Hi Iam having serious issues try to persist some serializable objects to a file on the local android file system. Iam getting a Bad file descriptor error and I think it is to do with my methods for creating the file. the file and checking if the file exists. i create a private file object in the class. Then, on write or read. I check file existance with the following code.
#Override
public boolean fileExists() {
File file = context.getFileStreamPath(filename);
return file.exists();
}
this doesnt instantiate my file object called "objectfile"!! but does check the "filename" exists.
to create the file I call this method if "filename" doesnt exist.
public void createFile()
{
objectfile = new File(context.getFilesDir(), filename);
objectfile.setReadable(true);
objectfile.setWritable(true);
}
Iam not sure if this will give me back my previously created file which would be ideally what I want to do. Is there a way i can just get the old file or create a new one and pass it to "objectfile" variable in the constructor??
Iam also wondering what the best way to do this is??
Or should i just use the mysqlite db? using object file persistance doesn't seem to be working out for me right now and iam working to a deadline. Also this method is mention in the gooogle docs so I thought it would be legit was to do it.
http://developer.android.com/training/basics/data-storage/files.html
here is my method for reading the serializable objects
public synchronized ArrayList<RoomItem> readObjects() {
final ArrayList<RoomItem> readlist = new ArrayList<>();
if(!fileExists())
return readlist;
if(objectfile == null)
createFile();
try {
finputstream = new FileInputStream(objectfile);
instream = new ObjectInputStream(finputstream);
readwritethread = new Thread(new Runnable() {
#Override
public void run() {
try {
final ArrayList<RoomItem> readitems = (ArrayList<RoomItem>) instream.readObject();
instream.close();
finputstream.close();
handler.post(new Runnable() {
#Override
public void run() {
listener.updateList(readitems);
}
});
} catch(IOException e)
{
e.printStackTrace();
}
catch(ClassNotFoundException e)
{
e.printStackTrace();
Log.d("read failed", "file read failed");
}
}
});
}
catch(Exception e)
{
e.printStackTrace();
}
timeOutReadWrite(readwritethread);
readwritethread.start();
try {
readwritethread.join();
} catch (InterruptedException e) {
e.printStackTrace();
}
Log.d("read from file", "file read");
return readlist;
if anyone could suggest any improvements id really appreciate it. I use a handler to pass back to my activity and implement a listener interface on my activity thats call the activity when all the obj are read. Thanks again!
1#: Yes, it will return the original file you created.
2#: Depends on the thing you want to store, seems File is more flex from description
hope helpful.
We have used
FileOutputStream fos = context.openFileOutput("file.ser", Context.MODE_PRIVATE);
to write our serialized files.This will carete files in /data/data/app.package.name/files/. In fact, this path is returned by getFilesDir().
And while deserializing, use
//make sure you pass the same file that was passed to openFileOutput()..
FileInputStream fis = context.openFileInput("file.ser");
Also, to avoid confusing between file names you can use name of class that is being serialized.
Ex:
public static <T> void serialize(final Context context, final T objectToSerialize) {
....
....
Strin fileName = objectToSerialize.getClass().getSimpleName();
...
}
Do this and keep the method in util so it can be used for any type of objects (T type) to serialize.
Related
Here I try to use getClassLoader().getResources() to get my .model file, however it returns null. I'm not sure where goes wrong!
And when I try to print out the urls, it gives me java.lang.TwoEnumerationsInOne#5fd1900, what does this means?
public Activity(MainActivity activity) {
MainActivity activity = new MainActivity();
try {
// Open stream to read trained model from file
InputStream is = null;
// this .model file is save under my /project/app/src/main/res/
Enumeration<URL> urls = Activity.class.getClassLoader().getResources("file.model");
// System.out.println("url:"+urls);
if (urls.hasMoreElements()) {
URL element = urls.nextElement();
is = element.openStream();
}
// deserialize the model
classifier = (J48) SerializationHelper.read(is);
is.close();
} catch (Exception e) {
e.printStackTrace();
}
}
In Android, resources put under src/main/res are not visible in the class path and can only be accessed via the android resources API. Try to put the file into src/main/resources.
I just started to learn developping android and I have a (probably) basic questions, but I didn't find anything clear.
I'm trying to store data in a JSON file, well, I've understood the logic to store it, my way is:
public boolean writeFileJson(JSONObject jobj) {
try {
FileOutputStream fOut = openFileOutput(file, Context.MODE_PRIVATE);
fOut.write(jobj.toString().getBytes());
fOut.close();
Toast.makeText(getBaseContext(), "file saved", Toast.LENGTH_SHORT).show();
} catch (Exception e1) {
e1.printStackTrace();
}
return true;
}
But my problem is to read, and concretely for the first time, because the way I do it is:
public String readFileJson() {
int c;
String temp = "";
try {
FileInputStream fin = openFileInput(file);
while ((c = fin.read()) != -1) {
temp = temp + Character.toString((char) c);
}
Toast.makeText(getBaseContext(), "file read", Toast.LENGTH_SHORT).show();
} catch (Exception e2) {
}
return temp;
}
So wen I read it for the first time and I want to acces to a parameter of my JSON is obvious that any JSON Object already exist in the file.
So I try to save a first JSON Object with my parameters in onCreate() method and save it in the file, but wen I run the app, and I stop it, it returns again to execute onCreate() and deletes all data stored during the run time.
So my question is: There is any way to init only for one time the parameters of the JSON file to could access for the first time unlike it's empty???
I hope that I'd explained well!!
Thanxxxx!!!!
You can create your own flag boolean and check when you start.
Well I don't understand well why you can use a flag if the flag is set to init value in onCreate(), but I've tried a basic method: check each time if the json file is null. But it's like so basic no? Is there any ther way, or trying to understand how to use flags without reset their values?
msgjson = readFileJson();
if(msgjson == "") {
json.put("ARRAY", jsonArray);
}else{
json = new JSONObject(msgjson);
}
Thanx!!
I have a custom object class Record that implements Parcelable and I'm creating a ListView via ArrayAdapter<Record> I want to be able to save that list so that it automatically loads the next time the user opens the app. The list is populated dynamically and I'm calling my save method everytime a record is added. Then I have a SharedPreference with a boolean value that I set to true so that I know the user has saved some data to load the next time the app is open. Here are my save and load methods:
public void writeRecordsToFile(ArrayAdapter<Record> records) {
String fileName = Environment.getExternalStorageDirectory().getPath() + "/records.dat";
try {
File file = new File(fileName);
if(!file.exists()){
file.createNewFile();
}
ObjectOutputStream stream = new ObjectOutputStream(new FileOutputStream(file));
stream.writeObject(records);
stream.flush();
stream.close();
}
catch (IOException e){
Log.e("MyApp","IO Exception: " + e);
}
writeSavedState();
}
the writeSavedState() is for my SP
public void readRecordsList() {
String fileName = Environment.getExternalStorageDirectory().getPath() + "/records.dat";
try {
ObjectInputStream inputStream = new ObjectInputStream(getApplicationContext().openFileInput(fileName));
adapter = (ArrayAdapter<Record>)inputStream.readObject();
inputStream.close();
}
catch (Exception e){
Log.e("MyApp" , "File Not Found: " + e);
}
}
When I first open the app I get a message:
E/MyApp﹕ File Not Found: java.lang.IllegalArgumentException: File /storage/emulated/0/records.dat contains a path separator
and then when I add a Record to my list I get the message:
E/MyApp﹕ IO Exception: java.io.IOException: open failed: EACCES (Permission denied)
The second message I'm assuming I'm getting because of the first message. This is my first time working with I/O in Android so any help would be appreciated!
EDIT
After adding the permissions to the manifest I'm now only getting an error:
E/MyApp﹕ IO Exception: java.io.NotSerializableException: android.widget.ArrayAdapter
As I said, my custom object is Parcelable and the rest of this is being done in my MainActivity. Do I need to make a new class that is Serializable to build my ArrayAdapter?
I would suggest to save the records in internal storage in private mode,which can be accessed by your app only.If you store it in External storage, there is no guarantee that it will be available next time you load your app.
Also, you should save array of record objects rather than ArrayAdapter object.
Parcel and Parcelable are fantastically quick, but its documentation says you must not use it for general-purpose serialization to storage, since the implementation varies with different versions of Android (i.e. an OS update could break an app which relied on it). So use Serializable in this case instead of Parcalable (from this SO thread)
You can use global variables to pass data from one activity to another. Also you can read/ write records when app starts using global class which extends Applicaion class.
You can try following code,
public class GlobalClass extends Application {
public static Object objectToBePassed; // global variable
final static private RECORDS_FILENAME = "myRecords.txt"
#Override
public void onCreate(){
super.onCreate();
readRecordsFromFile(); // read records when app starts
}
public boolean writeRecordsToFile(ArrayList<Record> records){
FileOutputStream fos;
ObjectOutputStream oos=null;
try{
fos = getApplicationContext().openFileOutput(RECORDS_FILENAME, Context.MODE_PRIVATE);
oos = new ObjectOutputStream(fos);
oos.writeObject(records);
oos.close();
return true;
}catch(Exception e){
Log.e(getClassName(), "Cant save records"+e.getMessage());
return false;
}
finally{
if(oos!=null)
try{
oos.close();
}catch(Exception e){
Log.e(getClassName(), "Error while closing stream "+e.getMessage());
}
}
}
private boolean readRecordsFromFile(){
FileInputStream fin;
ObjectInputStream ois=null;
try{
fin = getApplicationContext().openFileInput(RECORDS_FILENAME);
ois = new ObjectInputStream(fin);
ArrayList<Record> records = (ArrayList<Record>) ois.readObject();
ois.close();
Log.v(getClassName(), "Records read successfully");
return true;
}catch(Exception e){
Log.e(getClassName(), "Cant read saved records"+e.getMessage());
return false;
}
finally{
if(ois!=null)
try{
ois.close();
}catch(Exception e){
Log.e(getClassName(), "Error in closing stream while reading records"+e.getMessage());
}
}
}
}
So to pass any object from activity A to activity B, use following code in activity A ,
Intent intent = new Intent(this,B.class);
GlobalClass.objectToBePassed = obj;
startActivity(intent);
in activity B,
MyClass object = (MyClass) GlobalClass.objectToBePassed;
so to pass a Record class object, replace MyClass with Record.
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".
I have code that is inside my Activity class to load and save a file. It works fine. The code saves the contents of cFavretClass. I'm trying to clean up the code, so I moved the file i/o into the cFavret Class.
I cannot get the code to compile. Now I get an error saying openFileOutput is undefined in type cFavrets.
I'm assuming that this method was declared in Googles Activity Class?
Does this mean all file I/O must be in the activity class?
boolean Save()
{
String FILENAME = "hello_file";
try {
FileOutputStream fos = openFileOutput(FILENAME, Context.MODE_PRIVATE );
fos.write(buffer);
fos.close();
}
// just catch all exceptions and return false
catch (Throwable t) {
return false;
}
return true;
}
boolean Load()
{
String FILENAME = "hello_file";
try {
FileInputStream fos = openFileInput(FILENAME);
buffer[0]=0;
fos.read(buffer);
fos.close();
}
// just catch all exceptions and return false
catch (Throwable t) {
// maybe file does not exist, try creating it
return false;
}
return true;
}
Does this mean all file I/O must be in the activity class?
No, but the method in question is called from a context - just pass a context into the constructor of this cFavretClass (or to the method itself, if you prefer):
Context mContext;
public cFavretClass(Context context) {
mContext = context;
}
...
// in your methods:
mContext.openFileOutput(FILENAME);
As JRaymond explained, openFileOutput is a method of Context of which Activity is derived. This method is special insofar as it allows you to create files, which are private to your application.
You can use the normal Java I/O classes too, to write to the external storage (SD-Card) but those files will be readable by everybody.