Getting IP inside activity causes crash

Question

im creating an android app, to link and transfer socket packets to the server through wifi. In order for this, the IP address of the connected server has to be defined. I used the below function to get the IP address. When i hardcode the SERVER_IP = "0"; the app runs normally. Please help!

    private final String SERVER_IP = getIpAddr();

    public String getIpAddr() {
    WifiManager wifiManager = (WifiManager) getApplicationContext().getSystemService(WIFI_SERVICE);
    WifiInfo wifiInfo = wifiManager.getConnectionInfo();
    int ip = wifiInfo.getIpAddress();

    String ipString = String.format(
            "%d.%d.%d.%d",
            (ip & 0xff),
            (ip >> 8 & 0xff),
            (ip >> 16 & 0xff),
            (ip >> 24 & 0xff));

    return ipString;
}

After Setting up the above, I ran the below codes on the OnCreate function.

class ClientThread implements Runnable {

    @Override
    public void run() {
        try {
            InetAddress serverAddr = InetAddress.getByName(SERVER_IP);
            socket = new Socket(serverAddr, SERVERPORT);
        } catch (UnknownHostException e1) {
            e1.printStackTrace();
        } catch (IOException e1) {
            e1.printStackTrace();
        }
    }
}

As soon as that happens, my android application will stop working.

This is the error that appears:

Java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context android.content.Context.getApplicationContext()' on a null object reference

Try using ActivityName.this.getApplicationContext() to get the context — Saranga Athukorale, May 03 '17 at 04:00

score 3 · Accepted Answer · answered May 03 '17 at 03:49

Use this function to get IP (v4 or v6) inside an activity:

// Get IP address from first non-localhost interface
// @param ipv4  true returns ipv4
//              false returns ipv6
// @return address or empty string
public static String getLocalIpAddress(boolean useIPv4) {
    try {
        List<NetworkInterface> interfaces = Collections.list(NetworkInterface.getNetworkInterfaces());
        for (NetworkInterface intf : interfaces) {
            List<InetAddress> addrs = Collections.list(intf.getInetAddresses());
            for (InetAddress addr : addrs) {
                if (!addr.isLoopbackAddress()) {
                    String sAddr = addr.getHostAddress();
                    //boolean isIPv4 = InetAddressUtils.isIPv4Address(sAddr);
                    boolean isIPv4 = sAddr.indexOf(':')<0;
                    if (useIPv4) {
                        if (isIPv4)
                            return sAddr;
                    } else {
                        if (!isIPv4) {
                            int delim = sAddr.indexOf('%'); // drop ip6 zone suffix
                            return delim<0 ? sAddr.toUpperCase() : sAddr.substring(0, delim).toUpperCase();
                        }
                    }
                }
            }
        }
    } catch (Exception ex) { } // for now eat exceptions
    return "";
}

When you need it, just call it this way: getLocalIpAddress(true). You will get the IP as a string for you to use.

socket = new Socket(serverAddr, SERVERPORT); Just wondering, if I am connecting to my server correctly with this function? My serverAddr will be the ip address of my wifi i used to connect between my android phone and my hardware. — mctjl_1997, May 04 '17 at 10:10
I think you are right. The "real" IP of your phone is revealed only when behind a regular data conn, like 3G, 4G and so on. — statosdotcom, May 04 '17 at 10:30
Thanks for the reply! I see, so it looks like my TCP client set up is correct. However, my current server connection failure might be due to my hardware not setting up the socket server correctly. I will look into this! Thank you very much! :) I am pretty new to all of these — mctjl_1997, May 04 '17 at 11:47

score 1 · Answer 2 · answered May 03 '17 at 03:43

Here are a few things that you have to check

Does the Application has the permission to access the WiFi ?
If you are using the getApplicationContext() in a different class , pass a context from the Parent Activity.
Test in on a mobile device not on the emulator.

Getting IP inside activity causes crash

2 Answers2