This library allows you to manage your openhab3 instance without tough process of writing java client by yourself
Connecting to your openhab is simple as it:
var client = new OpenhabClientImpl("http://192.168.0.257:8080", "login", "password");
client.item().setState("LivingRoom_Light", "OFF");
client.item().getState("LivingRoom_Light"); //will return "ON" or "OFF"
client.event().all().subscribe(event->{
System.out.println(event);
});