How do you implement custom exception handling with Thymeleaf?

Table of Contents

• Introduction
  • Steps to Implement Custom Exception Handling with Thymeleaf
    • 1. Create a Custom Exception Class
    • 2. Define a Global Exception Handler Using **@ControllerAdvice**
    • 3. Create Custom Error Pages Using Thymeleaf
    • 4. Handling Other Common Exceptions
    • 5. Configure the Error Page for HTTP Errors
• Conclusion

Introduction

In a Spring Boot application, exception handling is crucial to ensure that errors are managed properly and users are presented with friendly and informative messages when something goes wrong. Thymeleaf is commonly used to display error pages in Spring Boot, and it can be integrated with custom exception handling to present error messages in a clear and user-friendly way. This guide explains how to implement custom exception handling in Spring Boot and use Thymeleaf templates to display error pages to users.

Steps to Implement Custom Exception Handling with Thymeleaf

1. Create a Custom Exception Class

First, you'll need to define your custom exceptions. These can be any exceptions that you expect to occur in your application and want to handle in a specific way.

Example: Custom Exception Class

In this example, ResourceNotFoundException is a custom exception that might be thrown when a resource (like a user or an item) is not found.

2. Define a Global Exception Handler Using **@ControllerAdvice**

In Spring Boot, @ControllerAdvice is a powerful feature that allows you to handle exceptions globally across your application. You can create a global exception handler class to catch specific exceptions and provide custom error responses.

Example: Global Exception Handler

In this example:

• The @ControllerAdvice annotation allows the GlobalExceptionHandler class to handle exceptions globally.
• The @ExceptionHandler(ResourceNotFoundException.class) method is used to handle the ResourceNotFoundException and provide a custom error page.
• The model.addAttribute("errorMessage", ex.getMessage()) adds the exception message to the model, which will be accessible in the Thymeleaf template.

3. Create Custom Error Pages Using Thymeleaf

Next, create the Thymeleaf templates that will be displayed when the exception is thrown. These templates will show the error messages or any other relevant information.

Example: Thymeleaf Error Page (**resourceNotFound.html**)

In this template:

• The th:text="${errorMessage}" expression is used to display the error message passed from the controller.
• You can customize the error page further to match your application's style and provide helpful links to the user.

4. Handling Other Common Exceptions

You can extend your exception handling to cover other common exceptions like NullPointerException, IllegalArgumentException, etc. Simply add more methods to the @ControllerAdvice class to handle these exceptions and display corresponding error pages.

Example: Handling Generic Exceptions

Example: Generic Error Page (**genericError.html**)

This will catch any uncaught exceptions and show a generic error page with a message like “An unexpected error occurred. Please try again later.”

5. Configure the Error Page for HTTP Errors

In addition to custom exceptions, Spring Boot allows you to define error pages for common HTTP errors such as 404 (not found) and 500 (internal server error). You can create these error pages using Thymeleaf templates and configure them in application.properties.

Example: Create a **404.html** and **500.html** Thymeleaf Templates

• 404.html: Custom page for "Page Not Found"
• 500.html: Custom page for "Internal Server Error"

Example: **404.html**

Example: **500.html**

To enable these error pages in Spring Boot, make sure you configure server.error.whitelabel.enabled=false in application.properties to disable the default error page and use your custom pages instead.

Conclusion

Custom exception handling in Spring Boot with Thymeleaf allows you to handle errors gracefully and present clear, user-friendly messages to users. By using @ControllerAdvice, you can globally manage exceptions, and Thymeleaf templates provide an easy way to display these errors. You can customize your error pages, handle specific exceptions like ResourceNotFoundException, and even manage HTTP error pages like 404 and 500. This approach ensures that your application remains robust and user-friendly, even when unexpected errors occur.