build-frontend fails when file is duplicated in frontend and META-INF/frontend

[javier-godoy]

Description of the bug

I have a WAR project that contains the same file in /src/main/frontend/styles and /src/main/resources/META-INF/frontend/styles (because of reasons*)

When I run mvn clean package -Pproduction it fails with the following error

Failed to find the following css files in the node_modules or C:\Users\Javier\Downloads\skeleton-starter-flow-24\src\main\frontend directory tree:
[ERROR] - ./styles/image-crop-styles.css
[ERROR] Check that they exist or are installed. If you use a custom directory for your resource files instead of the default frontend folder then make sure it's correctly configured (e.g. set 'vaadin.frontend.frontend.folder' property)

Note that the file DOES exist.

* Yes, this setup is weird...

Expected behavior

The build should succeed (or it should fail with a different error, currently it complains that an existing file does not exist)

Minimal reproducible example

1. Build the attached project with mvn clean package -Pproduction
2. Observe that the build fails
3. Delete \src\main\resources\META-INF\frontend\styles
4. Rebuild: mvn clean package -Pproduction
5. Observe that the build succeeds

skeleton-starter-flow-24.zip

Versions

• Vaadin / Flow version: 24.6.5
• Java version: Corretto-17.0.0.35.1 (build 17+35-LTS)
• OS version: Windows 10.0.19045.5371

[mcollovati]

It works if you put the resources in /src/main/resources/META-INF/resources/frontend/.
This is the path that TaskCopyLocalFrontendFiles uses to copy local resources to the generated/jarResources folder.
The path can be configured by setting frontendResourcesDirectory in the vaadin-maven-plugin configuration

<frontendResourcesDirectory>${project.basedir}/src/main/resources/META-INF/frontend</frontendResourcesDirectory>

Edit: the parameter is documented here

/src/main/resources/META-INF/resources/frontend/ is the documented folder for CSS and JS resources in add-on projects (https://vaadin.com/docs/latest/flow/advanced/loading-resources)

[javier-godoy]

@mcollovati Thanks, I still have to move some things around, but it's a little bit more clear this way. The project was actually a non-spring project, so I tried to move resources according to Table 1 from https://vaadin.com/docs/latest/flow/advanced/loading-resources.

I still think there is an issue here, because I had put a file in directories A and B, then the build complained that A/file didn't exist (while it did exist), and I had to delete B/file so that A/file could be found.

[mcollovati]

The issue here is that the copy task considers only one folder, but the the piece of code that resolves the import also fallbacks to the META-INF/frontend folder.
So, the resource if "found" but not present in the generated folder

[mcollovati]

https://github.com/vaadin/flow/blob/main/flow-server%2Fsrc%2Fmain%2Fjava%2Fcom%2Fvaadin%2Fflow%2Fserver%2Ffrontend%2FAbstractUpdateImports.java#L515-L523

https://github.com/vaadin/flow/blob/main/flow-server%2Fsrc%2Fmain%2Fjava%2Fcom%2Fvaadin%2Fflow%2Fserver%2Ffrontend%2FAbstractUpdateImports.java#L845-L849