Скачать или смотреть Solving the 404 Error in Thymeleaf Forms with Spring Boot's GET Method

Learn how to effectively bind forms in Thymeleaf using Spring Boot's `GET` method, and prevent common errors like `404 Not Found`.
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Understanding the 404 Error in Thymeleaf Forms with Spring Boot

If you're building a web application with Spring Boot and Thymeleaf, you might encounter a 404 Not Found error when trying to bind a HTML form to a GET mapping. This error often leaves developers puzzled, especially when the configuration seems technically correct.

In this guide, we'll explore a common issue related to binding a form's GET method to a GetMapping controller, and provide a straightforward solution.

The Problem

Let's take a look at the scenario:

You have an HTML form created using Thymeleaf that looks something like this:

[[See Video to Reveal this Text or Code Snippet]]

Your Spring Boot controller is set up as follows:

[[See Video to Reveal this Text or Code Snippet]]

However, upon submitting the form, you receive an error message stating No mapping for GET /SearchTerminal, ultimately resulting in a 404 error.

Why the Error Occurs

The error arises because of a fundamental misunderstanding of how @ GetMapping and URL parameters work in Spring Boot:

URL Parameters Are Not Path Variables: When you append parameters to a URL after a ?, they are considered query parameters, not path variables. Hence, the syntax in your controller's @ GetMapping is not valid for handling these parameters.

The Solution

To fix this issue, you need to adjust your controller's method to correctly accept the query parameter. Here's how you can do it:

Change the Mapping: Remove the query parameter from the @ GetMapping value. Your method should simply map to /SearchTerminal.

Use @ RequestParam: Use @ RequestParam to retrieve the query parameter from the URL.

Updated Controller Code

Here's how your controller should be updated:

[[See Video to Reveal this Text or Code Snippet]]

Explanation of Changes

Removed Query Parameters from Mapping: By specifying only /SearchTerminal in the @ GetMapping, the controller is now correctly set to handle incoming requests to that URL without attempting to read parameters from the mapping pattern.

Using @ RequestParam: This annotation allows you to extract the value of the code parameter directly from the URL. When your form is submitted and the URL looks like http://localhost:8081/SearchTerminal?code=10101010, Spring will map the value 10101010 to the code variable in your method.

Summary

In summary, when working with Thymeleaf forms and Spring Boot's GET method, it's crucial to differentiate between path variables and query parameters. By adhering to the correct mapping conventions and utilizing the appropriate annotations, you can effectively prevent common errors like the 404 Not Found.

Key Takeaways

Always Use @ RequestParam for Query Parameters: This will allow your controller to receive parameters passed in the URL after the ?.

Correct Mapping is Critical: Make sure your method mappings precisely reflect the expected request patterns.

With these adjustments, you should find that your form submissions work smoothly without encountering 404 errors.

Thank you for reading! Happy coding!