Handling not found exceptions in Spring Boot - Mini Project: Build & Apply

Handling not found exceptions in Spring Boot

📖 Scenario: You are building a simple Spring Boot REST API for managing books in a library. Users can request details of a book by its ID.Sometimes, the requested book ID does not exist in the system. You want to handle this case gracefully by sending a clear error message and a proper HTTP status code.

🎯 Goal: Build a Spring Boot REST controller that returns book details by ID. If the book is not found, return a 404 Not Found response with a custom error message.

📋 What You'll Learn

Create a simple data structure to hold book information

Add a configuration variable for the error message

Implement a method to find a book by ID and throw a custom exception if not found

Create an exception handler to return a 404 status and error message

💡 Why This Matters

🌍 Real World

Handling not found exceptions is common in web APIs to inform clients clearly when requested data does not exist.

💼 Career

Backend developers often implement exception handling in Spring Boot to improve API reliability and user experience.

1

Create the initial book data structure

Create a Map<Integer, String> called books with these exact entries: 1 mapped to "Spring Boot Basics", 2 mapped to "Java Fundamentals", and 3 mapped to "REST API Design".

Spring Boot

// Create the books map with IDs and titles
// Your code here

Need a hint?

Use Map.of() to create an immutable map with the given entries.

2

Add a custom error message variable

Create a String variable called notFoundMessage and set it to "Book not found".

Spring Boot

Map books = Map.of(
    1, "Spring Boot Basics",
    2, "Java Fundamentals",
    3, "REST API Design"
);

// Create the notFoundMessage variable
// Your code here

Need a hint?

Just create a simple String variable with the exact name and value.

3

Implement method to find book or throw exception

Write a method called getBookById that takes an int id parameter and returns a String. Inside, use books.get(id) to get the book title. If the result is null, throw a new BookNotFoundException with notFoundMessage. Otherwise, return the book title.

Spring Boot

Map books = Map.of(
    1, "Spring Boot Basics",
    2, "Java Fundamentals",
    3, "REST API Design"
);

String notFoundMessage = "Book not found";

// Write getBookById method below
// Your code here

Need a hint?

Check if the book is null and throw the exception with the message variable.

4

Create exception handler for BookNotFoundException

Add a method annotated with @ExceptionHandler(BookNotFoundException.class) that returns a ResponseEntity<String>. The method should take a BookNotFoundException ex parameter and return ResponseEntity.status(HttpStatus.NOT_FOUND).body(ex.getMessage()).

Spring Boot

Map books = Map.of(
    1, "Spring Boot Basics",
    2, "Java Fundamentals",
    3, "REST API Design"
);

String notFoundMessage = "Book not found";

public String getBookById(int id) {
    String book = books.get(id);
    if (book == null) {
        throw new BookNotFoundException(notFoundMessage);
    }
    return book;
}

// Add exception handler method below
// Your code here

Need a hint?

Use @ExceptionHandler annotation and return a ResponseEntity with 404 status and the exception message.