Custom exception classes in Spring Boot - Mini Project: Build & Apply

Custom Exception Classes in Spring Boot

📖 Scenario: You are building a simple Spring Boot application that manages user accounts. You want to handle errors clearly by creating your own exception class for when a user is not found.

🎯 Goal: Create a custom exception class called UserNotFoundException that extends RuntimeException. Then configure a controller advice to handle this exception and return a friendly error message.

📋 What You'll Learn

Create a custom exception class named UserNotFoundException that extends RuntimeException

Add a constructor to UserNotFoundException that accepts a String message and passes it to the superclass

Create a Spring @RestControllerAdvice class named GlobalExceptionHandler

Add a method in GlobalExceptionHandler annotated with @ExceptionHandler(UserNotFoundException.class) that returns a ResponseEntity<String> with the exception message and HTTP status NOT_FOUND

💡 Why This Matters

🌍 Real World

Custom exceptions help make error handling clear and consistent in real applications, improving user experience and debugging.

💼 Career

Understanding how to create and handle custom exceptions is essential for backend developers working with Spring Boot to build robust APIs.

1

Create the custom exception class

Create a public class called UserNotFoundException that extends RuntimeException. Add a constructor that accepts a String message and passes it to super(message).

Spring Boot

public class UserNotFoundException extends RuntimeException {
    // Your code here
}

Need a hint?

Remember to extend RuntimeException and create a constructor that calls super(message).

2

Create the global exception handler class

Create a public class called GlobalExceptionHandler annotated with @RestControllerAdvice. This class will handle exceptions globally.

Spring Boot

public class UserNotFoundException extends RuntimeException {
    public UserNotFoundException(String message) {
        super(message);
    }
}

@RestControllerAdvice
public class GlobalExceptionHandler {
    // Your code here
}

Need a hint?

Use @RestControllerAdvice annotation on the class GlobalExceptionHandler.

3

Add exception handler method

Inside GlobalExceptionHandler, add a method annotated with @ExceptionHandler(UserNotFoundException.class). The method should return ResponseEntity<String> with the exception message and HTTP status NOT_FOUND. Use method signature: public ResponseEntity<String> handleUserNotFound(UserNotFoundException ex).

Spring Boot

import org.springframework.web.bind.annotation.RestControllerAdvice;

public class UserNotFoundException extends RuntimeException {
    public UserNotFoundException(String message) {
        super(message);
    }
}

@RestControllerAdvice
public class GlobalExceptionHandler {
    // Add exception handler method here
}

Need a hint?

Use @ExceptionHandler(UserNotFoundException.class) and return a ResponseEntity with the exception message and HttpStatus.NOT_FOUND.

4

Complete imports and class structure

Add all necessary import statements for ResponseEntity, HttpStatus, ExceptionHandler, and RestControllerAdvice at the top of the file to complete the code.

Spring Boot

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

// Your code here if any additional imports needed

Need a hint?

Make sure all required Spring annotations and classes are imported.