UDP Socket编程中服务端不存在时客户端消息去向及程序卡顿问题咨询

Hey there, let's tackle your two UDP socket questions clearly:

UDP is a connectionless protocol—unlike TCP, it doesn't verify if the server is online or listening before sending data. Here's what you can expect:

• If the target IP/port is unreachable (server doesn't exist or isn't accepting traffic), your operating system may send an ICMP "Destination Unreachable" packet back to the client. But this isn't guaranteed—some networks or firewalls will drop these error packets, so you might never get feedback that the message failed.
• Basic UDP clients don't listen for ICMP errors by default. The send() call will return immediately, making it seem like the message went through successfully, even though it was dropped somewhere in the network.
• If you used socket.connect() on your UDP socket (this just binds it to a specific remote address, not a real connection like TCP), some systems will route ICMP errors to your socket. In that case, a subsequent receive() call might throw a PortUnreachableException—but without calling receive(), you still won't know the message never reached a server.

This is almost certainly a UI thread blocking problem if you're using a GUI framework like Swing, JavaFX, or AWT. Here's the breakdown:

• All GUI frameworks handle user input, button clicks, and interface updates on a single dedicated thread (the Event Dispatch Thread for Swing, JavaFX Application Thread for JavaFX).
• If you put blocking network operations (like DatagramSocket.receive()—which waits indefinitely for a response if none comes) directly in the code that runs when you click the "Send" button, you're stopping the UI thread dead in its tracks. It can't process any other clicks, redraw the window, or respond to you until that blocking operation finishes.
• Even if the send() itself is fast, if you're waiting for a server response that never arrives (thanks to your first question's scenario), that receive() call will lock up your UI forever until you force-close the program.

Quick fix steps: #

• Move all network code (sending packets, listening for responses) to a separate background thread.
• For Swing, use SwingWorker to run background tasks and safely update the UI when done. For JavaFX, use Task or Service classes.
• Never call blocking I/O methods directly from a UI event handler (the code triggered by your "Send" button click).

Note: Since you mentioned an image, if your freeze looks like the window won't redraw, buttons won't click, and the app is completely unresponsive, this confirms the UI thread is blocked.

内容的提问来源于stack exchange，提问作者Apeal Acharya