检查nfc标签是否在附近

Question

我想让我的手机检测附近(表面附近(是否有nfc标签以下代码没有错误，但我一运行应用程序，它就崩溃了。如果你们中的某个人可以查看我的代码，检查是否有我看不到的东西，那将是非常有帮助的。下面是运行时间错误。

public class AccessControlActivity extends AppCompatActivity {
NfcAdapter nfcAdapter;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_access_control);
nfcAdapter = NfcAdapter.getDefaultAdapter(this);
// Checks if there is NFC function
if(nfcAdapter != null && nfcAdapter.isEnabled()) {
//Toast.makeText(this, "NFC works", Toast.LENGTH_SHORT).show();
}
else {
Toast.makeText(this, "NFC is not available!", Toast.LENGTH_SHORT).show();
//finish();
}

}

@Override
protected  void onNewIntent(Intent intent) {
Toast.makeText(this, "NFC intent received", Toast.LENGTH_LONG).show();
super.onNewIntent(intent);
}
@Override
protected void onResume() {
Intent intent = new Intent(this, AccessControlActivity.class);
intent.addFlags(Intent.FLAG_RECEIVER_REPLACE_PENDING);
PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, intent, 0);
IntentFilter[] intentFilters = new IntentFilter[]{};
nfcAdapter.enableForegroundDispatch(this, pendingIntent, intentFilters, null);

super.onResume();
}
@Override
protected void onPause() {
nfcAdapter.disableForegroundDispatch(this);

super.onPause();
}
}

运行时错误如下：

E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.nfc.netvision, PID: 5484
java.lang.RuntimeException: Unable to resume activity {com.nfc.netvision/com.nfc.netvision.AccessControlActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.nfc.NfcAdapter.enableForegroundDispatch(android.app.Activity, android.app.PendingIntent, android.content.IntentFilter[], java.lang.String[][])' on a null object reference
at android.app.ActivityThread.performResumeActivity(ActivityThread.java:4341)
at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:4373)
at android.app.servertransaction.ResumeActivityItem.execute(ResumeActivityItem.java:52)
at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:176)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:97)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2043)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:216)
at android.app.ActivityThread.main(ActivityThread.java:7464)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:549)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:955)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.nfc.NfcAdapter.enableForegroundDispatch(android.app.Activity, android.app.PendingIntent, android.content.IntentFilter[], java.lang.String[][])' on a null object reference
at com.nfc.netvision.AccessControlActivity.onResume(AccessControlActivity.java:116)
at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1456)
at android.app.Activity.performResume(Activity.java:8125)
at android.app.ActivityThread.performResumeActivity(ActivityThread.java:4331)
at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:4373) 
at android.app.servertransaction.ResumeActivityItem.execute(ResumeActivityItem.java:52) 
at android.app.servertransaction.TransactionExecutor.executeLifecycleState(TransactionExecutor.java:176) 
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:97) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2043) 
at android.os.Handler.dispatchMessage(Handler.java:106) 
at android.os.Looper.loop(Looper.java:216) 
at android.app.ActivityThread.main(ActivityThread.java:7464) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:549) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:955) 
I/Process: Sending signal. PID: 5484 SIG: 9

Answer 1

因此，如果您检查onCreate中的nfcAdapter != null并只显示toast，则您的应用程序将盲目地尝试在onResume中使用可能的null适配器。

这将解释onResume中的Attempt to invoke virtual methodon a null object reference，因为变量nfcAdapter可能是onResume中的null

您应该在onResume中再次检查null

此外，你的意向过滤器看起来也不太好，它们的代码要么导致没有意向发送给你，要么相反，导致所有非NFC意向都发送给你。

当出现任何类型的标签时，调用更正常的代码将是。

@Override
protected void onResume() {
super.onResume();
IntentFilter tagDetected = new IntentFilter(NfcAdapter.ACTION_TAG_DISCOVERED);
IntentFilter ndefDetected = new IntentFilter(NfcAdapter.ACTION_NDEF_DISCOVERED);
try {
ndefDetected.addDataType("*/*");
} catch (IntentFilter.MalformedMimeTypeException e) {}
IntentFilter techDetected = new IntentFilter(NfcAdapter.ACTION_TECH_DISCOVERED);
IntentFilter[] nfcIntentFilter = new IntentFilter[]{ndefDetected,techDetected,tagDetected};
PendingIntent pendingIntent = PendingIntent.getActivity(
this, 0, new Intent(this, getClass()).addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP), 0);
if(nfcAdapter!= null)
nfcAdapter.enableForegroundDispatch(this, pendingIntent, nfcIntentFilter, null);
}
@Override
protected void onPause() {
super.onPause();
if(nfcAdapter!= null)
nfcAdapter.disableForegroundDispatch(this);
}