Thank you for your reply.
Unfortunately, I tried this, this didn't work either.
However, meanwhile, I have found a solution:
- using Eclipse, choose export/Runnable JAR file/ and then select
"Extract required libraries into generated JAR" and not
(as I did ...) "Package required libraries into generated JAR" !
- of course, tell eclipse where to find the html files (through Build Path/Add Class Folder)
and then it is ok with:
URL address = getClass().getResource(indexName);
JFrame viewerFrameInputFile = new DocumentViewer(filePath);
I don't know exactly what Eclipse changed in building the archive,
but it is ok now !