TCP。 客户端即使服务器不接受也可以连接

Question

I have TCP server-client application. It works but sometime something happens. Client connects to server but server says he doesn't accepted him.

Server side code:

while(!stopped){
            try {
                AcceptClient();
            } catch(SocketTimeoutException ex){
                continue;
            } catch (IOException ex) {
                System.err.println("AppServer: Client cannot be accepted.\n"+ex.getMessage()+"\n");
                break;
            }
...

private void AcceptClient() throws IOException {
    clientSocket = serverSocket.accept();
    clientSocket.setSoTimeout(200);
    out = new ObjectOutputStream(clientSocket.getOutputStream());
    in = new ObjectInputStream(clientSocket.getInputStream());
    System.out.println("Accepted connection from "+clientSocket.getInetAddress());
}

Client side code:

    try {
        socket = new Socket(IPAddress, serverPort);
        socket.setSoTimeout(5000);
        out = new ObjectOutputStream(socket.getOutputStream());
        in = new ObjectInputStream(socket.getInputStream());
    } catch (IOException e1) {
        sendSystemMessage("DISCONNECTED");
        sendSystemMessage(e1.getMessage());
        return;
    }
    sendSystemMessage("CONNECTED");

If client connects the message:

Accepted connection from ... appears. But sometimes it doesn't appear even if client sends message "CONNECTED"

Server is still runing the loop trying to get client and it is catching socketTimeoutException. Client is connected, sends message and waits for response.

Answer 1

I suspect a missing 'flush' inside your client's 'sendSystemMessage()'. Unfortunately the constructor of ObjectInputStream attempts to read a header from the underlying stream (which is not very intuitive IMHO). So if the client fails to flush the data - the server may remain stuck on the line "in = new ObjectInputStream(socket.getInputStream())"...

As a side note it's usually better for a server to launch a thread per incoming client, but that's just a side remark (plus it obviously depends on requirements).

Answer 2

I found the problem. The communication on my net is too slow so it timeouts in getting inputstream. The solution has two parts. Flushing outputstream before getting inputstream. And set socket timout after streams are initialized.

serverside:

clientSocket = serverSocket.accept();
out = new ObjectOutputStream(clientSocket.getOutputStream());
out.flush()
in = new ObjectInputStream(clientSocket.getInputStream());
clientSocket.setSoTimeout(200);