Read NFC Tag ID on Android to Use as Unique User Identifier

This guide shows how to read the UID of an NFC card in an Android app and use it as a unique identifier for user login.

1. Add NFC permissions

<!-- NFC permission -->
<uses-permission android:name="android.permission.NFC" />
<!-- Require NFC hardware -->
<uses-feature android:name="android.hardware.nfc" android:required="true" />

2. Configure the activity launch mode

Set the main activity to singleTop so that a new intent is delivered to onNewIntent() instead of creating a new instance.

<activity android:name=".MainActivity" android:launchMode="singleTop">
    <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
</activity>

3. Design the layout

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#151414">
    <LinearLayout
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:orientation="horizontal"
        android:layout_centerInParent="true">
        <TextView
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:text="读取到的卡UID: "
            android:textColor="#fff" />
        <TextView
            android:id="@+id/tv_uid"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:textColor="#fff" />
    </LinearLayout>
</RelativeLayout>

4. Implement NFC handling in MainActivity

In onCreate() the app obtains an NfcAdapter, checks whether NFC is supported and enabled, creates a PendingIntent, and calls onNewIntent() because the launch mode is singleTop.

protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    tvUid = findViewById(R.id.tv_uid);
    nfcAdapter = NfcAdapter.getDefaultAdapter(this);
    pendingIntent = PendingIntent.getActivity(this, 0,
        new Intent(this, getClass()).addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP), 0);
    if (nfcAdapter == null) {
        Toast.makeText(this, "设备不支持NFC", Toast.LENGTH_LONG).show();
        return;
    }
    if (!nfcAdapter.isEnabled()) {
        Toast.makeText(this, "请在系统设置中先启用NFC功能", Toast.LENGTH_LONG).show();
        return;
    }
    onNewIntent(getIntent());
}

The overridden onNewIntent() forwards the received Intent to resolveIntent(), which extracts the Tag object.

@Override
protected void onNewIntent(Intent intent) {
    super.onNewIntent(intent);
    resolveIntent(intent);
}

void resolveIntent(Intent intent) {
    Tag tag = intent.getParcelableExtra(NfcAdapter.EXTRA_TAG);
    if (tag != null) {
        processTag(intent);
    }
}

processTag()

obtains the UID byte array, converts it to a hexadecimal string, and displays it.

public void processTag(Intent intent) {
    Tag tagFromIntent = intent.getParcelableExtra(NfcAdapter.EXTRA_TAG);
    byte[] uidBytes = tagFromIntent.getId();
    String uidHex = ByteArrayToHexString(uidBytes);
    tvUid.setText(uidHex);
}

private String ByteArrayToHexString(byte[] inarray) {
    String[] hex = {"0","1","2","3","4","5","6","7","8","9","A","B","C","D","E","F"};
    StringBuilder out = new StringBuilder();
    for (byte b : inarray) {
        int i = (b & 0xFF) >> 4;
        out.append(hex[i]);
        i = b & 0x0F;
        out.append(hex[i]);
    }
    return out.toString();
}

5. Manage foreground dispatch

Enable dispatch in onResume() and disable it in onPause() so the activity receives NFC intents while it is visible.

@Override
protected void onResume() {
    super.onResume();
    if (nfcAdapter != null) {
        nfcAdapter.enableForegroundDispatch(this, pendingIntent, null, null);
    }
}

@Override
protected void onPause() {
    super.onPause();
    if (nfcAdapter != null) {
        nfcAdapter.disableForegroundDispatch(this);
    }
}

6. Run and test

Build and run the app, enable NFC on the device, and bring an NFC card close to the phone. The UID appears in the TextView. Debugging can be done by setting breakpoints in the methods above.