Скачать или смотреть Solving the Exception evaluating OGNL expression in Thymeleaf's For Loops

Learn how to fix the `Exception evaluating OGNL expression` error in Thymeleaf when using a for loop with a Kotlin backend. Follow this guide for troubleshooting tips and effective solutions.
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This video is based on the question https://stackoverflow.com/q/69200536/ asked by the user 'Luís Soares' ( https://stackoverflow.com/u/819651/ ) and on the answer https://stackoverflow.com/a/69214045/ provided by the user 'andrewJames' ( https://stackoverflow.com/u/12567365/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Solving the Exception evaluating OGNL expression in Thymeleaf's For Loops

Thymeleaf is a powerful templating engine for Java applications, particularly useful in web development for generating dynamic web pages. However, developers often encounter issues, one of which is the frustrating Exception evaluating OGNL expression error when they try to use a for loop in their templates. If you're facing this problem, you're not alone, and this guide will walk you through understanding and solving it.

Understanding the Problem

You might see an error message like this when running your Thymeleaf application:

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

This indicates that Thymeleaf is having trouble evaluating the expression used for the vote object in your HTML template. The source of the issue often lies in how variable names are defined and accessed in both your Kotlin controller and Thymeleaf templates.

Example Code Snippet

Here's the simplified version of the code structure you're likely working with:

Kotlin Controller

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

Thymeleaf Template (my-list.html)

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

Diagnosing the Issue

The problem stems from the way Thymeleaf interprets variable names. In your controller, you are passing a map entry where the key is "votes-positive". However, the hyphen in the key might cause Thymeleaf to interpret it as a minus sign in an OGNL expression. This leads to the evaluation failure and subsequently the error you're encountering.

Solution: Rename Your Key

The solution is straightforward: change the map key to avoid using hyphens. Consider replacing "votes-positive" with a name that doesn't contain special characters. Here is how you can modify your Kotlin code:

Updated Kotlin Controller

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

Updated Thymeleaf Template

Then, update your Thymeleaf template accordingly:

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

Conclusion

By removing the hyphen from your variable name, you can avoid confusion in the OGNL expression processing within Thymeleaf. This small adjustment will help clear errors and allow your template to render the vote.name successfully.

If you encounter any further issues or have additional questions about Thymeleaf debugging or model trees, feel free to explore more about these topics in the Thymeleaf documentation or consult community forums. Happy coding!