无法将套接字设置为非阻塞

Question

I'm trying to get to grips with java networking and have a server talking to a client. I have found an error in trying to make a socket nonblocking. Could someone have a look through my code and try and find the bug?

public static void main(String[] args) throws IOException {

    ServerSocket serverSocket = null;
    boolean listening = true;   

    try {
        serverSocket = new ServerSocket(4444);
        serverSocket.configureBlocking(false);
        System.out.println("Server started");
    } catch (IOException e) {
        System.out.println("Could not listen on port: 4444.");
        System.exit(-1);
    }

    while (listening){

        Socket s = serverSocket.accept();

        long id = clients_id++;
        ServerThread st = new ServerThread(s, id);
        addClient(id, st);
        st.start();
    }

    serverSocket.close();
}

Answer 1

You are confusing two (given: rather confusing) concepts of Java IO:

• (Old) Stream-IO: Streams are always blocking, no matter what options you might set.
• (New) NIO : This can be non-blocking if used correctly but is more complicated, however you need to use a Selector instead of calling accept on your own.

If you plan on using non-blocking IO, you should definitely read through the documentation first and experiment a bit to understand that concept properly. However once you do understand it, it allows you to write very fast IO-code.

Answer 2

It's not a 'bug', it's just a compile error. You have invented a method that doesn't exist, so the compiler won't let you call it. See java.nio.channels.ServerSocketChannel. It does have a configureBlocking() method, as does SocketChannel.

However, given the code you have written, there is no reason to set non-blocking mode at all. What you have written is a typical framework for a blocking-mode TCP server.

Also, when you get an exception, don't just print a message of your own devising. Print the one that comes with the exception.

Answer 3

NIO is blocking by default so you don't need to set it.

I use blocking NIO and I suggest using that unless you have thousands of connections.

What I suggest you do is use a thread pool like a ExecutorService.

I have found an error in trying to make a socket nonblocking.

Since you haven't said what the error is, I suspect the error is not in the code you provided.

Why do you want to make it non-blocking?