Activity 在运行时检查权限时出现上下文空指针错误

Question

我试图确保在闹钟应用程序的第一个运行上，具有读取较新设备的外部存储的权限。

我不断收到上下文的空指针错误。它在这一行：

if (ContextCompat.checkSelfPermission(context, Manifest.permission.READ_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED)

我收到这个错误：

 Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'int android.content.Context.checkPermission(java.lang.String, int, int)' on a null object reference

我尝试了 (Activity) context、getParent()、this.context、getApplicationContext，正如您在下面看到的只是上下文。他们都得到了相同的结果。

在此先感谢您的帮助。

这是完整的代码：

public class MainActivity extends AppCompatActivity implements FragmentAddAlarms.onTabChangedListener {

    public static final int MY_PERMISSIONS_REQUEST_READ_EXTERNAL_STORAGE = 123;
    Context context;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    }
    boolean result = checkPermission();
    if (result) {
        //writeCalendarEvent();
    }

}

@TargetApi(Build.VERSION_CODES.JELLY_BEAN)
public boolean checkPermission()
{
    int currentAPIVersion = Build.VERSION.SDK_INT;
    if(currentAPIVersion>=android.os.Build.VERSION_CODES.M)
    {
        if (ContextCompat.checkSelfPermission(context, Manifest.permission.READ_EXTERNAL_STORAGE) != PackageManager.PERMISSION_GRANTED) {
            if (ActivityCompat.shouldShowRequestPermissionRationale((Activity) context, Manifest.permission.READ_EXTERNAL_STORAGE)) {
                AlertDialog.Builder alertBuilder = new AlertDialog.Builder(context);
                alertBuilder.setCancelable(true);
                alertBuilder.setTitle("Permission necessary");
                alertBuilder.setMessage("External storage is necessary to for alarms!!!");
                alertBuilder.setPositiveButton(android.R.string.yes, new DialogInterface.OnClickListener() {
                    @TargetApi(Build.VERSION_CODES.JELLY_BEAN)
                    public void onClick(DialogInterface dialog, int which) {
                        ActivityCompat.requestPermissions((Activity)context, new String[]{Manifest.permission.READ_EXTERNAL_STORAGE}, MY_PERMISSIONS_REQUEST_READ_EXTERNAL_STORAGE);
                    }
                });
                AlertDialog alert = alertBuilder.create();
                alert.show();

            } else {
                ActivityCompat.requestPermissions((Activity)context, new String[]{Manifest.permission.READ_EXTERNAL_STORAGE}, MY_PERMISSIONS_REQUEST_READ_EXTERNAL_STORAGE);
            }
            return false;
        } else {
            return true;
        }
    } else {
        return true;
    }
}

Answer 1

您的变量 context 是 null 且无用。如果您需要在内部 class（例如您的 AlertDialog' s onClick 方法）。

Activity 在运行时检查权限时出现上下文空指针错误

Activity context null pointer error when checking for permissions at runtime

android

nullpointerexception

android-permissions

android-context