We need to program a multithreaded chat programm. I have already started to program it, but the server only processes the first message

Main task: Fix the code so that multiple clients can log on and server receives and sends all messages to all clients or one specific client if message starts with @username.

All other TODOs in the file can be ignored.

Please do not use any code from web as my teacher will check for plagiarism with an MIT software.