Today, I tried out the Ble Bootloader from the Sdk examples.
As a central client, I'd used the »nRF Loader App« on an iPhone and also »nRF Mcp« on an Android Tablet (I'd not tried out the C# Windows client yet).
Is there also a Linux client available (maybe in Java)?.
No, there are no such application currently available, and if you need this, it is something you'll have to make yourself. It is yet not possible to implement a custom GATT Client over BlueZ's D-Bus interface, so this would most likely have to be done by making a C-application, similar to gatttool, which is part of upstream BlueZ.
Thanx for that very useful hint. That really saves me some research effort about this topic.