How do you implement a custom error handler in Spring Boot?

Table of Contents

• Introduction
• How Spring Boot Handles Errors
  • Common Exception Handling Strategies
• Steps to Implement a Custom Error Handler in Spring Boot
  • Step 1: Create a Custom Error Response Class
    • Example: Custom Error Response Class
  • Step 2: Create a Global Exception Handler Using @ControllerAdvice
    • Example: Global Exception Handler
  • Step 3: Define Custom Exceptions (Optional)
    • Example: Custom Exception Class
  • Step 4: Throw Exceptions in Your Controller
    • Example: Throwing the Custom Exception in a Controller
  • Step 5: Customizing HTTP Status Codes (Optional)
    • Example: Custom HTTP Status Codes
• Step 6: Customize Error Pages (Optional)
  • • Example: Custom Error Page
• Conclusion

Introduction

Handling errors effectively is crucial in any web application to provide clear and useful feedback to users or clients. In Spring Boot, you can implement custom error handling by using the @ControllerAdvice annotation, which allows you to centralize the handling of exceptions across all controllers. Additionally, you can customize the error response to provide detailed messages, error codes, or other useful information. This guide will walk you through how to implement a custom error handler in Spring Boot.

How Spring Boot Handles Errors

Spring Boot has a default error handling mechanism that automatically returns a generic response when an error occurs, such as an HTTP 400 or 404 status code. However, for more control over error handling (e.g., custom error messages, logging, or complex error responses), Spring Boot allows you to customize error handling through a combination of exception classes and global exception handlers.

Common Exception Handling Strategies

• Using **@ControllerAdvice** for Global Error Handling: Centralizes exception handling across all controllers.
• Customizing Error Responses: Modify the structure and content of the error responses.
• Handling Specific Exceptions: Customize how particular exceptions are handled, such as IllegalArgumentException, EntityNotFoundException, etc.

Steps to Implement a Custom Error Handler in Spring Boot

Step 1: Create a Custom Error Response Class

You can define a custom class to represent the error response structure. This class can include fields like error code, message, timestamp, and more.

Example: Custom Error Response Class

In this example:

• The ErrorResponse class includes fields for timestamp, status, error, message, and path to provide a detailed error response.

Step 2: Create a Global Exception Handler Using @ControllerAdvice

The @ControllerAdvice annotation in Spring allows you to define global exception handling logic. This class will capture exceptions thrown by controllers and return a custom response.

Example: Global Exception Handler

In this example:

• @ControllerAdvice is used to define a global exception handler.
• handleResourceNotFound() handles a custom ResourceNotFoundException and sends a 404 Not Found response.
• handleGeneralException() handles any other generic exceptions and returns a 500 Internal Server Error.

Step 3: Define Custom Exceptions (Optional)

You can define your own exception classes to handle specific error cases in your application. For example, you might create a ResourceNotFoundException to indicate when a resource cannot be found.

Example: Custom Exception Class

This class extends RuntimeException and can be thrown when a resource is not found.

Step 4: Throw Exceptions in Your Controller

You can now throw the custom exceptions in your controllers whenever a specific error condition occurs, and it will be handled by the global exception handler.

Example: Throwing the Custom Exception in a Controller

In this example:

• If the id is invalid (e.g., null or negative), a ResourceNotFoundException is thrown.
• This exception is caught by the handleResourceNotFound method in the GlobalExceptionHandler class.

Step 5: Customizing HTTP Status Codes (Optional)

You can also customize HTTP status codes based on the exception type. For example, you might return a 400 Bad Request for invalid input or a 401 Unauthorized for authentication failures.

Example: Custom HTTP Status Codes

In this example:

• InvalidInputException results in a 400 Bad Request error, and the response includes a custom error message.

Step 6: Customize Error Pages (Optional)

If you want to display custom error pages (e.g., for 404 Not Found or 500 Internal Server Error), you can configure custom error pages in Spring Boot.

Example: Custom Error Page

Create an HTML file in the src/main/resources/templates directory:

• src/main/resources/templates/error/404.html (for 404 errors)
• src/main/resources/templates/error/500.html (for 500 errors)

Spring Boot will automatically use these templates for the corresponding HTTP errors.

Conclusion

In Spring Boot, custom error handling is essential for providing meaningful error messages and improving the user experience. By using @ControllerAdvice, you can implement global exception handling, define custom error responses, and provide more detailed error information to clients. Custom exceptions, HTTP status codes, and error pages can further enhance error handling in your application, ensuring that users receive clear and actionable feedback when something goes wrong.