
            inverseJoinColumns = @JoinColumn(name = "project_id"))
    @JsonManagedReference
    private Set<Project> assignedProjects = new HashSet<>();

part of code Project Entity:

    @JsonBackReference
    @ManyToMany(mappedBy = "assignedProjects")
    private Set<Employee> employeeSet = new HashSet<>();

✅When sending POST with Postman or Curl to save new Project, works fine.

🔴When sending POST with Postman or Curl to save new Employee, I get

"timestamp": "2025-08-28T17:12:19.998+00:00",
    "status": 415,
    "error": "Unsupported Media Type",

nota bene, when sending GET request for Employee, I would like to see all the Projects that employee is assigned on, and when sending GET request for Project, I would like to see all the Employees on that project. My question naturally is, why am I getting that 415 error

Code: https://github.com/beaverDamDemo/for-stackoverflow-question/tree/master (you would need to set the package in these files to make it work)

I tried examples from three different youtube tutorials and one example, taken from some published code on github

Vy Do · Accepted Answer · 2025-08-29 03:57:41Z

0

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.*;
import java.util.List;

@RestController
@RequestMapping("/youtube-dev-2-prod-coding/employee")
public class EmployeeController {

    @Autowired
    private EmployeeService employeeService;

    @PostMapping("/save")
    public ResponseEntity<Employee> saveEmployee(@RequestBody Employee empObj) {
        employeeService.saveEmployee(empObj);
        return new ResponseEntity<>(HttpStatus.CREATED); // <-- focus at here.
    }

problem at https://github.com/beaverDamDemo/for-stackoverflow-question/blob/master/EmployeeController.java#L18

return new ResponseEntity<>(HttpStatus.CREATED);

best practice, but may be quite hard for new comer. employeeService.saveEmployee(empObj); it needs return created object. But if is hard for you, try to return just a simple Employee object in body, even dummy employee, it will removing the error.

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.*;
import java.util.List;

@RestController
@RequestMapping("/youtube-dev-2-prod-coding/employee")
public class EmployeeController {

    @Autowired
    private EmployeeService employeeService;

    @PostMapping("/save")
    public ResponseEntity<Employee> saveEmployee(@RequestBody Employee empObj) {
        employeeService.saveEmployee(empObj);
        Employee employee = new Empoyee();
        employee.set...
        employee.set...
        return new ResponseEntity<>(HttpStatus.CREATED).body(employee); // <-- focus at here.
    }

see this line

return ResponseEntity.created(location).header("MyResponseHeader", "MyValue").body("Hello World");

at https://docs.spring.io/spring-framework/docs/4.1.0.RC1_to_4.1.0.RC2/Spring%20Framework%204.1.0.RC2/org/springframework/http/ResponseEntity.html#:~:text=return%20ResponseEntity.created(location).header(%22MyResponseHeader%22%2C%20%22MyValue%22).body(%22Hello%20World%22)%3B

answered Aug 29 at 3:57

Vy Do

1

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2 Comments

king_potato Aug 29 at 10:26

Update 🔴It continues to throw 415 if I use this code: ``` @PostMapping("/save") public void saveEmployee(@RequestBody Employee empObj) { employeeService.saveEmployee(empObj); } ``` as well if I use this code: ``` @PostMapping("/save") public ResponseEntity<Employee> saveEmployee(@RequestBody Employee empObj) { employeeService.saveEmployee(empObj); Employee dummy = new Employee(); dummy.setEmpId(0L); dummy.setEmpName("Dummy Name"); return new ResponseEntity<>(dummy, HttpStatus.CREATED); } ```

king_potato Aug 29 at 10:36

If I remove both @JsonManagedReference, it manages to save the new employee. But it will crash afterwards when trying to assign a project to an employee.

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