Why doses this code sometimes throw UnknownHostException but other times throw SocketTimeoutException

Question

    URL realUrl = new URL("xxx.xxxx.com");
    URLConnection conn = realUrl.openConnection();
    conn.setRequestProperty("accept", "*/*");
    conn.setRequestProperty("connection", "Keep-Alive");
    conn.setRequestProperty("user-agent",
            "Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1;SV1)");
    conn.setDoOutput(true);
    conn.setDoInput(true);
    out = new PrintWriter(new OutputStreamWriter(conn.getOutputStream()));
    out.print(param);
    out.flush();
    in = new BufferedReader(
                    new InputStreamReader(conn.getInputStream()));
    ByteArrayOutputStream baos = new ByteArrayOutputStream();
    byte[] buf = new byte[1024];
    int len = 0;
    InputStream is = conn.getInputStream();
    while ((len = is.read(buf)) != -1) {
        System.out.println(len);
        baos.write(buf, 0, len);
    }

Linux OS,Tomcat server,"xxx.xxxx.com" is an incorrect URL.

I am expecting a UnknownHostException all the time, but sometimes what I am getting is UnknownHostException.

UnknownHostException error message

java.net.UnknownHostException: xxx.xxxx.com at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:178) at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392) at java.net.Socket.connect(Socket.java:579) at sun.net.NetworkClient.doConnect(NetworkClient.java:178) at sun.net.www.http.HttpClient.openServer(HttpClient.java:432)

SocketTimeoutException error message

java.net.SocketTimeoutException: connect timed out at java.net.PlainSocketImpl.socketConnect(Native Method) at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:339) at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:200) at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:182) at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392) at java.net.Socket.connect(Socket.java:579) at sun.net.NetworkClient.doConnect(NetworkClient.java:178) at sun.net.www.http.HttpClient.openServer(HttpClient.java:432)

score 0 · Answer 1 · answered Mar 09 '16 at 04:17

Just a guess - is it possible that sometimes your app can not connect to the DNS server? That would explain why you receive a SocketTimeoutException instead of an UnknownHostException - your app is trying to look up the hostname but it times out before it can determine that the host does not exist.

Why doses this code sometimes throw UnknownHostException but other times throw SocketTimeoutException

1 Answers1