当我调用NfcAdapter.getDefaultAdapter(this)时,我得到null.为什么会这样？

码：

public class ForegroundDispatch extends Activity {
private NfcAdapter mAdapter;
private PendingIntent mPendingIntent;
private IntentFilter[] mFilters;
private String[][] mTechLists;
private TextView mText;
private int mCount = 0;

@Override
public void onCreate(Bundle savedState) {
    super.onCreate(savedState);

    setContentView(R.layout.foreground_dispatch);
    mText = (TextView) findViewById(R.id.text);
    mText.setText("Scan a tag");

    mAdapter = NfcAdapter.getDefaultAdapter(this);

    // Create a generic PendingIntent that will be deliver to this activity. The NFC stack
    // will fill in the intent with the details of the discovered tag before delivering to
    // this activity.
    mPendingIntent = PendingIntent.getActivity(this,new Intent(this,getClass()).addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP),0);

    // Setup an intent filter for all MIME based dispatches
    IntentFilter ndef = new IntentFilter(NfcAdapter.ACTION_NDEF_DISCOVERED);
    try {
        ndef.addDataType("*/*");
    } catch (MalformedMimeTypeException e) {
        throw new RuntimeException("fail",e);
    }
    mFilters = new IntentFilter[] {
            ndef,};

    // Setup a tech list for all NfcF tags
    mTechLists = new String[][] { new String[] { NfcF.class.getName() } };
}

@Override
public void onResume() {
    super.onResume();
    mAdapter.enableForegroundDispatch(this,mPendingIntent,mFilters,mTechLists); //CRASHES HERE BECAUSE mAdapter IS NULL
}

@Override
public void onNewIntent(Intent intent) {
    Log.i("Foreground dispatch","Discovered tag with intent: " + intent);
    mText.setText("Discovered tag " + ++mCount + " with intent: " + intent);
}

@Override
public void onPause() {
    super.onPause();
    mAdapter.disableForegroundDispatch(this);
 }
  }

我的清单：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
  package="com.neka.znacka"
  android:versionCode="1"
  android:versionName="1.0">
<uses-sdk android:minSdkVersion="10" />
<uses-permission android:name="android.permission.NFC"></uses-permission>
<uses-feature android:name="android.hardware.nfc" android:required="true" />

<application android:icon="@drawable/icon" android:label="@string/app_name">
    <activity android:name=".Uvodna"
              android:label="@string/app_name">
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>

    <activity android:name="Simulator">
    </activity>

</application>

有任何想法吗？