I'm trying to implement a Credit Card emulating app and setup a HostApduService-class that runs like expected.
Now I'm trying to load card data from a file (can be in internal or external storage, fixed name and folder) to be more flexible with the card data I'm going to emulate.
Although HostApduService is a service with a context I'm not been able to access files from the service - I'm always getting a "java.lang.NullPointerException":
E/AndroidRuntime: FATAL EXCEPTION: main
Process: de.androidcrypto.hcecreditcardemulator, PID: 26574
java.lang.RuntimeException: Unable to instantiate service de.androidcrypto.hcecreditcardemulator.HceService: java.lang.NullPointerException: Attempt to invoke virtual method 'java.io.File android.content.Context.getFilesDir()' on a null object reference
at android.app.ActivityThread.handleCreateService(ActivityThread.java:3529)
at android.app.ActivityThread.-wrap4(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1786)
at android.os.Handler.dispatchMessage(Handler.java:105)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6944)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.io.File android.content.Context.getFilesDir()' on a null object reference
at android.content.ContextWrapper.getFilesDir(ContextWrapper.java:237)
at de.androidcrypto.hcecreditcardemulator.HceService.<init>(HceService.java:13)
at java.lang.Class.newInstance(Native Method)
at android.app.ActivityThread.handleCreateService(ActivityThread.java:3526)
at android.app.ActivityThread.-wrap4(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1786)
at android.os.Handler.dispatchMessage(Handler.java:105)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6944)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)
Edit March 15th: I checked that the getBaseContext() returns nullbut getApplicationContext() forced the NullPointerException. Due to this the getFiles() fails with the NPE as well.
This is the (very shortened) HceService.java class I'm using:
package de.androidcrypto.hcecreditcardemulator;
import android.nfc.cardemulation.HostApduService;
import android.os.Bundle;
import java.io.File;
public class HceService extends HostApduService {
public HceService() {
// check for a specific file in internal storage
System.out.println("start HceService");
File file = getFilesDir();
//File file2 = getExternalFilesDir(null);
System.out.println("file exists: " + file.exists());
}
@Override
public byte[] processCommandApdu(byte[] bytes, Bundle bundle) {
return new byte[0];
}
@Override
public void onDeactivated(int i) {
}
}
It will be started from AndroidManifest.xml:
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools">
<!-- Card emulation was introduced in API 19. -->
<!-- Min/target SDK versions (<uses-sdk>) managed by build.gradle -->
<uses-feature android:name="android.hardware.nfc.hce" android:required="true" />
<uses-permission android:name="android.permission.NFC" />
<application
android:allowBackup="true"
android:dataExtractionRules="@xml/data_extraction_rules"
android:fullBackupContent="@xml/backup_rules"
android:icon="@mipmap/ic_launcher"
android:label="@string/app_name"
android:supportsRtl="true"
android:theme="@style/Theme.HceCreditCardEmulator"
tools:targetApi="31">
<activity
android:name=".MainActivity"
android:exported="true">
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<!-- BEGIN_INCLUDE(CardEmulationManifest) -->
<!-- Service for handling communication with NFC terminal. -->
<service android:name=".HceService"
android:exported="true"
android:permission="android.permission.BIND_NFC_SERVICE">
<!-- Intent filter indicating that we support card emulation. -->
<intent-filter>
<action android:name="android.nfc.cardemulation.action.HOST_APDU_SERVICE"/>
<category android:name="android.intent.category.DEFAULT"/>
</intent-filter>
<!-- Required XML configuration file, listing the AIDs that we are emulating cards
for. This defines what protocols our card emulation service supports. -->
<meta-data android:name="android.nfc.cardemulation.host_apdu_service"
android:resource="@xml/aid_list"/>
</service>
<!-- END_INCLUDE(CardEmulationManifest) -->
</application>
</manifest>
and has this aid.xml-file:
<?xml version="1.0" encoding="utf-8"?>
<aid-group android:description="@string/card_title" android:category="payment">
<aid-filter android:name="F222222222"/>
<aid-filter android:name="325041592E5359532E4444463031"/>
<aid-filter android:name="A0000000031010"/>
</aid-group>
<!-- END_INCLUDE(CardEmulationXML) -->
</host-apdu-service>
As it has the category "payments" the service can be easily switched in the device's settings:
Settings -> connections -> NFC -> Tap and pay -> tab "payments" select "HCE"
At this time the NPE is thrown on this line 13 in HceService.java:
File file = getFilesDir();
So the main question is: how can I access to a file from a Service or better HostApduService class ?. Is there a way to provide a non-null context to the service ?
Thanks in advance.
I'm answering my own question with the solution that is working.
The main error was to try to get the context in the constructor of the service:
public HceService() {
// check for a specific file in internal storage
System.out.println("start HceService");
System.out.println("getBaseContext: " + getBaseContext());
System.out.println("getApplicationContext: " + getApplicationContext());
File file = getFilesDir();
//File file2 = getExternalFilesDir(null);
System.out.println("file exists: " + file.exists());
}
At this time the context is not available yet and I received the Null Pointer Exception.
The solution is to move the code from the constructor to the onCreate-method:
public HceService() {
/*
// move these lines to onCreate method
// check for a specific file in internal storage
System.out.println("start HceService");
System.out.println("getBaseContext: " + getBaseContext());
System.out.println("getApplicationContext: " + getApplicationContext());
File file = getFilesDir();
//File file2 = getExternalFilesDir(null);
System.out.println("file exists: " + file.exists());
*/
}
@Override
public void onCreate() {
super.onCreate();
// check for a specific file in internal storage
System.out.println("start HceService");
System.out.println("getBaseContext: " + getBaseContext());
System.out.println("getApplicationContext: " + getApplicationContext());
File file = getFilesDir();
//File file2 = getExternalFilesDir(null);
System.out.println("file exists: " + file.exists());
}
This is the result:
I/System.out: start HceService
I/System.out: getBaseContext: android.app.ContextImpl@ecd5e6
I/System.out: getApplicationContext: android.app.Application@3e75727
I/System.out: file exists: true