How do you create a custom exception handler in Spring Boot?

Table of Contents

Introduction

In Spring Boot applications, it’s essential to handle exceptions gracefully, providing users with meaningful error messages and managing errors in a structured manner. Creating a custom exception handler allows you to centralize and manage exception handling in a Spring Boot application. This ensures that your application remains robust, user-friendly, and error-resistant.

This guide will explain how to create custom exception handlers in Spring Boot, using @ExceptionHandler and @ControllerAdvice to handle errors at the controller level or globally across the application.

Creating a Custom Exception Handler

1. Define a Custom Exception Class

First, create a custom exception that you want to handle. For instance, let’s define an exception for when a resource is not found:

2. Creating a Custom Exception Handler using **@ExceptionHandler**

Now that you have a custom exception, you can create a custom handler method inside your controller. The @ExceptionHandler annotation will help you specify which exceptions to handle.

Here’s an example of handling the ResourceNotFoundException inside a controller:

Explanation:

  • Custom Exception: The ResourceNotFoundException is thrown when a resource is not found based on the given id.
  • **@ExceptionHandler(ResourceNotFoundException.class)**: This method handles ResourceNotFoundException. It is triggered whenever this exception is thrown in the controller.
  • **@ResponseStatus(HttpStatus.NOT_FOUND)**: The 404 NOT_FOUND HTTP status code is returned with the custom error message when the exception is caught.

3. Using **@ControllerAdvice** for Global Exception Handling

While @ExceptionHandler works well for handling exceptions at the controller level, it’s often beneficial to manage exceptions globally. Spring provides the @ControllerAdvice annotation, which allows you to define global exception handlers that apply to all controllers in the application.

Here’s how you can handle exceptions globally in a Spring Boot application:

Explanation:

  • **@ControllerAdvice**: This annotation allows the GlobalExceptionHandler to be applied globally across all controllers.
  • **@ExceptionHandler(ResourceNotFoundException.class)**: This handler will handle ResourceNotFoundException globally and return a 404 NOT_FOUND status along with the custom message.
  • **@ExceptionHandler(Exception.class)**: A generic exception handler that will catch all other exceptions, providing a 500 INTERNAL_SERVER_ERROR status.

4. Returning JSON Responses for API Applications

For RESTful APIs, instead of returning views (like in the examples above), you often need to return JSON responses with error details. You can achieve this by creating a custom error response object and returning it as JSON.

Here’s an example of a global exception handler that returns a custom error response in JSON format:

ErrorResponse Class:

Explanation:

  • ErrorResponse: A custom class to hold the error details, including the HTTP status and message.
  • **ResponseEntity<ErrorResponse>**: The custom error response is returned as a ResponseEntity, which Spring will convert into a JSON response automatically.

Conclusion

Creating a custom exception handler in Spring Boot improves the maintainability of your application by centralizing error handling. You can use @ExceptionHandler to handle exceptions within a specific controller or use @ControllerAdvice for global exception handling. For REST APIs, returning JSON error responses provides a better experience for API consumers.

Key Points:

  • **@ExceptionHandler**: Used to handle exceptions at the controller level.
  • **@ControllerAdvice**: Used for global exception handling across all controllers.
  • Custom Error Responses: For REST APIs, it’s common to return custom error responses in JSON format.
  • Best Practices: Handle specific exceptions, provide meaningful error messages, and use @ResponseStatus for appropriate HTTP status codes.
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