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
}

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
}

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
}

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

Hint

Make sure all required Spring annotations and classes are imported.

Practice

1. What is the main purpose of creating a custom exception class in Spring Boot?

easy

A. To automatically fix errors without any code changes

B. To replace all built-in exceptions with your own versions

C. To make the application run faster by skipping error checks

D. To handle specific error cases clearly and separately in your application

Solution

1. Step 1: Understand the role of custom exceptions

   Custom exceptions help you identify and manage specific error situations clearly in your code.

2. Step 2: Compare with other options

   Replacing all exceptions or auto-fixing errors is not the goal; custom exceptions improve clarity and control.

3. Final Answer:

   To handle specific error cases clearly and separately in your application -> Option D

4. Quick Check:

   Custom exceptions clarify error handling = C [OK]

Hint: Custom exceptions clarify specific errors clearly [OK]

Common Mistakes:

• Thinking custom exceptions replace all errors
• Believing they fix errors automatically
• Assuming they improve performance directly

2. Which of the following is the correct way to declare a custom exception class in Spring Boot?

easy

A. public class MyException extends RuntimeException { public MyException(String message) { super(message); } }

B. public interface MyException extends RuntimeException {}

C. public class MyException implements RuntimeException {}

D. public class MyException extends Exception {}

Solution

1. Step 1: Check correct inheritance

   Custom exceptions usually extend RuntimeException to avoid mandatory try-catch blocks.

2. Step 2: Verify constructor usage

   Providing a constructor that calls super(message) is standard to pass error messages.

3. Final Answer:

   public class MyException extends RuntimeException { public MyException(String message) { super(message); } } -> Option A

4. Quick Check:

   Extend RuntimeException with constructor = D [OK]

Hint: Extend RuntimeException and add constructor with message [OK]

Common Mistakes:

• Using implements instead of extends
• Extending Exception instead of RuntimeException
• Declaring interface instead of class

3. Given this custom exception class:

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

What will happen if this exception is thrown in a Spring Boot controller without any handler?

medium

A. The exception will be ignored and the request will succeed

B. The application will return a 500 Internal Server Error response

C. The application will automatically return a 404 Not Found response

D. The server will crash and stop running

Solution

1. Step 1: Understand default Spring Boot behavior

   Without a handler, Spring Boot treats RuntimeExceptions as server errors and returns HTTP 500.

2. Step 2: Check other options

   Ignoring exceptions or auto 404 responses require explicit handling; server crash does not happen.

3. Final Answer:

   The application will return a 500 Internal Server Error response -> Option B

4. Quick Check:

   Unhandled RuntimeException = HTTP 500 error [OK]

Hint: Unhandled RuntimeException returns HTTP 500 by default [OK]

Common Mistakes:

• Assuming automatic 404 response
• Thinking exceptions are ignored
• Believing server crashes on exception

4. Identify the error in this custom exception class:

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

medium

A. The class should extend Exception, not RuntimeException

B. The first constructor should accept a message parameter

C. The second method should be a constructor but is declared as void

D. There is no error; the class is correct

Solution

1. Step 1: Check method declarations

   The second method has a return type void, so it is not a constructor but a normal method.

2. Step 2: Understand constructor syntax

   Constructors must not have a return type; this causes the message constructor to be invalid.

3. Final Answer:

   The second method should be a constructor but is declared as void -> Option C

4. Quick Check:

   Constructor must not have return type = B [OK]

Hint: Constructors have no return type, not even void [OK]

Common Mistakes:

• Adding void to constructor
• Confusing methods with constructors
• Extending wrong exception class

5. How can you handle a custom exception ResourceMissingException globally in Spring Boot to return a 404 status with a custom message?

hard

A. Create a @ControllerAdvice class with a method annotated @ExceptionHandler(ResourceMissingException.class) that returns ResponseEntity with status 404 and message

B. Throw ResourceMissingException inside a try-catch block and catch it in every controller method separately

C. Override the default error page in Spring Boot without using @ExceptionHandler

D. Use @ResponseStatus(HttpStatus.INTERNAL_SERVER_ERROR) on the exception class

Solution

1. Step 1: Use @ControllerAdvice for global handling

   This annotation allows centralized exception handling across all controllers.

2. Step 2: Define @ExceptionHandler method for ResourceMissingException

   This method can build a ResponseEntity with HTTP 404 and a custom message to send back.

3. Final Answer:

   Create a @ControllerAdvice class with a method annotated @ExceptionHandler(ResourceMissingException.class) that returns ResponseEntity with status 404 and message -> Option A

4. Quick Check:

   Global handler with @ExceptionHandler = A [OK]

Hint: Use @ControllerAdvice and @ExceptionHandler for global exception handling [OK]

Common Mistakes:

• Catching exceptions in every controller method
• Using wrong HTTP status annotation
• Overriding error page without exception handler