What could be the reason for p:fileUpload not calling the listener function following an upgrade from jsf-2.1 to jsf-2.3 (myfaces-2.1 to myfaces-2.3) and primefaces-3.1 to primefaces-8.0?

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There could be several reasons why the listener function is not being called after upgrading to a newer version of JSF and PrimeFaces:

1. Check if the PrimeFaces version you have upgraded to is compatible with the version of JSF you are using. Make sure that you have the correct dependencies and configurations set up.

2. Verify that the listener function is properly defined in your backing bean and is correctly bound to the fileUpload component.

3. Check if any changes have been made to the p:fileUpload component in the newer version of PrimeFaces. Make sure that the component attributes are set correctly and that any new or updated attributes are used as intended.

4. Ensure that any additional libraries or dependencies required by the newer version of PrimeFaces are properly included in your project.

5. Check the server logs for any errors or warnings that might indicate an issue with the fileUpload component or its listener function.

6. Try debugging the issue by adding breakpoints in the listener function to see if it is being called or not.

7. If all else fails, try reaching out to the PrimeFaces or JSF community for help or filing a bug report.