NSD missing NSDManager in DiscoveryListener-callback, when calling resolveService

Question

I want to exchange data between android devices, therefore NSD should be used to find other devices. I followed the example on Android Developers

The service is registered successfully on the network by the server-device, the client-device recognize the service and enter the discoveryListener callback method onServiceFound(). Then I initialize the resolveListener and call resolveService(), but the app crashes with

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.net.nsd.NsdManager.resolveService(android.net.nsd.NsdServiceInfo, android.net.nsd.NsdManager$ResolveListener)' on a null object reference
                  at de.niklasdahlheimer.oboeapitest.oboeapitest.NSDFinder$1.onServiceFound(NSDFinder.java:59)
                  at android.net.nsd.NsdManager$ServiceHandler.handleMessage(NsdManager.java:333)
                  at android.os.Handler.dispatchMessage(Handler.java:102)
                  at android.os.Looper.loop(Looper.java:154)
                  at android.os.HandlerThread.run(HandlerThread.java:61)

My mNsdManager is not null in the NSDFinder class but somehow it is null in the callback-methods of the DiscoveryListener. Seems like I have a problem with different threads or a missunderstanding in working with contexts. Should my NSDFinder class extend service or something? I do not want the NSD-Code in the main-activity to keep the code well structured.

---

(shortened) main Activity

public class MainActivity extends AppCompatActivity {

    //Variable Declarations
    NSDFinder my_nsdfinder;

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        button_discoverserver =(Button) findViewById(R.id.button_discoverServer);



        //Variables Instantiations
        my_nsdfinder = new NSDFinder();


                    button_discoverserver.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                my_nsdfinder.initializeDiscoveryListener();
                my_nsdfinder.startDiscovery(getApplicationContext());
                Toast.makeText(getApplicationContext(),"Suche nach Servern...",Toast.LENGTH_LONG).show();
            }
        });

    } 
}

NSDFinder class (NSDFinder.java)

public class NSDFinder{
    String TAG = "NSDFINDER";
    NsdManager mNsdManager;
    NsdManager.DiscoveryListener mDiscoveryListener;
    String mServiceName;
    String SERVICE_TYPE = "_http._tcp.";
    Context mContext;
    int mDiscoveryActive = 0;
    FragmentManager m_DialogManager;
    NsdManager.ResolveListener mResolveListener;
    NsdServiceInfo mService;
    int mServiceport;
    InetAddress mServicehostAdress;


    public void initializeDiscoveryListener() {
        // Instantiate a new DiscoveryListener
        mDiscoveryListener = new NsdManager.DiscoveryListener() {

            // Called as soon as service discovery begins.
            @Override
            public void onDiscoveryStarted(String regType) {
                Log.d(TAG, "Service discovery started");
                mDiscoveryActive = 1;

            }

            @Override
            public void onServiceFound(NsdServiceInfo service) {
                // A service was found! Do something with it.
                Log.d(TAG, "Service discovery success\n" + 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(mServiceName)) {
                    // 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: " + mServiceName);
                } else if (service.getServiceName().contains("LTCTool")){
                    Toast.makeText(mContext,"LTCTool Server gefunden! "+service,Toast.LENGTH_LONG).show();
                    initializeResolveListener();
                    mNsdManager.resolveService(service,mResolveListener);
                    //mNsdManager.stopServiceDiscovery(mDiscoveryListener);
                }
            }

            @Override
            public void onServiceLost(NsdServiceInfo service) {
                // When the network service is no longer available.
                // Internal bookkeeping code goes here.
                Log.e(TAG, "service lost" + service);
            }

            @Override
            public void onDiscoveryStopped(String serviceType) {
                Log.i(TAG, "Discovery stopped: " + serviceType);
                mDiscoveryActive = 0;
            }

            @Override
            public void onStartDiscoveryFailed(String serviceType, int errorCode) {
                Log.e(TAG, "Discovery failed: Error code:" + errorCode);
                mNsdManager.stopServiceDiscovery(this);
            }

            @Override
            public void onStopDiscoveryFailed(String serviceType, int errorCode) {
                Log.e(TAG, "Discovery failed: Error code:" + errorCode);
                mNsdManager.stopServiceDiscovery(this);
            }
        };
    }

    public void startDiscovery(Context _c){
        mContext = _c;
        NsdManager mNsdManager = (NsdManager) _c.getSystemService(Context.NSD_SERVICE);
        mNsdManager.discoverServices(SERVICE_TYPE, mNsdManager.PROTOCOL_DNS_SD, mDiscoveryListener);

    }


    public void initializeResolveListener() {
        Log.d(TAG, "start initalizing ResolveListener");
        mResolveListener = new NsdManager.ResolveListener() {

            @Override
            public void onResolveFailed(NsdServiceInfo serviceInfo, int errorCode) {
                // Called when the resolve fails. Use the error code to debug.
                Log.e(TAG, "Resolve failed" + errorCode);
            }

            @Override
            public void onServiceResolved(NsdServiceInfo serviceInfo) {
                Log.e(TAG, "Resolve Succeeded. " + serviceInfo);

                if (serviceInfo.getServiceName().equals(mServiceName)) {
                    Log.d(TAG, "Same IP.");
                    return;
                }
                mService = serviceInfo;
                mServiceport = mService.getPort();
                mServicehostAdress = mService.getHost();
                Toast.makeText(mContext,"Dienstinformationen aufgelöst! Port: "+mServiceport+" Adresse: "+mServicehostAdress,Toast.LENGTH_LONG).show();
            }
        };

        Log.d(TAG, "ResolveListener initialized");
    }

}

Answer 1

After a few more months of coding in Java, I'm looking at this question again.

The answer is clear now. In

public void startDiscovery(Context _c)

I called

NsdManager mNsdManager = (NsdManager) _c.getSystemService(Context.NSD_SERVICE);

which, instead of initialize the member-variable of the class, declared a new, local variable in the function startDiscovery(). So mNsdManagerwas only present in startDiscovery(). Changing the line to

mNsdManager = (NsdManager) _c.getSystemService(Context.NSD_SERVICE);

would probably have solved the problem.

Maybe this post will help another 'newbie' someday.

Good to see, that sometimes "mysterious problems" will be solved by time and individual progress :)