How do you create custom exceptions in Spring Boot?

Table of Contents

Introduction

In Spring Boot, handling errors and exceptions effectively is crucial for building robust and user-friendly applications. While Spring provides a variety of built-in exceptions, there are scenarios where you may need to create your own custom exceptions. Custom exceptions allow you to define specific error scenarios, provide meaningful error messages, and improve the overall error handling strategy in your application.

This guide covers how to create custom exceptions in Spring Boot, how to throw them when necessary, and how to handle them gracefully.

1. Creating Custom Exceptions in Spring Boot

1.1. Basic Structure of a Custom Exception Class

Creating a custom exception in Spring Boot involves defining a new class that extends either Exception or RuntimeException. Custom exceptions can include custom error messages, error codes, or additional information to make the exception more meaningful.

Example: Custom Checked Exception

A checked exception must be explicitly declared in the method signature using the throws keyword. It is typically used for conditions that the caller should handle.

Example: Custom Unchecked Exception

An unchecked exception extends RuntimeException and does not need to be declared in the method signature. It is generally used for programming errors or situations that do not require explicit handling by the caller.

In these examples:

  • ResourceNotFoundException is a checked exception used for situations where a requested resource could not be found.
  • InvalidInputException is an unchecked exception used to signal invalid user input or data.

1.2. Adding Custom Fields to Custom Exceptions

You can enhance your custom exceptions by adding custom fields to store additional information, such as an error code or additional context about the exception.

Example: Custom Exception with Error Code

In this example:

  • The CustomExceptionWithCode class includes an errorCode field to provide more specific information about the error.
  • The custom exception can then be thrown with a specific error code, which can be helpful in API responses or logging.

2. Throwing Custom Exceptions

Once your custom exceptions are created, you can throw them in your service or controller methods whenever a specific error condition is encountered.

2.1. Throwing a Custom Exception in a Service Layer

You can throw your custom exceptions in the service layer based on certain conditions, such as when a resource is not found or invalid data is provided.

Example: Throwing a ResourceNotFoundException

In this case:

  • The findUserById method throws a ResourceNotFoundException if the provided userId is invalid or if the user is not found in the database.

2.2. Throwing a Custom Exception in a Controller Layer

In your controller layer, you can also throw custom exceptions to signal errors in user input or to provide meaningful error responses in case of failures.

Example: Throwing an InvalidInputException

Here:

  • The getUser method checks if the id is valid. If it's not, it throws an InvalidInputException.

3. Handling Custom Exceptions in Spring Boot

Spring Boot provides multiple ways to handle exceptions gracefully, including handling custom exceptions in a centralized way using @ControllerAdvice or @ExceptionHandler within controllers.

3.1. Using @ExceptionHandler for Handling Custom Exceptions in Controllers

You can use the @ExceptionHandler annotation to catch and handle specific custom exceptions within a controller.

Example: Handling ResourceNotFoundException

In this example:

  • If the ResourceNotFoundException is thrown, the handleResourceNotFound method will catch it and return a 404 Not Found response with the exception message.

3.2. Using @ControllerAdvice for Global Exception Handling

Instead of defining individual exception handlers for each controller, you can centralize exception handling with @ControllerAdvice. This will allow you to handle exceptions across the entire application.

Example: Global Exception Handling with @ControllerAdvice

With @ControllerAdvice, you can handle exceptions globally:

  • handleResourceNotFound handles ResourceNotFoundException and returns a 404 response.
  • handleInvalidInput handles InvalidInputException and returns a 400 response.
  • A generic handler catches any other exceptions and returns a 500 response.

3.3. Returning Custom Response for REST APIs

For REST APIs, you can also return structured error responses in JSON format, which makes it easier for clients to consume and understand the error.

Example: Returning Custom JSON Response

This example returns a JSON response with error and message properties for a ResourceNotFoundException.

4. Conclusion

Creating and handling custom exceptions in Spring Boot is an essential practice for effective error management. Custom exceptions provide more meaningful error reporting, allow you to catch specific conditions, and make your code more robust and maintainable. By combining custom exceptions with Spring’s @ExceptionHandler and @ControllerAdvice, you can ensure that your application handles errors gracefully and provides a better user experience. Whether you're building a traditional web application or a REST API, custom exceptions allow you to offer more detailed, clear, and structured error messages.

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