Works rather well, except that whenever I open the serial monitor window, it seems to reset the UNO.
Yep, that happens.
I read this is normal for Linux and Mac ( which means for me "not for Windows 7" )
and guess it has to do with some serial control signal ( DTR ?)
It happens on Windows, too. Yes, it does.
My question: Can I control this behavior on the PC side ?
Yes, and no. Yes, you can control it in your application. No, the serial monitor is not your application.
Any hints for beginners to get started with Java JDK and COM, in respect to communicating with the UNO
Look in the Playground for the Interfacing with Software section. Scroll down to find the Java section.
a) How to start my own app without resetting the UNO ?
Generally, this is done with a simple hardware mod. There are several different possibilities - some permanent, some removable.