Android's backup service allows you to copy your persistent
application data to 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 don't need 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.
During a backup operation (which your application can request), Android's Backup Manager (BackupManager) queries your application for backup data, then hands it to
a backup transport, which then delivers the data to the cloud storage. During a
restore operation, the Backup Manager retrieves the backup data from the backup transport and
returns it to your application so your application can restore the data to the device. It's
possible for your application to request a restore, but that shouldn't be necessary—Android
automatically performs a restore operation when your application is installed and there exists
backup data associated with the user. The primary scenario in which backup data is restored is when
a user resets their device or upgrades to a new device and their previously installed
applications are re-installed.
Note: The backup service is not designed for synchronizing application data with other clients or saving data that you'd like to access during the normal application lifecycle. You cannot read or write backup data on demand and cannot access it in any way other than through the APIs provided by the Backup Manager.
The backup transport is the client-side component of Android's backup framework, which is customizable by the device manufacturer and service provider. The backup transport may differ from device to device and which backup transport is available on any given device is transparent to your application. The Backup Manager APIs isolate your application from the actual backup transport available on a given device—your application communicates with the Backup Manager through a fixed set of APIs, regardless of the underlying transport.
Data backup is not guaranteed to be available on all Android-powered devices. However, your application is not adversely affected in the event that a device does not provide a backup transport. If you believe that users will benefit from data backup in your application, then you can implement it as described in this document, test it, then publish your application without any concern about which devices actually perform backup. When your application runs on a device that does not provide a backup transport, your application operates normally, but will not receive callbacks from the Backup Manager to backup data.
Although you cannot know what the current transport is, you are always assured that your backup data cannot be read by other applications on the device. Only the Backup Manager and backup transport have access to the data you provide during a backup operation.
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 always be cautious about using backup to store sensitive data, such as usernames and passwords.
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 (using the backup transport) and your backup agent handles all your data transactions on the device.
To implement a backup agent, you must:
android:backupAgent attribute.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
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.
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.
Google provides a backup transport with Android Backup Service for most Android-powered devices running Android 2.2 or greater.
In order for you application to perform backup using Android Backup Service, you must register your application with the service to receive a Backup Service Key, then declare the Backup Service Key in your Android manifest.
To get your Backup Service Key, register for Android Backup Service.
When you register, you will be provided a Backup Service Key and the appropriate <meta-data> XML code for your Android manifest file, which you must include as a child of the
<application> element. For example:
<application android:label="MyApplication"
android:backupAgent="MyBackupAgent">
...
<meta-data android:name="com.google.android.backup.api_key"
android:value="AEdPqrEAAAAIDaYEVgU6DJnyJdBmU7KLH3kszDXLv_4DIsEIyQ" />
</application>
The android:name must be "com.google.android.backup.api_key" and
the android:value must be the Backup Service Key received from the Android Backup
Service registration.
If you have multiple applications, you must register each one, using the respective package name.
Note: The backup transport provided by Android Backup Service is
not guaranteed to be available
on all Android-powered devices that support backup. Some devices might support backup
using a different transport, some devices might not support backup at all, and there is no way for
your application to know what transport is used on the device. However, if you implement backup for
your application, you should always include a Backup Service Key for Android Backup Service so
your application can perform backup when the device uses the Android Backup Service transport. If
the device does not use Android Backup Service, then the <meta-data> element with the
Backup Service Key is ignored.
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:
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.
When you create a backup agent by extending BackupAgent, you
must implement the following callback methods:
onBackup()onRestore()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:
oldStateParcelFileDescriptor 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.dataBackupDataOutput object, which you use to deliver your backup
data to the Backup Manager.newStateParcelFileDescriptor 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:
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.
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:
writeEntityHeader(), passing a unique string key for the data you're about to write and the data
size.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.writeUTF(mPlayerName); outWriter.writeInt(mPlayerScore); // 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).
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.
The following example saves a representation of the current data into newState using
the file's last-modified timestamp:
FileOutputStream outstream = new FileOutputStream(newState.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.
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:
dataBackupDataInput, which allows you to read your backup
data.appVersionCodeandroid: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.newStateParcelFileDescriptor 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() on the
data to iterate
through all entities in the data set. For each entity found, do the following:
getKey().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:
getDataSize() and create a byte array of that size.readEntityData() and pass it the byte array, which is where the data will go, and specify the
start offset and the size to read.newState parameter the same as you do during onBackup().
For example, here's how you can restore the data backed up by the example in the previous section:
@Override
public void onRestore(BackupDataInput data, int appVersionCode,
ParcelFileDescriptor newState) throws IOException {
// There should be only one entity, but the safest
// way to consume it is using a while loop
while (data.readNextHeader()) {
String key = data.getKey();
int dataSize = data.getDataSize();
// If the key is ours (for saving top score). Note this key was used when
// we wrote the backup entity header
if (TOPSCORE_BACKUP_KEY.equals(key)) {
// Create an input stream for the BackupDataInput
byte[] dataBuf = new byte[dataSize];
data.readEntityData(dataBuf, 0, dataSize);
ByteArrayInputStream baStream = new ByteArrayInputStream(dataBuf);
DataInputStream in = new DataInputStream(baStream);
// Read the player name and score from the backup data
mPlayerName = in.readUTF();
mPlayerScore = in.readInt();
// Record the score on the device (to a file or something)
recordScore(mPlayerName, mPlayerScore);
} else {
// We don't know this entity key. Skip it. (Shouldn't happen.)
data.skipEntityData();
}
}
// Finally, write to the state blob (newState) that describes the restored data
FileOutputStream outstream = new FileOutputStream(newState.getFileDescriptor());
DataOutputStream out = new DataOutputStream(outstream);
out.writeUTF(mPlayerName);
out.writeInt(mPlayerScore);
}
In this example, the appVersionCode parameter passed to onRestore() is not used. However, you might want to use
it if you've chosen to perform backup when the user's version of the application has actually moved
backward (for example, the user went from version 1.5 of your app to 1.0). For more information, see
the section about Checking the Restore Data Version.
For an example implementation of BackupAgent, see the ExampleAgent class in the Backup and Restore sample
application.
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:
addHelper()
to add the helper to your BackupAgentHelper.The following sections describe how to create a backup agent using each of the available helpers.
When you instantiate a SharedPreferencesBackupHelper, you must
include 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
@Override
public 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.
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:
Interesting Fact:
A zero-length array is lighter-weight than a normal Object, so it's great for an intrinsic lock.
// 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.
For an example implementation of BackupAgentHelper with FileBackupHelper, see the
FileHelperExampleAgent class in the Backup and Restore sample
application.
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.
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.
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.
Once you've implemented your backup agent, you can test the backup and restore functionality
with the following procedure, using bmgr.
tools/ path:
adb shell bmgr enable true
If you've properly implemented backup in your application, then it should request a
backup each time the data changes. For example, each time the user changes some data, your app
should call dataChanged(), which adds a backup request to
the Backup Manager queue. For testing purposes, you can also make a request with the following
bmgr command:
adb shell bmgr backup your.package.name
adb shell bmgr run
This forces the Backup Manager to perform all backup requests that are in its queue.
adb uninstall your.package.name
If your backup agent is successful, all the data you initialized in step 4 is restored.