我想在android设备之间交换数据，因此应使用NSD查找其他设备。

服务器设备已在网络上成功注册了该服务，客户端设备识别了该服务并在onServiceFound()上输入了DiscoveryListener回调方法。 然后，我初始化resolveListener并调用resolveService()，但应用程序崩溃，

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)

我的mNsdManager在NSDFinder类中不为null，但在DiscoveryListener的回调方法中以某种方式为null。 似乎我在使用不同的线程时遇到了问题，或者在处理上下文时出现了误解。 我的NSDFinder类应该扩展服务还是什么？ 我不希望main-activity中的NSD代码保持结构良好。

(缩短)主要活动

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类(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");

}

}