java - 请求相机权限会导致 String.equals 中出现 NullPointerException

Question

我正在尝试请求相机权限以在我的 Android 应用中使用 Google OCR，但在大多数设备上，它会在安装时崩溃。

我读过类似的帖子，说这是由于将 null 上下文传递到 ContextCompat.checkSelfPermission() 引起的，但我看不出上下文如何为 null。

这是我的代码:

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

    StrictMode.VmPolicy.Builder builder = new StrictMode.VmPolicy.Builder();
    StrictMode.setVmPolicy(builder.build());

    surfaceView = findViewById(R.id.surfaceView);

    if (checkForPermissions(Manifest.permission.CAMERA,
            Manifest.permission.READ_EXTERNAL_STORAGE,
            Manifest.permission.INTERNET)) {
        surfaceView.getHolder().addCallback(new SurfaceCallback());
    }
}
...
private boolean checkForPermissions(String ... permissions) {
    List<String> permissionsToRequest = new ArrayList<>(Arrays.asList(permissions));

    for (Iterator<String> it = permissionsToRequest.iterator(); it.hasNext();) {
        if (ContextCompat.checkSelfPermission(this, it.next()) == PackageManager.PERMISSION_GRANTED) {
            it.remove();
        }
    }
    if (permissionsToRequest.isEmpty()) {
        return true;
    }

    ActivityCompat.requestPermissions(MainActivity.this, permissionsToRequest.toArray(permissions), 0);
    return false;
}

@Override
public void onRequestPermissionsResult(int requestCode,
                                       @NonNull String[] permissions,
                                       @NonNull int[] results) {
    if (requestCode != 0) {
        return;
    }

    for (int i = 0; i < results.length; i++) {
        if (results[i] != PackageManager.PERMISSION_GRANTED && permissions[i] != null) {
            AlertDialog alertDialog = new AlertDialog.Builder(MainActivity.this).create();
            alertDialog.setTitle("Permissions needed");
            alertDialog.setMessage("This app requires camera permissions to function properly.");
            alertDialog.setButton(AlertDialog.BUTTON_POSITIVE, "OK",
                    new DialogInterface.OnClickListener() {
                        public void onClick(DialogInterface dialog, int which) {
                            dialog.dismiss();
                            ActivityCompat.requestPermissions(MainActivity.this,
                                    new String[] {
                                        Manifest.permission.CAMERA,
                                        Manifest.permission.READ_EXTERNAL_STORAGE,
                                        Manifest.permission.INTERNET
                                    },
                                    0);
                        }
                    });
            alertDialog.show();
            return;
        }
    }
    surfaceView.getHolder().addCallback(new SurfaceCallback());
}

SurfaceCallback 实现 SurfaceHolder.Callback，这就是相机实际初始化的地方。

这是异常(exception):

4606/? E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.google.android.packageinstaller, PID: 4606
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.google.android.packageinstaller/com.android.packageinstaller.permission.ui.GrantPermissionsActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.equals(java.lang.Object)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2646)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707)
at android.app.ActivityThread.-wrap12(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6077)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'boolean java.lang.String.equals(java.lang.Object)' on a null object reference
at com.android.packageinstaller.permission.ui.GrantPermissionsActivity.computePermissionGrantState(GrantPermissionsActivity.java:314)
at com.android.packageinstaller.permission.ui.GrantPermissionsActivity.updateDefaultResults(GrantPermissionsActivity.java:363)
at com.android.packageinstaller.permission.ui.GrantPermissionsActivity.onCreate(GrantPermissionsActivity.java:122)
at android.app.Activity.performCreate(Activity.java:6662)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2599)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2707) 
at android.app.ActivityThread.-wrap12(ActivityThread.java) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1460) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:154) 
at android.app.ActivityThread.main(ActivityThread.java:6077) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:866) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:756) 

Answer 1

通过查看源代码，我最好的猜测是，您最终将 null 作为您要传递给的 String[] 中的条目之一ActivityCompat.requestPermissions()。

这可能与您在数组和列表之间转换的方式有关。特别是，permissionsToRequest.toArray(permissions) 有点可怕，因为您试图重用作为参数传入的 String[] 。另外，as Andy pointed out , toArray() 将未使用的数组元素设置为 null。

我会像我一样分配一个新的String[] here :

  private boolean hasPermission(String perm) {
    return(ContextCompat.checkSelfPermission(this, perm)==
      PackageManager.PERMISSION_GRANTED);
  }

  private String[] netPermissions(String[] wanted) {
    ArrayList<String> result=new ArrayList<String>();

    for (String perm : wanted) {
      if (!hasPermission(perm)) {
        result.add(perm);
      }
    }

    return(result.toArray(new String[result.size()]));
  }