Develop Reference

new BlobStoreManager read write on Android 11

I previously used external storage to store specific data that I would like to share between my applications (without having any contentprovider "host")
File folder = new File(Environment.getExternalStorageDirectory(), "FOLDER_NAME");
File file = new File(folder, "FILE_NAME.dat");
FileOutputStream outputStream = new FileOutputStream(file);
That is why I am trying to use BlobStoreManager, as suggested in google's recommendation for targeting 30 (https://developer.android.com/training/data-storage/shared/datasets)
The read & write are based on a BlobHandle with 4 parameters, one being MessageDigest based on a "content". BlobHandle must use the same 4 parameters, or read will fail (SecurityException).
I managed to write data, and to read it, but it makes no sense:
It seems that in order to write, I need to use the data I want to write to generate the BlobHandle.
Then, to read, as BlobHandle must use the same 4 parameters, I also need the data I wrote to be able to read.
Totally illogic, as I wanted to read this data, I don't have it!
I must miss something or just do not understand how it work. If someone can help :)
Here are my sample:
If I set the following:
createBlobHandle: content = "mydata"
write: data = "mydata"
Then write will success, and read will success too. But it I can not know the value before reading it in a normal usecase :(
If I set the following (which would be logic, at least to me):
createBlobHandle: content = "somekey"
write: data = "mydata"
Then write will fail :(
#RequiresApi(api = Build.VERSION_CODES.R)
private BlobHandle createBlobHandle() {
//Transfer object
String content = "SomeContentToWrite";
String label = "label123";
String tag = "test";
//Sha256 summary of the transmission object
try {
byte[] contentByte = content.getBytes("utf-8");
MessageDigest md = MessageDigest.getInstance("sha256");
byte[] contentHash = md.digest(contentByte);
return BlobHandle.createWithSha256(contentHash, label,0, tag);
} catch (NoSuchAlgorithmException e) {
e.printStackTrace();
} catch (UnsupportedEncodingException e) {
e.printStackTrace();
}
return null;
}
private void write() {
String data = "SomeContentToWrite";
#SuppressLint("WrongConstant") final BlobStoreManager blobStoreManager = ((BlobStoreManager) applicationContext.getSystemService(Context.BLOB_STORE_SERVICE));
//Generate the session of this operation
try {
BlobHandle blobHandle = createBlobHandle();
if (blobHandle == null)
return;
long sessionId = blobStoreManager.createSession(blobHandle);
try (BlobStoreManager.Session session = blobStoreManager.openSession(sessionId)) {
try (OutputStream pfd = new ParcelFileDescriptor.AutoCloseOutputStream(session.openWrite(0, data.getBytes().length))) {
//The abstract of the written object must be consistent with the above, otherwise it will report SecurityException
Log.d(TAG, "writeFile: >>>>>>>>>>text = " + data);
pfd.write(data.getBytes());
pfd.flush();
//Allow public access
session.allowPublicAccess();
session.commit(applicationContext.getMainExecutor(), new Consumer<Integer>() {
#Override
public void accept(Integer integer) {
//0 success 1 failure
Log.d(TAG, "accept: >>>>>>>>" + integer);
}
});
}
}
} catch (IOException e) {
e.printStackTrace();
}
}
private String read() {
String data = "";
#SuppressLint("WrongConstant") final BlobStoreManager blobStoreManager = ((BlobStoreManager) applicationContext.getSystemService(Context.BLOB_STORE_SERVICE));
BlobHandle blobHandle = createBlobHandle();
if (blobHandle != null) {
try (InputStream pfd = new ParcelFileDescriptor.AutoCloseInputStream(blobStoreManager.openBlob(createBlobHandle()))) {
//Read data
byte[] buffer = new byte[pfd.available()];
pfd.read(buffer);
String text = new String(buffer, Charset.forName("UTF-8"));
Log.d(TAG, "readFile: >>>>>>>>>>>>>>>>>>>>" + text);
} catch (IOException e) {
e.printStackTrace();
} catch (SecurityException e) {
e.printStackTrace();
}
}
return data;
}

According to the official training documentation linked in the question, the missing piece of information, at the time of the question having been asked, is that the four pieces of data contained in the BlobHandler need to be uploaded to a server owned by the client application then subsequently downloaded by which ever other application wants to access the blob via the BlobStorageManager.
So it would seem that on-device blob discovery is not supported. There could also be a solution possible using a Content Provider which could offer up the four required pieces of data, thus circumventing the need for the server infrastructure.

Can't get path to /data/data/my_app/files

I'm working on an app which stores small amounts of data in /data/data/my_app/files using this code:
private void buildFileFromPreset(String fileName) {
fileName = fileName.toLowerCase();
StandardData data = StandardData.getInstance();
String[] list = data.getDataByName(fileName);
FileOutputStream fos = null;
try {
fos = openFileOutput(fileName, Context.MODE_PRIVATE);
PrintWriter writer = new PrintWriter(fos);
for (int i = 0; i < list.length; i++) {
writer.println(list[i]);
}
writer.close();
} catch (FileNotFoundException e) {
} catch (IOException e) {
e.printStackTrace();
} finally {
try {
fos.close();
} catch (IOException e) {
e.printStackTrace();
}
}
}
All works fine when the app gets started, in the onCreate() function of the main activity, I want to check if the files that were created last time are still present:
private String getAppFilesDir() {
ContextWrapper c = new ContextWrapper(this);
return c.getFilesDir().toString();
}
which returns something like:
/data/user/0/my_app/files
I've read some older posts (2012) suggesting this method must work but it doesn't, probably since jellybean.
So my question:
How can I check if the files I created using FileOutputStream and PrintWriter in a previous session still exist?
I hope I provided enough info for you guys to answer (:

I still have not found a solution for this specific problem.
Instead I am now using SQLite so I don't have to worry about these kinds of things.

Android Studio - saving user data

I want to save a file "xp.txt" to keep track of the user's experience in a game. If the file doesn't exist, I'd like to create it and write "0" in the file (to indicate 0 experience). If the file exists I'd like to read from it and save the exp to an int variable in the java class. And I'd also like to know how to change the value from "0" to some other value.
Thanks in advance!
My current code is:
try {
OutputStreamWriter out = new OutputStreamWriter(openFileOutput(XP, 0));
out.write("0");
out.close();
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
try {
InputStream in = openFileInput(XP);
if (in != null) {
InputStreamReader temp = new InputStreamReader(in);
BufferedReader reader = new BufferedReader(temp);
String str;
StringBuilder buf = new StringBuilder();
while ((str = reader.readLine()) != null) {
xp = Integer.parseInt(str);
System.out.println(xp);
}
in.close();
}
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
But I cannot find the text file and it doesn't work as I intended.

You can use the following ways to store data
Shared Preferences - Store private primitive data in key-value
pairs.
Internal Storage - Store private data on the device memory.
External Storage - Store public data on the shared external storage.
SQLite Databases - Store structured data in a private database.
Network Connection - Store data on the web with your own network
server.
I think for your use case SharedPreferences will do good. A quick example would be
To Save a value :
SharedPreferences.Editor editor = getSharedPreferences("unique_name", MODE_PRIVATE).edit();
editor.putInt("xp", 10);
editor.commit();
To Retrieve value
SharedPreferences prefs = getSharedPreferences("unique_name", MODE_PRIVATE);
int xp = prefs.getInt("xp", 0); // will return 0 if no value is saved
You can read more from this here.

Serialization of Application Object

At first I have strong Java knowledege, but however just started with Android.
My Android app is downloading some fairly complex data (texts, dates, images) which I am saving in a custom object. The data need to be refresh from time to time. However usually the data downloaded will not change.
In order to keep the data in memory I am using the Application Object. Unfortunately, it looks like the application object instance is destroyed when the app is killed.
Hence, I was wondering if it would be of good practice to serialize and save my custom object (which is contained in the application object) in the internal storage during onPause(). Obviously, I would then first read from the file in onResume() before reloading from the internet. The idea is also to enable offline viewing.
In longer term the plan is to move the code downloading the date in a background service. As there seems to be many different ways to keep application state in Android, I would like to be be sure that this is the correct way to go.

Try using those methods class to save the Object(s) (implements serialize) you need:
public synchronized boolean save(String fileName, Object objToSave)
{
try
{
// save to file
File file = new File(CONTEXT.getDir("filesdir", Context.MODE_PRIVATE) + "/file.file");
if (file.exists())
{
file.delete();
}
file.getParentFile().mkdirs();
file.createNewFile();
ObjectOutputStream oos = new ObjectOutputStream(new FileOutputStream(file));
oos.writeObject(objToSave);
oos.close();
return true;
}
catch (FileNotFoundException e)
{
e.printStackTrace();
return false;
}
catch (IOException e)
{
e.printStackTrace();
return false;
}
}
public synchronized Object load(String fileName)
{
try
{
File file = new File(CONTEXT.getDir("filesdir", Context.MODE_PRIVATE) + "/file.file");
if (!file.exists())
{
return null;
}
ObjectInputStream ois = new ObjectInputStream(new FileInputStream(file));
savedObj = ois.readObject();
ois.close();
return savedObj;
}
catch (FileNotFoundException e)
{
e.printStackTrace();
return null;
}
catch (Exception e)
{
e.printStackTrace();
return null;
}
}
You'll need to cast the Object you load().
CONTEXT is an Activity or ApplicationContext to get access to the cachedir.
Your could use Environment.getExternalStorageState() instead to get a directory path. DOn't forget to add it "/filename".

Is there any way to store a 2D array in a persistant storage?

I want to load a 2D array like this one:
[
[false, true, false, false],
[true, false, false, false],
[false, false, false, true],
[false, false, true, false],
[false, false, true, false]
]
Actually, these are the radio button states, false indicating radiobutton is uncheck while true indicating that the radio button is checked.
To explain the whole scenario, I am creating a quiz, which has a question and options for this i have created a radiobuttonStates[][] 2D array. The first [] indicating number of questions versus number of options in second []. There is a save and exit button which saves the quiz (saving the question number from where the user left and the selected radio buttons, previous as well as present). So the 2D array which i have created in the first attempt of the quiz, I want to load this same array when the user again comes back and resumes the quiz. Right now i am thinking to store this array in some persistant storage (Database or any kind). But I am not getting any way on how to store. The user clicks the resume button and I am showing the current question number from the database but not able to show the radio button selection. Please help me.

Well you can always transform your boolean array into a string(or more strings), and store it in SharedPreferences.
For example: ResultString = "false,true,false,false";
When you need the result back from the SharedPreference just split your strings, using a certain separator (for instance ",") and reuse your saved data.
Like this you get faster store/restore data backup. Using SQLite to store this kind of data is not a good idea.
Good luck,
Arkde

Well, you can make some kind of table structure where you put each individual array value in a specific row/column. However, if your array contains a small number of elements (like the one you showcase in your question), you can just serialize the instances and save the bytes in the database. This way you won't have to make all the boiler-plate code that thakes each array value and puts it in some row/column and then builds an array out of various row/column values.

An easy and instant way is to store the array or multiple 2d arrays is to use file to read and write your array object.
here is the code sample below which reds my custom objects from a file and writes them to the file. the only thing in the methods below are when reading it reds all objects and when writing it writes all objects again means you can not append single object with previous file or read single object from file. so if you hae to append another array you have to read all previous ones and then write them again with the increment of one you want to append.
public static boolean writeBlockedMessagesInFile(Context context,
ArrayList<BlockedMessages> blockedMessages) {
boolean status = false;
FileOutputStream fos = null;
ObjectOutputStream oos = null;
try {
fos = context.openFileOutput("BlockedMessagesFile.dat",
Context.MODE_WORLD_WRITEABLE);
oos = new ObjectOutputStream(fos);
if (blockedMessages != null && blockedMessages.size() != 0) {
for (int i = 0; i < blockedMessages.size(); i++)
oos.writeObject(blockedMessages.get(i));
}
oos.flush();
oos.close();
fos.close();
status = true;
} catch (IOException e) {
e.printStackTrace();
}
return status;
}
public static ArrayList<BlockedMessages> readBlockedMessagesFromFile(Context context) {
ArrayList<BlockedMessages> blockedMessages = new ArrayList<BlockedMessages>();
FileInputStream fis = null;
ObjectInputStream ois = null;
Object object = null;
try {
fis = context.openFileInput("BlockedMessagesFile.dat");
ois = new ObjectInputStream(fis);
BlockedMessages blo;
Object temp;
try {
while ((blo = (BlockedMessages) ois.readObject()) != null) {
blockedMessages.add(blo);
}
} catch (NullPointerException npe) {
npe.printStackTrace();
} catch (EOFException eof) {
eof.printStackTrace();
} catch (FileNotFoundException fnfe) {
fnfe.printStackTrace();
}
} catch (Exception e) {
e.printStackTrace();
} finally {
try {
if (ois != null) {
ois.close();
}
} catch (IOException e) {
e.printStackTrace();
}
try {
if (fis != null) {
fis.close();
}
} catch (IOException e) {
e.printStackTrace();
}
}
return blockedMessages;
}