Android's {@link android.app.backup 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 ({@link android.app.backup.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:
The {@link android.app.backup.BackupAgent} class provides the central interface with which your application communicates with the Backup Manager. If you extend this class directly, you must override {@link android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor) onBackup()} and {@link android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor) onRestore()} to handle the backup and restore operations for your data.
Or
The {@link android.app.backup.BackupAgentHelper} class provides a convenient wrapper around the {@link android.app.backup.BackupAgent} class, which minimizes the amount of code you need to write. In your {@link android.app.backup.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 {@link android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor) onBackup()} and {@link android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor) onRestore()}.
Android currently provides backup helpers that will backup and restore complete files from {@link android.content.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 {@code
android:backupAgent} attribute in the {@code
For example:
<manifest ... >
...
<application android:label="MyApplication"
android:backupAgent="MyBackupAgent">
<activity ... >
...
</activity>
</application>
</manifest>
Another attribute you might want to use is {@code 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 "{@code 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 {@code android:minSdkVersion} attribute to "8".
Google provides a backup transport with Android Backup Service for most Android-powered devices running Android 2.2 or greater.
In order for your 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 {@code
<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 {@code
Most applications shouldn't need to extend the {@link android.app.backup.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 {@link android.app.backup.BackupAgent} directly if you need to:
If you don't need to perform any of the tasks above and want to back up complete files from {@link android.content.SharedPreferences} or internal storage, you should skip to Extending BackupAgentHelper.
When you create a backup agent by extending {@link android.app.backup.BackupAgent}, you must implement the following callback methods:
When it's time to back up your application data, the Backup Manager calls your {@link android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor) 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 {@link android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor) onBackup()} method. Each time that your application data changes and you want to perform a backup, you must request a backup operation by calling {@link android.app.backup.BackupManager#dataChanged()} (see Requesting Backup for more information). A backup request does not result in an immediate call to your {@link android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor) 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 {@code bmgr} tool.
When the Backup Manager calls your {@link android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor) onBackup()} method, it passes three parameters:
Using these parameters, you should implement your {@link android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor) onBackup()} method to do the following:
// 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.
You must write each chunk of data as an "entity" in the {@link android.app.backup.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:
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).
The following example saves a representation of the current data into {@code 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 {@link android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor) 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 {@link android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor) 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 {@link android.app.backup.BackupManager#requestRestore(RestoreObserver) requestRestore()} (see Requesting restore for more information).
Note: While developing your application, you can also request a restore operation with the {@code bmgr} tool.
When the Backup Manager calls your {@link android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor) onRestore()} method, it passes three parameters:
In your implementation of {@link android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor) onRestore()}, you should call {@link android.app.backup.BackupDataInput#readNextHeader()} on the {@code data} to iterate through all entities in the data set. For each entity found, do the following:
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 {@code appVersionCode} parameter passed to {@link android.app.backup.BackupAgent#onRestore 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 {@link android.app.backup.BackupAgent}, see the {@code ExampleAgent} class in the Backup and Restore sample application.
You should build your backup agent using {@link android.app.backup.BackupAgentHelper} if you want to back up complete files (from either {@link android.content.SharedPreferences} or internal storage). Building your backup agent with {@link android.app.backup.BackupAgentHelper} requires far less code than extending {@link android.app.backup.BackupAgent}, because you don't have to implement {@link android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor) onBackup()} and {@link android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor) onRestore()}.
Your implementation of {@link android.app.backup.BackupAgentHelper} must use one or more backup helpers. A backup helper is a specialized component that {@link android.app.backup.BackupAgentHelper} summons to perform backup and restore operations for a particular type of data. The Android framework currently provides two different helpers:
You can include multiple helpers in your {@link android.app.backup.BackupAgentHelper}, but only one helper is needed for each data type. That is, if you have multiple {@link android.content.SharedPreferences} files, then you need only one {@link android.app.backup.SharedPreferencesBackupHelper}.
For each helper you want to add to your {@link android.app.backup.BackupAgentHelper}, you must do the following during your {@link android.app.backup.BackupAgent#onCreate()} method:
The following sections describe how to create a backup agent using each of the available helpers.
When you instantiate a {@link android.app.backup.SharedPreferencesBackupHelper}, you must include the name of one or more {@link android.content.SharedPreferences} files.
For example, to back up a {@link android.content.SharedPreferences} file named "user_preferences", a complete backup agent using {@link android.app.backup.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 {@link android.app.backup.SharedPreferencesBackupHelper} includes all the code needed to backup and restore a {@link android.content.SharedPreferences} file.
When the Backup Manager calls {@link android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor) onBackup()} and {@link android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor) onRestore()}, {@link android.app.backup.BackupAgentHelper} calls your backup helpers to perform backup and restore for your specified files.
Note: {@link android.content.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 {@link android.app.backup.FileBackupHelper}, you must include the name of one or more files that are saved to your application's internal storage (as specified by {@link android.content.ContextWrapper#getFilesDir()}, which is the same location where {@link android.content.Context#openFileOutput(String,int) openFileOutput()} writes files).
For example, to backup two files named "scores" and "stats," a backup agent using {@link android.app.backup.BackupAgentHelper} looks like this:
public class MyFileBackupAgent extends BackupAgentHelper {
// The name of the 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
@Override
public void onCreate() {
FileBackupHelper helper = new FileBackupHelper(this,
TOP_SCORES, PLAYER_STATS);
addHelper(FILES_BACKUP_KEY, helper);
}
}
The {@link android.app.backup.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();
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({@link android.content.Context#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 {@link android.app.backup.BackupAgentHelper}, you must override {@link android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor) onBackup()} and {@link android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor) onRestore()} to synchronize the backup and restore operations with the same intrinsic lock. For example, the {@code 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 {@link android.app.backup.FileBackupHelper} in the {@link android.app.backup.BackupAgent#onCreate()} method and override {@link android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor) onBackup()} and {@link android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor) onRestore()} to synchronize read and write operations.
For an example implementation of {@link android.app.backup.BackupAgentHelper} with {@link android.app.backup.FileBackupHelper}, see the {@code 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 {@code android:versionCode} attribute. Before the Backup Manager calls your backup agent to restore your data, it looks at the {@code 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 {@link android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor) onRestore()} method, because the restore set is considered meaningless to an older version.
You can override this behavior with the {@code android:restoreAnyVersion} attribute. This attribute is either "{@code true}" or "{@code false}" to indicate whether you want to restore the application regardless of the restore set version. The default value is "{@code false}". If you define this to be "{@code true}" then the Backup Manager will ignore the {@code android:versionCode} and call your {@link android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor) onRestore()} method in all cases. In doing so, you can manually check for the version difference in your {@link android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor) 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 {@link android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor) onRestore()} method passes you the version code included with the restore data set as the {@code appVersionCode} parameter. You can then query the current application's version code with the {@link android.content.pm.PackageInfo#versionCode PackageInfo.versionCode} field. For example:
PackageInfo info;
try {
String name = {@link android.content.ContextWrapper#getPackageName() getPackageName}();
info = {@link android.content.ContextWrapper#getPackageManager
getPackageManager}().{@link android.content.pm.PackageManager#getPackageInfo(String,int)
getPackageInfo}(name,0);
} catch (NameNotFoundException nnfe) {
info = null;
}
int version;
if (info != null) {
version = info.versionCode;
}
Then simply compare the {@code version} acquired from {@link android.content.pm.PackageInfo} to the {@code appVersionCode} passed into {@link android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor) onRestore()}.
Caution: Be certain you understand the consequences of setting {@code android:restoreAnyVersion} to "{@code true}" for your application. If each version of your application that supports backup does not properly account for variations in your data format during {@link android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor) 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 {@link android.app.backup.BackupManager#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 {@link android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor) 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 {@link android.app.backup.BackupManager#dataChanged()} several times consecutively, before the Backup Manager requests a backup from your agent, your agent still receives just one call to {@link android.app.backup.BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor) onBackup()}.
Note: While developing your application, you can request a backup and initiate an immediate backup operation with the {@code 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 {@link android.app.backup.BackupManager#requestRestore(RestoreObserver) requestRestore()}, if necessary. In which case, the Backup Manager calls your {@link android.app.backup.BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor) 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 {@code bmgr} tool.
Once you've implemented your backup agent, you can test the backup and restore functionality with the following procedure, using {@code bmgr}.
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 {@link android.app.backup.BackupManager#dataChanged()}, which adds a backup request to the Backup Manager queue. For testing purposes, you can also make a request with the following {@code 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.