调用 resolveService 时，NSD 在 DiscoveryListener-callback 中缺少 NSDManager

Question

我想在android设备之间交换数据，因此应该使用NSD来查找其他设备。 我按照Android Developers上的示例进行操作

服务端设备在网络上注册成功，客户端设备识别服务并进入discoveryListener回调方法onServiceFound()。然后我初始化resolveListener并调用resolveService()，但应用程序崩溃了

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 类中不为空，但在 DiscoveryListener 的回调方法中不知何故为空。 似乎我在使用不同的线程时遇到了问题，或者在使用上下文时存在误解。我的 NSDFinder 类应该扩展服务还是什么？我不希望主活动中的 NSD 代码保持代码结构良好。

---

（缩短的）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");
    }

}

Answer 1

在用 Java 编码几个月后，我又在看这个问题。

现在答案很清楚了。在

public void startDiscovery(Context _c)

我打过电话

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

它没有初始化类的成员变量，而是在函数startDiscovery() 中声明了一个新的本地变量。所以mNsdManager只存在于startDiscovery()。 将行更改为

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

可能已经解决了问题。

也许这篇文章有一天会帮助另一个“新手”。

很高兴看到，有时“神秘的问题”会随着时间和个人的进步而得到解决:)