Štěpán Roh

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Monday, March 3, 2008

Java Mystery Of (Un)Closed Sockets

by Štěpán Roh

Windows has a limit on number of existing client network sockets. I recently ran into this when I started to close sockets which were previously left to be timed out. That was certainly a suprprise, because naturally one would thought that closed socket will be, well, closed much earlier than unclosed one.

I created a small test case:

    public static void main(String[] args) throws Exception {
        int i = 0;
        while (true) {
            Socket sock = openSocket();
            OutputStream os = sock.getOutputStream();
            os.write("GET / HTTP/1.1\n\n".getBytes());
            InputStream is = sock.getInputStream();
            while (is.read() != -1) {
                // void
            System.out.println("" + i++);

    private static Socket openSocket() throws Exception {
        InetAddress address = InetAddress.getByName("localhost");
        Socket socket = new Socket(address, 80);
        socket.setSoLinger(true, 0);
        return socket;

The socket.setSoLinger(true, 0) should force the TCP connection to be dropped immediately with connection reset when socket is closed.

I tried that (and modifications without setting of SO_LINGER(0) and without closing the streams and socket) on Windows XP Professional connecting to Apache 2.0.59 and got this (the numbers differ between executions, but there is a general trend visible in a number of successfully allocated sockets):

closed with SO_LINGER(0)closed normallyunclosed
Java 5lessmoremore
Java 6morelessless

Fortunately Java 6 behaves more logically. But, of course, anyone allocating that many sockets should be punished.

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