I am learning how REST APIs work in Spring Boot and found an issue when sending JSON data to a controller endpoint.

My API returns:

HTTP 415 Unsupported Media Type

even though the JSON body itself seems valid.

Controller example:

@RestController
@RequestMapping("/users")
public class UserController {

    @PostMapping
    public String createUser(@RequestBody User user) {
        return "User created";
    }
}

Model:

public class User {

    private String name;
    private int age;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }
}

JSON body sent from Postman:

{
    "name": "Jeferson",
    "age": 20
}

Request configuration in Postman:

• Method: POST

• URL: http://localhost:8080/users

• Body type: raw

• Format selected: JSON

I already checked:

• the endpoint URL

• the HTTP method

• the JSON format

• the model fields and getters/setters

I also reviewed the HTTP 415 documentation related to unsupported media types, but I still do not fully understand why Spring Boot rejects the request body in this case.

I noticed that when the request fails, Postman sends this header:

Content-Type: text/plain

instead of:

Content-Type: application/json

My questions are:

• Why does Spring Boot reject the request when the JSON body itself is valid?

• Is the Content-Type header required for @RequestBody to work correctly?

• How does Spring Boot determine which media types are supported for a request?