Java 9 has just been released. As always, there are problems with brand new versions. In the case of JSesh, it looks most of the software runs fine.
However, the way a java software interacts with Mac-specific functions (such as the Application Preference, and the way to open a file by clicking on it), has dramatically changed from java 8 to java 9.
As a result, the current version of JSesh is not really usable with Java 9.
For this reason, I have decided to modify the Mac distribution of JSesh 6.5.10: I include Java 8 in the software itself. It's already fixed on my download page, but you can find the new version at https://files.qenherkhopeshef.org/jsesh/JSesh-6.5.10-jvm.dmg.