Both the RPM and JAR file types are basically archives, packages that can be extracted with the right tool. So you could simply use one of such tools, like the 7-zip to convert RPM to JAR. However it doesn't mean the resulting JAR will be a valid JAVA archive for the purposes of installing apps, it will but just a JAR archive that has the data from the RPM inside it.
Convert Linux package manager archive to Compressed archive file package for Java classes and data.
We have found one software suitable in our database for this conversion.
Microsoft Windows software - convert rpm to jar on Windows
Popular open source file archiver with a high compression ratio
Convert from rpm file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.