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JNI DETECTED ERROR IN APPLICATION: JNI GetMethodID called with pending exception java.lang.ClassNotFoundException: Didn't find class

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    Please I want to build an NSD ( network services discovery) app, using Jni with Qt, But I get the following error: JNI DETECTED ERROR IN APPLICATION: JNI GetStaticMethodID called with pending exception java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference

    I am trying to call the function: discover()

    Here is the code below:

    import android.content.Context;
    import android.os.Bundle;
    import android.widget.TextView;
    import android.net.*;
    import android.net.wifi.WifiManager;
    import android.net.wifi.*;
    import 	android.net.ConnectivityManager;
    import 	android.net.wifi.WifiInfo;
    import android.net.NetworkInfo.DetailedState;
    import android.net.DhcpInfo;
    import 	android.net.nsd.NsdManager;
    import android.net.nsd.NsdManager.DiscoveryListener;
    import android.util.Log;
    import android.net.nsd.NsdServiceInfo;
    import android.text.format.Formatter;
    import java.net.NetworkInterface;
    import java.net.SocketException;
    import java.util.Enumeration;
    import java.util.regex.Matcher;
    import java.util.regex.Pattern;
    import java.net.InetAddress;
    import 	java.io.IOException;
    import 	java.net.UnknownHostException;
    //import android.app.Activity;
    import java.lang.reflect.Method;
     public  class MusicList extends org.qtproject.qt5.android.bindings.QtActivity {
         final String TAG="Display Message";
         final String SERVICE_TYPE="_http._tcp.";
         final  String serviceName= "esp32";
          private static NsdManager.DiscoveryListener  discoveryListener;
          private static NsdManager nsdManager;
          private static Context context;
         public void initializeDiscoveryListener() {
             // Instantiate a new DiscoveryListener
               discoveryListener = new NsdManager.DiscoveryListener() {
                 // Called as soon as service discovery begins.
                 public void onDiscoveryStarted(String regType) {
                     Log.d(TAG, "Service discovery started");
                 public void onServiceFound(NsdServiceInfo service) {
                     // A service was found! Do something with it.
                     Log.d(TAG, "Service discovery success" + service);
                     if (!service.getServiceType().equals(SERVICE_TYPE)) {
                         // Service type is the string containing the protocol and
                         // transport layer for this service.
                         Log.d(TAG, "Unknown Service Type: " + service.getServiceType());
                     } else if (service.getServiceName().equals(serviceName)) {
                         // The name of the service tells the user what they'd be
                         // connecting to. It could be "Bob's Chat App".
                         Log.d(TAG, "Same machine: " + serviceName);
                     } else if (service.getServiceName().contains("NsdChat")){
                        // nsdManager.resolveService(service, resolveListener);
                 public void onServiceLost(NsdServiceInfo service) {
                     // When the network service is no longer available.
                     // Internal bookkeeping code goes here.
                     Log.e(TAG, "service lost: " + service);
                 public void onDiscoveryStopped(String serviceType) {
                     Log.i(TAG, "Discovery stopped: " + serviceType);
                 public void onStartDiscoveryFailed(String serviceType, int errorCode) {
                     Log.e(TAG, "Discovery failed: Error code:" + errorCode);
                 public void onStopDiscoveryFailed(String serviceType, int errorCode) {
                     Log.e(TAG, "Discovery failed: Error code:" + errorCode);
     public static void discover(){
         nsdManager = (NsdManager) context.getSystemService(Context.NSD_SERVICE);
         nsdManager.discoverServices("_http._tcp.", NsdManager.PROTOCOL_DNS_SD, discoveryListener);
         //nsdManager = (NsdManager)c.getSystemService(Context.NSD_SERVICE);
     public static String discoverDevices(Context context)throws IOException {
         //MusicList scan= new MusicList();
         return "Abdelhamid";

  • Moderators

    Do you set the context anywhere? Doesn't look like it. The error tells you that it is null, you need to set the context before you call any methods from it.

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