How can i find an open ports in range of ports?

Question

I want to find an open local port in some range.

How can I do that in the most efficient way, without connecting to the port.

Answer 1

If you want to find a local open port to bind a server to, then you can create a ServerSocket and if it does not throw an Exception, then it's open.

I did the following in one of my projects:

private int getAvailablePort() throws IOException {
    int port = 0;
    do {
        port = RANDOM.get().nextInt(20000) + 10000;
    } while (!isPortAvailable(port));

    return port;
}

private boolean isPortAvailable(final int port) throws IOException {
    ServerSocket ss = null;
    try {
        ss = new ServerSocket(port);
        ss.setReuseAddress(true);
        return true;
    } catch (final IOException e) {
    } finally {
        if (ss != null) {
            ss.close();
        }
    }

    return false;
}

RANDOM is a ThreadLocal here, but of course you can do an incrementing part there.

Answer 2

There's a little problem you may face in a multitasking windows/unix environment: if some isPortAvailable(final int port) from any of the answers returns a true for you, that doesn't mean, that at the moment when you will actually bind it it still will be available. The better solution would be create a method

ServerSocket tryBind(int portRangeBegin, int portRangeEnd) throws UnableToBindException;

So that you will just launch it and receive open socket for you, on some available port in the given range.