> Markdown version of [Converting Legacy UIs](https://vaadin.com/docs/latest/tools/mpr/introduction/3-legacy-uis). Section index: [llms.txt](https://vaadin.com/docs/latest/tools/llms.txt)

# Converting Legacy UIs

The UI object — which represents the `<body>` element in the page — is controlled by Flow when running the MPR. As a result, anything extending `UI` should be converted. Actually, `UI` class shouldn’t be used for layout or navigation handling. At the end of this part of the process, you shouldn’t have any `UI` class — unless you have a real use case for it.

Depending on what’s used in the project, you have several conversion paths:

- Application uses [Spring Boot](https://vaadin.com/docs/latest/tools/mpr/introduction/3-spring-boot.md)

- Application uses [CDI](https://vaadin.com/docs/latest/tools/mpr/introduction/3-cdi.md)

- Application uses [Navigator](https://vaadin.com/docs/latest/tools/mpr/introduction/3-navigator.md)

- Application doesn’t use any of those.

Continue to [Converting a UI](https://vaadin.com/docs/latest/tools/mpr/introduction/3-no-framework.md).

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