NullPointerException：嘗試調用虛擬方法'android.content.pm.PackageManager

Question

在應用啟動時收到此錯誤

日志貓：

-10594/com.example.messageswap E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.messageswap, PID: 10594
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.messageswap/com.example.messageswap.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.PackageManager android.content.Context.getPackageManager()' on a null object reference
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3194)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016)
        at android.os.Handler.dispatchMessage(Handler.java:107)
        at android.os.Looper.loop(Looper.java:214)
        at android.app.ActivityThread.main(ActivityThread.java:7356)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.PackageManager android.content.Context.getPackageManager()' on a null object reference
        at android.content.ContextWrapper.getPackageManager(ContextWrapper.java:98)
        at com.example.messageswap.MainActivity.<init>(MainActivity.java:63)
        at java.lang.Class.newInstance(Native Method)
        at android.app.AppComponentFactory.instantiateActivity(AppComponentFactory.java:95)
        at androidx.core.app.CoreComponentFactory.instantiateActivity(CoreComponentFactory.java:45)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1243)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3182)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3409) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2016) 
        at android.os.Handler.dispatchMessage(Handler.java:107) 
        at android.os.Looper.loop(Looper.java:214) 
        at android.app.ActivityThread.main(ActivityThread.java:7356) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930) 

我試圖讓我的 android 設備接收通過 NFC 發送數據的權限檢查。

MainActivity.java：

 PackageManager packageManager = getPackageManager();

    public void permissionCheck(){
        int permissionCheck = ContextCompat.checkSelfPermission(this, Manifest.permission.READ_PHONE_STATE);

        if (permissionCheck != PackageManager.PERMISSION_GRANTED) {
            ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.READ_PHONE_STATE}, REQUEST_READ_PHONE_STATE);
        } else {
            //TODO
        }
    }

    @Override
    public void onRequestPermissionsResult(int requestCode, String permissions[], int[] grantResults) {
        switch (requestCode) {
            case REQUEST_READ_PHONE_STATE:
                if ((grantResults.length > 0) && (grantResults[0] == PackageManager.PERMISSION_GRANTED)) {
                    //TODO
                }
                break;

            default:
                break;
        }
    }

據我了解，從 Android 6.0 開始，這是必需的。 該應用程序的目的是通過 NFC 在 Android 設備之間發送和接收字符串。

提前致謝。

Answer 1

根據堆棧跟蹤，您正在嘗試在MainActivity實例的初始化處檢索 PackageManager 實例。 這是不可能的。

為了從 Activity 中獲取 PackageManager，您的 Activity 必須附加到 Application class。 這是在attachBaseContext生命周期方法中完成的。

所以問題在於這一行：

PackageManager packageManager = getPackageManager();

為了解決這個問題，請將該行代碼修改為：

PackageManager packageManager;

並添加到您的onCreate方法中：

@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    packageManager = getPackageManager();
}

這將解決您的致命異常。

請注意：該解決方案只是您問題的基本解決方案。 我建議您不要在每次需要訪問PackageManager時將PackageManager存儲在該字段中 - 改用getPackageManager()方法。

Answer 2

NFC 未被列為危險權限https://developer.android.com/reference/android/Manifest.permission#NFC因此它不需要任何運行時權限檢查（如 6.0 及更高版本中的“危險”權限所必需的）

因此，明確的許可

<uses-permission android:name="android.permission.NFC" />

足夠的