nullpointerexception:尝试调用虚拟方法“android.content.pm.packagemanager”

nxagd54h 于 2021-07-06 发布在 Java

关注(0)|答案(2)|浏览(1083)

在应用程序启动时收到此错误
日志：

-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)

我正在尝试允许我的android设备接收通过nfc发送数据的权限检查。
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;
        }
    }

我知道自从安卓6.0以来，这是必须的。应用程序的目的是在android设备之间通过nfc发送和接收字符串。
提前谢谢。

Java android Permissions nfc

来源：https://stackoverflow.com/questions/65249754/nullpointerexception-attempt-to-invoke-virtual-method-android-content-pm-packa

2条答案

按热度按时间

kwvwclae1#

根据堆栈跟踪，您正在尝试在 MainActivity 示例。这是不可能做到的。
为了从活动中获取packagemanager，您的活动必须附加到应用程序类。这是在 attachBaseContext 生命周期方法。
所以问题就在于：

PackageManager packageManager = getPackageManager();

为了解决这个问题，请将代码行修改为：

PackageManager packageManager;

再加上 onCreate 方法：

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

这将修复致命的异常。
请注意：该解决方案只是您问题的基本解决方案。我建议你不要储存 PackageManager 在你需要的任何时候 PackageManager -使用 getPackageManager() 方法。

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rur96b6h2#

nfc未被列为危险许可https://developer.android.com/reference/android/manifest.permission#nfc 因此，它不需要任何运行时权限检查（如6.0及更高版本中“危险”权限所需）
因此 <uses-permission android:name="android.permission.NFC" /> 够了吗

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nullpointerexception:尝试调用虚拟方法“android.content.pm.packagemanager”

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