NullPointerException：尝试获取具有正确权限的空数组的长度

Question

I make this app for school and need to hand it in tomorrow. Little problem, it keeps crashing at a certain point.

My app downloads a .zip from Firebase and unzips it:

-- Cache directory

 --images
     --image01.jpg
     --image02.jpg
     --image03.jpg
     --and so on...
 --info.txt
 --scan234702640.zip // this is the file that includes the /images and the info.txt

So I have the following code in my class imageHandler:

// Get the file
    gsReference.getFile(localFile).addOnSuccessListener(new OnSuccessListener<FileDownloadTask.TaskSnapshot>() {
        @Override
        public void onSuccess(FileDownloadTask.TaskSnapshot taskSnapshot) {

            String path = "";
            File f = new File(context.getExternalCacheDir().toURI());
            File[] files1 = f.listFiles();
            for (File inFile1 : files1) {
                if (inFile1.getName().contains("scan")) {
                    path = context.getExternalCacheDir() + "/" +  inFile1.getName();
                }
            }

            //Decompress zip and build bitmap array
            try {
                ZipFile zip = new ZipFile(path);
                zip.extractAll(context.getExternalCacheDir() + "/");
            } catch (ZipException e){
                Toast.makeText(context, "ZIP error " + e.getCode() + " : " + e.getMessage(), Toast.LENGTH_SHORT).show();
            }

            //Construct bitmap array
            ArrayList<Bitmap> pictures = new ArrayList<Bitmap>();
            File f3 = new File(context.getExternalCacheDir().toString() + "/images/");
            if (f3.exists()) {
                File[] files3 = f3.listFiles(); //THE PROBLEM IS SOMEWHERE HERE

                if (files3 != null) {
                    for (File inFile3 : files3) {
                        if (inFile3.exists()) {
                            BitmapFactory.Options options = new BitmapFactory.Options();
                            options.inPreferredConfig = Bitmap.Config.ARGB_8888;
                            Bitmap bitmap = BitmapFactory.decodeFile(inFile3.getPath(), options);
                            pictures.add(bitmap);
                        }
                    }
                }
            }

            // DEBUG: Show what we have in our cache location

            String message = "";
            File f2 = new File(context.getExternalCacheDir().toURI() + "/images");
            File[] files2 = f2.listFiles();
            for (File inFile2 : files2) {
                if (inFile2.isDirectory()) {
                    message += "/" + inFile2.getName() + "\n";
                } else {
                    message += inFile2.getName() + "\n";
                }
            }

            AlertDialog dialog = new AlertDialog.Builder(context)
                    .setTitle("Test Message")
                    .setMessage(message)
                    .show();

            // END OF DEBUG


        }
    });

    return scan;

So, it crashes at the part where it says "THE PROBLEM IS SOMEWHERE HERE". I have asked all EXTERNAL_STORAGE permissions and I've checked if the files exist. I found out, in debug mode, that it successfully adds the bitmaps to the array, but when I have for example 10 images, it crashes after it loaded all 10 in. So pls help me ;)

Thx a lot, Jules

EDIT Here is the console log:

10-31 00:07:31.060 7536-7536/com.julescitronic.meeting E/AndroidRuntime: FATAL EXCEPTION: main
                                                                     Process: com.julescitronic.meeting, PID: 7536
                                                                     java.lang.NullPointerException: Attempt to get length of null array
                                                                         at com.julescitronic.meeting.imageHandler$2.onSuccess(imageHandler.java:152)
                                                                         at com.julescitronic.meeting.imageHandler$2.onSuccess(imageHandler.java:127)
                                                                         at com.google.firebase.storage.StorageTask$1.zza(Unknown Source:9)
                                                                         at com.google.firebase.storage.StorageTask$1.zzl(Unknown Source:4)
                                                                         at com.google.firebase.storage.zze$2.run(Unknown Source:10)
                                                                         at android.os.Handler.handleCallback(Handler.java:789)
                                                                         at android.os.Handler.dispatchMessage(Handler.java:98)
                                                                         at android.os.Looper.loop(Looper.java:164)
                                                                         at android.app.ActivityThread.main(ActivityThread.java:6541)
                                                                         at java.lang.reflect.Method.invoke(Native Method)
                                                                         at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
                                                                         at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)

Answer 1

I have the same problem. Use this insted of Bitmap to load pictures because loading pictures by Bitmap takes a lot of memory and crash your app

Answer 2

I found out, in debug mode, that it successfully adds the bitmaps to the array, but when I have for example 10 images, it crashes after it loaded all 10 in.

Based on your last sentence, I believe that you have memory issues rather than permission issues.

 BitmapFactory.Options options = new BitmapFactory.Options();
 options.inPreferredConfig = Bitmap.Config.ARGB_8888;
 Bitmap bitmap = BitmapFactory.decodeFile(inFile3.getPath(), options);
 pictures.add(bitmap);

With this code you load each bitmap in its full size and add it to a list of bitmaps, depriving GC to free the native object associated with the bitmap. This can be a big problem if you load an image with let's say 10 MB * 10 images. That will easily lead to memory issues.

I would suggest to load the bitmaps more efficiently. This would mean, that you load a downsampled bitmap into memory and use that one, since having a Bitmap with size for example 3000x4000 makes no sense if it will be displayed on an ImageView with size 512x384. This link is a good guide and can be of help:

https://developer.android.com/topic/performance/graphics/load-bitmap.html

Answer 3

listFiles();

is a file permission, I was getting similar crash in my code on Android API 23 and above (as file storage permission is required during runtime), so I provided same as follow :

if (Build.VERSION.SDK_INT >= 23) {
            if (AppMain.mContext.checkSelfPermission(android.Manifest.permission.WRITE_EXTERNAL_STORAGE) == PackageManager.PERMISSION_GRANTED) {
                Log.v(TAG, "Permission is granted");
                return true;
            } else {
                ActivityCompat.requestPermissions(activityContext, new String[]{Manifest.permission.WRITE_EXTERNAL_STORAGE}, 1);
                return false;
            }

After this I didn't get null pointer on listFile operation.