Call java programs from sql instructions (like new load data)

As you can project ANY ClassMethod as SQL PROCEDURE I see no obvious limit to your intention.

Call yoru JAVA from IRIS with any of the available interfaces   (or nodjs, or Python, or .....)
inside a ClassMethod that you present them as  SQL_PROCEDURE.

You can. Two different ways but both utilize the External Java Server with gateway connections. That is the same mechanism employed by LOAD DATA. If you have an example of what you want to do then I can provide you with a demo - using both options.