ResponseEntityExceptionHandler in Spring Boot - Mini Project: Build & Apply

Handling Errors Gracefully with ResponseEntityExceptionHandler in Spring Boot

📖 Scenario: You are building a simple Spring Boot REST API for a bookstore. You want to handle errors like when a book is not found or when invalid data is sent by the user. Instead of showing raw error messages, you want to send clear, friendly error responses.

🎯 Goal: Create a custom exception handler by extending ResponseEntityExceptionHandler to catch specific exceptions and return meaningful HTTP responses with error messages.

📋 What You'll Learn

Create a custom exception class called BookNotFoundException.

Create a controller class BookController with a method to get a book by ID.

Create a class CustomExceptionHandler that extends ResponseEntityExceptionHandler.

Override the method to handle BookNotFoundException and return a ResponseEntity with status 404 and a custom error message.

💡 Why This Matters

🌍 Real World

In real applications, handling exceptions gracefully improves user experience by sending clear error messages instead of server errors or stack traces.

💼 Career

Knowing how to customize error handling with ResponseEntityExceptionHandler is a key skill for backend developers working with Spring Boot REST APIs.

1

Create the custom exception class

Create a public class called BookNotFoundException that extends RuntimeException. Add a constructor that accepts a String message and passes it to the superclass constructor.

Spring Boot

// Your code here

Need a hint?

Think of this as creating a special kind of error that you can throw when a book is not found.

2

Create the BookController with a method to get a book by ID

Create a class called BookController annotated with @RestController. Add a method getBookById that takes a Long id parameter annotated with @PathVariable. Inside the method, throw a BookNotFoundException with the message "Book not found with id " + id.

Spring Boot

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

// Your code here

Need a hint?

This simulates a request for a book that always fails by throwing your custom exception.

3

Create the CustomExceptionHandler class extending ResponseEntityExceptionHandler

Create a class called CustomExceptionHandler annotated with @ControllerAdvice that extends ResponseEntityExceptionHandler. Add a method handleBookNotFoundException annotated with @ExceptionHandler(BookNotFoundException.class) that takes a BookNotFoundException ex parameter. Return a ResponseEntity with status HttpStatus.NOT_FOUND and body containing ex.getMessage().

Spring Boot

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

import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class BookController {
    @GetMapping("/books/{id}")
    public String getBookById(@PathVariable Long id) {
        throw new BookNotFoundException("Book not found with id " + id);
    }
}

// Your code here

Need a hint?

This class catches your custom exception and sends a 404 response with the error message.

4

Complete the exception handler with a fallback for other exceptions

In the CustomExceptionHandler class, add a method handleAllExceptions annotated with @ExceptionHandler(Exception.class) that takes an Exception ex parameter. Return a ResponseEntity with status HttpStatus.INTERNAL_SERVER_ERROR and body "An unexpected error occurred".

Spring Boot

import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.ControllerAdvice;
import org.springframework.web.bind.annotation.ExceptionHandler;
import org.springframework.web.servlet.mvc.method.annotation.ResponseEntityExceptionHandler;

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

import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RestController;

@RestController
public class BookController {
    @GetMapping("/books/{id}")
    public String getBookById(@PathVariable Long id) {
        throw new BookNotFoundException("Book not found with id " + id);
    }
}

@ControllerAdvice
public class CustomExceptionHandler extends ResponseEntityExceptionHandler {
    @ExceptionHandler(BookNotFoundException.class)
    public ResponseEntity handleBookNotFoundException(BookNotFoundException ex) {
        return new ResponseEntity(ex.getMessage(), HttpStatus.NOT_FOUND);
    }
}

// Your code here

Need a hint?

This fallback method catches any other errors and sends a 500 response with a friendly message.