Jabaco compiles the Jsrc sources into Java bytecode. You therefore can call/use many Jabaco structures directly from your Java classes.
It helps to decompile the generated Jabaco java code using tools like JAD. You then get an insight about the generated Java classes.
This should help how to use and call them.

To establish a callback facility, you could define a Jabaco handler class and pass a handler object to your Java class.

Code of handler class clsCallBackhandler In Jabaco:

Option Explicit

Public Sub callBackRoutine(someArgument As String)
Debug.Print "called with " & someArgument
End Sub

External class/routine in Java:

...

public void rexxStart(String name, clsCallBackHandler handler) {
...
handler.callBackRoutine("hello!");
...
}