Custom exception classes in Spring Boot - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a custom exception class in Spring Boot?

A custom exception class is a user-defined class that extends Java's Exception or RuntimeException to represent specific error conditions in a Spring Boot application.

beginner

Why create custom exception classes instead of using standard exceptions?

Custom exceptions help clearly identify and handle specific error cases, making the code easier to read, maintain, and debug.

beginner

How do you define a simple custom exception class in Spring Boot?

Create a class that extends RuntimeException and add constructors. For example:<br><pre>public class ResourceNotFoundException extends RuntimeException {<br>  public ResourceNotFoundException(String message) {<br>    super(message);<br>  }<br>}</pre>

intermediate

How can custom exceptions improve REST API error responses in Spring Boot?

By using @ControllerAdvice and @ExceptionHandler, you can catch custom exceptions and return meaningful HTTP status codes and messages to clients.

intermediate

What is the difference between checked and unchecked exceptions in the context of custom exceptions?

Checked exceptions extend Exception and must be declared or caught. Unchecked exceptions extend RuntimeException and do not require explicit handling. Custom exceptions are usually unchecked for cleaner code.

Which class should you extend to create an unchecked custom exception in Spring Boot?

A Exception

B RuntimeException

C Throwable

D Error

Unchecked exceptions extend RuntimeException, so extending RuntimeException creates an unchecked custom exception.

What annotation is commonly used to globally handle custom exceptions in Spring Boot?

A @ControllerAdvice

B @RestController

C @Service

D @Component

@ControllerAdvice allows centralized exception handling across all controllers.

Which method is used inside a custom exception class to pass an error message?

A toString()

B getMessage()

C printStackTrace()

D super(message)

Calling super(message) in the constructor passes the error message to the parent Exception class.

Why might you prefer unchecked exceptions for custom exceptions in Spring Boot?

A They force callers to handle exceptions explicitly.

B They are slower to execute.

C They simplify code by not requiring try-catch blocks.

D They cannot be caught.

Unchecked exceptions do not require explicit handling, making code cleaner and easier to maintain.

What HTTP status code is typically returned when a ResourceNotFoundException is thrown in a REST API?

A 404 Not Found

B 400 Bad Request

C 500 Internal Server Error

D 200 OK

404 Not Found indicates the requested resource does not exist.

Explain how to create and use a custom exception class in a Spring Boot REST API.

Think about how exceptions improve error clarity and API responses.

Describe the benefits of using custom exceptions over standard exceptions in Spring Boot applications.

Consider how custom exceptions help both developers and users.

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