Exception: Failure while trying to resolve exception[org.springframework.http.converter.HttpMessageNotWritableException]

The HttpMessageNotWritableException in Spring typically occurs when the framework is unable to convert the response body into a suitable format that can be written to the HTTP response. This can happen for various reasons, such as issues with message converters or serialization problems. Here are a few common reasons and possible solutions:

1. Missing Message Converter:
   • Ensure that you have the necessary message converters configured in your Spring application context. Spring uses message converters to convert objects to the appropriate response format (e.g., JSON, XML).
   • For JSON responses, you typically need the MappingJackson2HttpMessageConverter. Make sure it is present in your project’s dependencies.
2. Object Serialization Issue:
   • Check that the object you are returning from your controller method can be successfully serialized to the desired response format.
   • Ensure that the object has a suitable structure and no circular references, as these can cause serialization issues.
3. Custom Exception Handling:
   • If you have custom exception handling in your application, ensure that it does not interfere with the serialization process.
   • Double-check any exception handling mechanisms, and make sure they do not cause issues with writing the response.
4. Logging the Exception:
   • You might want to log the exception stack trace to get more details about the root cause of the HttpMessageNotWritableException. This can help you identify the specific problem.

Here’s a generic example of how you can log the exception in your exception handling block, Suppose you have a simple Spring MVC controller that returns an object as JSON. For this example, let’s consider a UserController with a method that returns a User object:

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

    @GetMapping("/example")
    public User getUser() {
        return new User(1, "John Doe", "john@example.com");
    }
}

public class User {
    private int id;
    private String name;
    private String email;

    // Constructor, getters, and setters
}

Now, if everything is set up correctly, accessing the /users/example endpoint should return a JSON representation of the User object. However, let’s introduce a problem intentionally to trigger the HttpMessageNotWritableException. Suppose you accidentally make the User class non-serializable. Now, when you try to access the /users/example endpoint, you might encounter the HttpMessageNotWritableException because Spring cannot serialize the User object into JSON. To handle this exception and provide a meaningful response, you can create a global exception handler in your application. Create a class annotated with @ControllerAdvice:

@ControllerAdvice
public class GlobalExceptionHandler {

    @ExceptionHandler(HttpMessageNotWritableException.class)
    @ResponseStatus(HttpStatus.INTERNAL_SERVER_ERROR)
    public ResponseEntity<String> handleHttpMessageNotWritableException(HttpMessageNotWritableException ex) {
        
        System.err.println("HttpMessageNotWritableException: " + ex.getMessage());

        // Return an appropriate error response
        return ResponseEntity.status(HttpStatus.INTERNAL_SERVER_ERROR).body("Internal Server Error");
    }
}

Remember to fix the serialization issue by making the User class implement Serializable.