Open Java .app application bundles with double click on a Mac without Apple's Java 6
From replies to a question I asked on Apple's java-dev list it would appear that on a Mac with Java 8 from Oracle installed, but not Java 6 from Apple, there is no current way to open a Java application bundle that was created using Apple's own old jar bundler technology. I suspect this is because the JavaApplicationStub is written to expect some aspect of Apple's Java not present in Oracle's Java.

From a user's point of view the only thing to do would seem to be to download and install Java 6 from https://support.apple.com/kb/DL1572 while it is still available.

From a developer's point of view the safest bet would be to embed Java within one's application bundle using a tool such as jwrapper (www.jwrapper.com). Alternatively there may be tools for creating a bundle that will run with Oracle's Java 8+.