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Learn how to effectively send `Flux` or `Mono` objects to FreeMarker templates in a Spring Boot application, and avoid common pitfalls.
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This video is based on the question https://stackoverflow.com/q/67850113/ asked by the user 'Сергей В' ( https://stackoverflow.com/u/8764425/ ) and on the answer https://stackoverflow.com/a/67851555/ provided by the user 'ddekany' ( https://stackoverflow.com/u/606679/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Send Flux or Mono to Freemarker Templates in Spring Boot

In a Spring Boot application, you might find yourself in situations where you need to render a list of data fetched from a repository in a web page. When using reactive programming with Project Reactor, you often work with types like Flux or Mono. However, sending these types directly to Freemarker templates can lead to confusion and errors, particularly when trying to access their content. In this post, we’ll explore how to correctly send Flux or Mono to Freemarker and avoid common pitfalls.

Understanding the Problem

Suppose you have a user repository that returns a Flux<User>, and you want to display the list of users on a webpage using Freemarker. Your initial approach in the controller looks correct:

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However, when you attempt to access this data in your Freemarker template like this:

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You may encounter an error: reemarker.core.InvalidReferenceException. This means your template cannot find the user attribute, making it seem as if the data is null or missing. So, how can we resolve this?

Solution: Accessing Flux Data in Freemarker

Highlighting the Issue

The root of the problem lies in how Freemarker handles objects. By default, Freemarker expects to work with plain Java objects that can be easily manipulated. When you attempt to reference user.content, Freemarker looks for methods like user.getContent() or properties associated with the object, but Flux doesn’t conform to these expectations directly.

Customizing Freemarker Configuration

To resolve this, you can customize the way Freemarker processes the Flux object. Here are the steps to achieve this:

Extend the DefaultObjectWrapper: Create a custom implementation that is aware of the Flux and Mono APIs.

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Set the Custom Object Wrapper in Freemarker Configuration: In your Spring Boot configuration, set up the Freemarker template engine to use your custom object wrapper.

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Accessing Data in Freemarker: After customizing the wrapper, you can now access the user list in your Freemarker template as follows:

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

Conclusion

By following the steps outlined above, you can effectively send Flux or Mono types to Freemarker templates in your Spring Boot applications. This approach not only solves the InvalidReferenceException problem but also ensures that you leverage the power of reactive programming while building your web applications.

If you want to incorporate advanced functionality in your templates, consider extending this custom object wrapper further to handle other reactive types or provide more complex behaviors. With this knowledge, you are now equipped to handle similar situations in your development journey!