java - 检查 nfc 标签是否在附近

Question

我希望我的手机能够检测附近(靠近其表面)是否有 nfc 标签 以下代码没有错误，但一旦我运行该应用程序，它就会崩溃。如果你们中的某个人可以查看我的代码并检查是否有我没有看到的内容，那将非常有帮助。下面是运行时错误。

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();
    }
}

运行时错误如下所示:

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，您检查 onCreate 中的 nfcAdapter != null 并仅显示一个 toast ，然后您的应用程序将盲目地尝试并使用可能的 null onResume 中的 适配器。

这将解释 onResume 中的尝试调用虚拟方法 在空对象引用上作为变量nfcAdapter 在 onResume

中可能为 null

您应该在 onResume 中再次检查 null

此外，您的 Intent 过滤器看起来也不正确，它们的代码要么导致没有 Intent 发送给您，要么相反导致所有 Intent 发送给您，包括非 NFC Intent。

当出现任何类型的标签时，会调用更正常的代码。

    @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);
    }