Data Backup

Android's backup service allows you to copy your persistent application data to a remote "cloud" storage, in order to provide a restore point for the application data and settings. If a user performs a factory reset or converts to a new Android-powered device, the system automatically restores your backup data when the application is re-installed. This way, your users are not required to reproduce their previous data or application settings. This process is completely transparent to the user and does not affect the functionality or user experience in your application.

Android-powered devices that support the backup service provide a cloud storage area that saves your backup data and a backup transport that delivers your data to the storage area and back to the device. During a backup operation, Android's Backup Manager requests backup data from your application, then delivers it to the cloud storage using the backup transport. During a restore operation, the Backup Manager retrieves the backup data from the backup transport and returns it to your application so it can restore the data to the device. The backup service is not designed for data synchronization (you do not have access the backup data, except during a restore operation on the device).

The cloud storage used for backup won't necessarily be the same on all Android-powered devices. The cloud storage and backup transport may differ between devices and service providers. Where the backup data is stored is transparent to your application, but you are assured that your application data cannot be read by other applications.

Caution: Because the cloud storage and transport service can differ from device to device, Android makes no guarantees about the security of your data while using backup. You should be cautious about using backup to store sensitive data, such as usernames and passwords.

The Basics

To backup your application data, you need to implement a backup agent. Your backup agent is called by the Backup Manager to provide the data you want to back up. It is also called to restore your backup data when the application is re-installed. The Backup Manager handles all your data transactions with the cloud storage and your backup agent handles all your data transactions on the device.

To implement a backup agent, you must:

1. Declare your backup agent in your manifest file with the android:backupAgent attribute.
2. Define a backup agent by either:
   1. Extending BackupAgent

      The BackupAgent class provides the central interface with which your application communicates with the Backup Manager. If you extend this class directly, you must override onBackup() and onRestore() to handle the backup and restore operations for your data.

      Or

   2. Extending BackupAgentHelper

      The BackupAgentHelper class provides a convenient wrapper around the BackupAgent class, which minimizes the amount of code you need to write. In your BackupAgentHelper, you must use one or more "helper" objects, which automatically backup and restore certain types of data, so that you do not need to implement onBackup() and onRestore().

      Android currently provides backup helpers that will backup and restore complete files from SharedPreferences and internal storage.

Declaring the Backup Agent in Your Manifest

This is the easiest step, so once you've decided on the class name for your backup agent, declare it in your manifest with the android:backupAgent attribute in the <application> tag.

For example:

<manifest ... >
    <application android:label="MyApplication"
                 android:backupAgent="MyBackupAgent">
        <activity ... >
            ...
        </activity>
    </application>
</manifest>

Another attribute you might want to use is android:restoreAnyVersion. This attribute takes a boolean value to indicate whether you want to restore the application data regardless of the current application version compared to the version that produced the backup data. (The default value is "false".) See Checking the Restore Data Version for more information.

Note: The backup service and the APIs you must use are available only on devices running API Level 8 (Android 2.2) or greater, so you should also set your android:minSdkVersion attribute to "8". However, if you implement proper backward compatibility in your application, you can support this feature for devices running API Level 8 or greater, while remaining compatible with older devices.

Extending BackupAgent

Most applications shouldn't need to extend the BackupAgent class directly, but should instead extend BackupAgentHelper to take advantage of the built-in helper classes that automatically backup and restore your files. However, you might want to extend BackupAgent directly if you need to:

• Version your data format. For instance, if you anticipate the need to revise the format in which you write your application data, you can build a backup agent to cross-check your application version during a restore operation and perform any necessary compatibility work if the version on the device is different than that of the backup data. For more information, see Checking the Restore Data Version.
• Instead of backing up an entire file, you can specify the portions of data the should be backed up and how each portion is then restored to the device. (This can also help you manage different versions, because you read and write your data as unique entities, rather than complete files.)
• Back up data in a database. If you have an SQLite database that you want to restore when the user re-installs your application, you need to build a custom BackupAgent that reads the appropriate data during a backup operation, then create your table and insert the data during a restore operation.

If you don't need to perform any of the tasks above and want to back up complete files from SharedPreferences or internal storage, you should skip to Extending BackupAgentHelper.

Required Methods

When you create a backup agent by extending BackupAgent, you must implement the following callback methods:

onBackup()

The Backup Manager calls this method after you request a backup. In this method, you read your application data from the device and pass the data you want to back up to the Backup Manager, as described below in Performing backup.

onRestore()

The Backup Manager calls this method during a restore operation (you can request a restore, but the system automatically performs restore when the user re-installs your application). When it calls this method, the Backup Manager delivers your backup data, which you then restore to the device, as described below in Performing restore.

Performing backup

When it's time to back up your application data, the Backup Manager calls your onBackup() method. This is where you must provide your application data to the Backup Manager so it can be saved to cloud storage.

Only the Backup Manager can call your backup agent's onBackup() method. Each time that your application data changes and you want to perform a backup, you must request a backup operation by calling dataChanged() (see Requesting Backup for more information). A backup request does not result in an immediate call to your onBackup() method. Instead, the Backup Manager waits for an appropriate time, then performs backup for all applications that have requested a backup since the last backup was performed.

Tip: While developing your application, you can initiate an immediate backup operation from the Backup Manager with the bmgr tool.

When the Backup Manager calls your onBackup() method, it passes three parameters:

oldState

An open, read-only ParcelFileDescriptor pointing to the last backup state provided by your application. This is not the backup data from cloud storage, but a local representation of the data that was backed up the last time onBackup() was called (as defined by newState, below, or from onRestore()—more about this in the next section). Because onBackup() does not allow you to read existing backup data in the cloud storage, you can use this local representation to determine whether your data has changed since the last backup.

data

A BackupDataOutput object, which you use to deliver your backup data to the Backup Manager.

newState

An open, read/write ParcelFileDescriptor pointing to a file in which you must write a representation of the data that you delivered to data (a representation can be as simple as the last-modified timestamp for your file). This object is returned as oldState the next time the Backup Manager calls your onBackup() method. If you do not write your backup data to newState, then oldState will point to an empty file next time Backup Manager calls onBackup().

Using these parameters, you should implement your onBackup() method to do the following:

1. Check whether your data has changed since the last backup by comparing oldState to your current data. How you read data in oldState depends on how you originally wrote it to newState (see step 3). The easiest way to record the state of a file is with its last-modified timestamp. For example, here's how you can read and compare a timestamp from oldState:

   // Get the oldState input stream
   FileInputStream instream = new FileInputStream(oldState.getFileDescriptor());
   DataInputStream in = new DataInputStream(instream);
   
   try {
       // Get the last modified timestamp from the state file and data file
       long stateModified = in.readLong();
       long fileModified = mDataFile.lastModified();
   
       if (stateModified != fileModified) {
           // The file has been modified, so do a backup
           // Or the time on the device changed, so be safe and do a backup
       } else {
           // Don't back up because the file hasn't changed
           return;
       }
   } catch (IOException e) {
       // Unable to read state file... be safe and do a backup
   }
   

   If nothing has changed and you don't need to back up, skip to step 3.

2. If your data has changed, compared to oldState, write the current data to data to back it up to the cloud storage.

   You must write each chunk of data as an "entity" in the BackupDataOutput. An entity is a flattened binary data record that is identified by a unique key string. Thus, the data set that you back up is conceptually a set of key-value pairs.

   To add an entity to your backup data set, you must:

   1. Call writeEntityheader(), passing a unique string key for the data you're about to write and the data size.
   2. Call writeEntityData(), passing a byte buffer that contains your data and the number of bytes to write from the buffer (which should match the size passed to writeEntityHeader()).

   For example, the following code flattens some data into a byte stream and writes it into a single entity:

   // Create buffer stream and data output stream for our data
   ByteArrayOutputStream bufStream = new ByteArrayOutputStream();
   DataOutputStream outWriter = new DataOutputStream(bufStream);
   // Write structured data
   outWriter.writeString(playerName);
   outWriter.writeInt(playerScore);
   // Send the data to the Backup Manager via the BackupDataOutput
   byte[] buffer = bufStream.toByteArray();
   int len = buffer.length;
   data.writeEntityHeader(TOPSCORE_BACKUP_KEY, len);
   data.writeEntityData(buffer, len);
   

   Perform this for each piece of data that you want to back up. How you divide your data into entities is up to you (and you might use just one entity).

3. Whether or not you perform a backup (in step 2), write a representation of the current data to the newState ParcelFileDescriptor. The Backup Manager retains this object locally as a representation of the data that is currently backed up. It passes this back to you as oldState the next time it calls onBackup() so you can determine whether another backup is necessary (as handled in step 1). If you do not write the current data state to this file, then oldState will be empty during the next callback.

   Again, the following example saves a representation of the data using the file's last-modified timestamp:

   FileOutputStream outstream = new FileOutputStream(stateFile.getFileDescriptor());
   DataOutputStream out = new DataOutputStream(outstream);
   
   long modified = mDataFile.lastModified();
   out.writeLong(modified);
   

Caution: If your application data is saved to a file, make sure that you use synchronized statements while accessing the file so that your backup agent does not read the file while an Activity in your application is also writing the file.

Performing restore

When it's time to restore your application data, the Backup Manager calls your backup agent's onRestore() method. When it calls this method, the Backup Manager delivers your backup data so you can restore it onto the device.

Only the Backup Manager can call onRestore(), which happens automatically when the system installs your application and finds existing backup data. However, you can request a restore operation for your application by calling requestRestore() (see Requesting restore for more information).

Note: While developing your application, you can also request a restore operation with the bmgr tool.

When the Backup Manager calls your onRestore() method, it passes three parameters:

data

A BackupDataInput, which allows you to read your backup data.

appVersionCode

An integer representing the value of your application's android:versionCode manifest attribute, as it was when this data was backed up. You can use this to cross-check the current application version and determine if the data format is compatible. For more information about using this to handle different versions of restore data, see the section below about Checking the Restore Data Version.

newState

An open, read/write ParcelFileDescriptor pointing to a file in which you must write the final backup state that was provided with data. This object is returned as oldState the next time onBackup() is called. Recall that you must also write the same newState object in the onBackup() callback—also doing it here ensures that the oldState object given to onBackup() is valid even the first time onBackup() is called after the device is restored.

In your implementation of onRestore(), you should call readNextHeader() to iterate through all entities in the data set. For each entity found, do the following:

1. Get the entity key with getKey().
2. Compare the entity key to a list of known key values that you should have declared as static final strings inside your BackupAgent class. When the key matches one of your known key strings, enter into a statement to extract the entity data and save it to the device:
   1. Get the entity data size with getDataSize() and create a byte array of that size.
   2. Call readEntityData() and pass it the byte array, which is where the data will go, and specify the start offset and the size to read.
   3. Your byte array is now full and you can read the data and write it to the device however you like.
3. After you read and write your data back to the device, write the state of your data to the newState parameter the same as you do during onBackup().

Extending BackupAgentHelper

You should build your backup agent using BackupAgentHelper if you want to back up complete files (from either SharedPreferences or internal storage). Building your backup agent with BackupAgentHelper requires far less code than extending BackupAgent, because you don't have to implement onBackup() and onRestore().

Your implementation of BackupAgentHelper must use one or more backup helpers. A backup helper is a specialized component that BackupAgentHelper summons to perform backup and restore operations for a particular type of data. The Android framework currently provides two different helpers:

• SharedPreferencesBackupHelper to backup SharedPreferences files.
• FileBackupHelper to backup files from internal storage.

You can include multiple helpers in your BackupAgentHelper, but only one helper is needed for each data type. That is, if you have multiple SharedPreferences files, then you need only one SharedPreferencesBackupHelper.

For each helper you want to add to your BackupAgentHelper, you must do the following during your onCreate() method:

1. Instantiate in instance of the desired helper class. In the class constructor, you must specify the appropriate file(s) you want to backup.
2. Call addHelper() to add the helper to your BackupAgentHelper.

The following sections describe how to create a backup agent using each of the available helpers.

Backing up SharedPreferences

When you instantiate a SharedPreferencesBackupHelper, you must the name of one or more SharedPreferences files.

For example, to back up a SharedPreferences file named "user_preferences", a complete backup agent using BackupAgentHelper looks like this:

public class MyPrefsBackupAgent extends BackupAgentHelper {
    // The name of the SharedPreferences file
    static final String PREFS = "user_preferences";

    // A key to uniquely identify the set of backup data
    static final String PREFS_BACKUP_KEY = "prefs";

    // Allocate a helper and add it to the backup agent
    void onCreate() {
        SharedPreferencesBackupHelper helper = new SharedPreferencesBackupHelper(this, PREFS);
        addHelper(PREFS_BACKUP_KEY, helper);
    }
}

That's it! That's your entire backup agent. The SharedPreferencesBackupHelper includes all the code needed to backup and restore a SharedPreferences file.

When the Backup Manager calls onBackup() and onRestore(), BackupAgentHelper calls your backup helpers to perform backup and restore for your specified files.

Note: SharedPreferences are threadsafe, so you can safely read and write the shared preferences file from your backup agent and other activities.

Backing up other files

When you instantiate a FileBackupHelper, you must include the name of one or more files that are saved to your application's internal storage (as specified by getFilesDir(), which is the same location where openFileOutput() writes files).

For example, to backup two files named "scores" and "stats," a backup agent using BackupAgentHelper looks like this:

public class MyFileBackupAgent extends BackupAgentHelper {
    // The name of the SharedPreferences file
    static final String TOP_SCORES = "scores";
    static final String PLAYER_STATS = "stats";

    // A key to uniquely identify the set of backup data
    static final String FILES_BACKUP_KEY = "myfiles";

    // Allocate a helper and add it to the backup agent
    void onCreate() {
        FileBackupHelper helper = new FileBackupHelper(this, TOP_SCORES, PLAYER_STATS);
        addHelper(FILES_BACKUP_KEY, helper);
    }
}

The FileBackupHelper includes all the code necessary to backup and restore files that are saved to your application's internal storage..

However, reading and writing to files on internal storage is not threadsafe. To ensure that your backup agent does not read or write your files at the same time as your activities, you must use synchronized statements each time you perform a read or write. For example, in any Activity where you read and write the file, you need an object to use as the intrinsic lock for the synchronized statements:

// Object for intrinsic lock
static final Object[] sDataLock = new Object[0];

Then create a synchronized statement with this lock each time you read or write the files. For example, here's a synchronized statement for writing the latest score in a game to a file:

try {
    synchronized (MyActivity.sDataLock) {
        File dataFile = new File(getFilesDir(), TOP_SCORES);
        RandomAccessFile raFile = new RandomAccessFile(dataFile, "rw");
        raFile.writeInt(score);
    }
} catch (IOException e) {
    Log.e(TAG, "Unable to write to file");
}

You should synchronize your read statements with the same lock.

Then, in your BackupAgentHelper, you must override onBackup() and onRestore() to synchronize the backup and restore operations with the same intrinsic lock. For example, the MyFileBackupAgent example from above needs the following methods:

@Override
public void onBackup(ParcelFileDescriptor oldState, BackupDataOutput data,
          ParcelFileDescriptor newState) throws IOException {
    // Hold the lock while the FileBackupHelper performs backup
    synchronized (MyActivity.sDataLock) {
        super.onBackup(oldState, data, newState);
    }
}

@Override
public void onRestore(BackupDataInput data, int appVersionCode,
        ParcelFileDescriptor newState) throws IOException {
    // Hold the lock while the FileBackupHelper restores the file
    synchronized (MyActivity.sDataLock) {
        super.onRestore(data, appVersionCode, newState);
    }
}

That's it. All you need to do is add your FileBackupHelper in the onCreate() method and override onBackup() and onRestore() to synchronize read and write operations.

Checking the Restore Data Version

When the Backup Manager saves your data to cloud storage, it automatically includes the version of your application, as defined by your manifest file's android:versionCode attribute. Before the Backup Manager calls your backup agent to restore your data, it looks at the android:versionCode of the installed application and compares it to the value recorded in the restore data set. If the version recorded in the restore data set is newer than the application version on the device, then the user has downgraded their application. In this case, the Backup Manager will abort the restore operation for your application and not call your onRestore() method, because the restore set is considered meaningless to an older version.

You can override this behavior with the android:restoreAnyVersion attribute. This attribute is either "true" or "false" to indicate whether you want to restore the application regardless of the restore set version. The default value is "false". If you define this to be "true" then the Backup Manager will ignore the android:versionCode and call your onRestore() method in all cases. In doing so, you can manually check for the version difference in your onRestore() method and take any steps necessary to make the data compatible if the versions conflict.

To help you handle different versions during a restore operation, the onRestore() method passes you the version code included with the restore data set as the appVersionCode parameter. You can then query the current application's version code with the PackageInfo.versionCode field. For example:

PackageInfo info;
try {
    String name = getPackageName();
    info = getPackageManager().getPackageInfo(name,0);
} catch (NameNotFoundException nnfe) {
    info = null;
}

int version;
if (info != null) {
    version = info.versionCode;
}

Then simply compare the version acquired from PackageInfo to the appVersionCode passed into onRestore().

Caution: Be certain you understand the consequences of setting android:restoreAnyVersion to "true" for your application. If each version of your application that supports backup does not properly account for variations in your data format during onRestore(), then the data on the device could be saved in a format incompatible with the version currently installed on the device.

Requesting Backup

You can request a backup operation at any time by calling dataChanged(). This method notifies the Backup Manager that you'd like to backup your data using your backup agent. The Backup Manager then calls your backup agent's onBackup() method at an opportune time in the future. Typically, you should request a backup each time your data changes (such as when the user changes an application preference that you'd like to back up). If you call dataChanged() several times consecutively, before the Backup Manager requests a backup from your agent, your agent still receives just one call to onBackup().

Note: While developing your application, you can request a backup and initiate an immediate backup operation with the bmgr tool.

Requesting Restore

During the normal life of your application, you shouldn't need to request a restore operation. They system automatically checks for backup data and performs a restore when your application is installed. However, you can manually request a restore operation by calling requestRestore(), if necessary. In which case, the Backup Manager calls your onRestore() implementation, passing the data from the current set of backup data.

Note: While developing your application, you can request a restore operation with the bmgr tool.