服务类（HostApduService）无权访问文件（）NPE NullPointerException）

问题描述 投票：0回答：1

我正在尝试实现一个信用卡模拟应用程序并设置一个按预期运行的 HostApduService 类。

现在我正在尝试从文件（可以在内部或外部存储、固定名称和文件夹中）加载卡数据，以便更灵活地处理我要模拟的卡数据。

尽管 HostApduService 是一项具有上下文的服务我无法从该服务访问文件 - 我总是收到“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) 

编辑 3 月 15 日： 我检查了 

getBaseContext()
返回 
null
但是 
getApplicationContext()
强制了 
NullPointerException
。因此，
getFiles()
也因 NPE 而失败。

这是我正在使用的（非常短的）HceService.java 类：

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) {

    }
}

会从AndroidManifest.xml开始：

<?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>

并有这个 aid.xml 文件：

<?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>

由于它具有“付款”类别，因此可以在设备的设置中轻松切换服务：

Settings -> connections -> NFC -> Tap and pay -> tab "payments" select "HCE"

此时在HceService.java中的这一行13处抛出了NPE：

File file = getFilesDir();

所以主要问题是：如何从服务或更好的 HostApduService 类访问文件？。有没有办法为服务提供非空上下文？

提前致谢。

java android service nfc hce
1个回答
0
投票

我正在用有效的解决方案回答我自己的问题。

主要错误是试图在服务的构造函数中获取上下文：

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());
}

此时上下文还不可用，收到空指针异常

解决方案是将代码从构造函数移动到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());
}

这是结果：

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
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