I attach a source.jar file for reproduction, but you should understand, how to launch standart DnD application from toys (run two times, close source window in one, and a target window in the other) in two jvms, to have the same effect.

In console move to a source.jar file folder, and execute commands:
java -jar source.jar lightgreen onlySource
java -jar source.jar gray onlyTarget

So, you will have source window in one VM, and the target window in the other vm.

In source window: select customBytes, customString and customClass and click a "clipboard" button.

In the target window (having selected customBytes, customString and customClass) click clipboard too, and no content will be transfered.

This is not actual, when you do clipboard'ed transfer in the same VM.