[英]java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.PackageManager
[英]NullPointerException: Attempt to invoke virtual method 'android.content.pm.PackageManager
在应用启动时收到此错误
日志猫:
-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)
我试图让我的 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;
}
}
据我了解,从 Android 6.0 开始,这是必需的。 该应用程序的目的是通过 NFC 在 Android 设备之间发送和接收字符串。
提前致谢。
根据堆栈跟踪,您正在尝试在MainActivity
实例的初始化处检索 PackageManager 实例。 这是不可能的。
为了从 Activity 中获取 PackageManager,您的 Activity 必须附加到 Application class。 这是在attachBaseContext
生命周期方法中完成的。
所以问题在于这一行:
PackageManager packageManager = getPackageManager();
为了解决这个问题,请将该行代码修改为:
PackageManager packageManager;
并添加到您的onCreate
方法中:
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
packageManager = getPackageManager();
}
这将解决您的致命异常。
请注意:该解决方案只是您问题的基本解决方案。 我建议您不要在每次需要访问PackageManager
时将PackageManager
存储在该字段中 - 改用getPackageManager()
方法。
NFC 未被列为危险权限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 及更高版本中的“危险”权限所必需的)
因此,明确的许可
<uses-permission android:name="android.permission.NFC" />
足够的
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