I’m coding a custom exception where if the username already exists, the response will be the custom exception, i.e., UserException in json.

UsernameAlreadyExistException.java

package com.crawler.exception;

public class UsernameAlreadyExistException extends RuntimeException {
    public UsernameAlreadyExistException(String message) {
        super(message);
    }
    public UsernameAlreadyExistException(String message, Throwable cause) {
        super(message, cause);
    }
}

UserException.java

package com.crawler.exception;

import org.springframework.http.HttpStatus;

public class UserException {
    private final String message;
    private final Throwable throwable;
    private final HttpStatus httpStatus;

    public UserException(String message, Throwable throwable, HttpStatus httpStatus) {
        this.message = message;
        this.throwable = throwable;
        this.httpStatus = httpStatus;
    }

    public String getMessage() {
        return message;
    }

    public Throwable getThrowable() {
        return throwable;
    }

    public HttpStatus getHttpStatus() {
        return httpStatus;
    }
}

UserExceptionHandler

package com.crawler.exception;

import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.ControllerAdvice;
import org.springframework.web.bind.annotation.ExceptionHandler;

@ControllerAdvice
public class UserExceptionHandler {

    @ExceptionHandler(value = {UsernameAlreadyExistException.class})
    public ResponseEntity<Object> handleUsernameAlreadyExistException(UsernameAlreadyExistException usernameAlreadyExistException) {
        UserException userException = new UserException(
            usernameAlreadyExistException.getMessage(),
            usernameAlreadyExistException.getCause(),
            HttpStatus.CONFLICT
        );
        return new ResponseEntity<>(userException, HttpStatus.CONFLICT);
    }
}

In the service implementation, I coded to throw a UsernameAlreadyExistException.

UserServiceImplementation.java

 @Override
    public String createUser(User user) {
        if (userRepository.findByUsername(user.getUsername()) != null) {
            throw new UsernameAlreadyExistException("Username already exist.");
        }
        userRepository.save(user);
        return "User created successfully.";
    }

This is the output from the console.

2024-09-24T13:41:43.809Z ERROR 1 --- [backend] [nio-8080-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet]    : Servlet.service() for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed: com.crawler.exception.UsernameAlreadyExistException: Username already exist.] with root cause

The response from POSTMAN.

{
    "timestamp": "2024-09-24T13:41:43.878+00:00",
    "status": 500,
    "error": "Internal Server Error",
    "path": "/api/v1/users"
}

Expected response from POSTMAN.

{
    "message": "Username already exist",
    "throwable": null,
    "httpStatus": "CONFLICT"
}