BackupAgent
public
abstract
class
BackupAgent
extends ContextWrapper
java.lang.Object | |||
↳ | android.content.Context | ||
↳ | android.content.ContextWrapper | ||
↳ | android.app.backup.BackupAgent |
Provides the central interface between an
application and Android's data backup infrastructure. An application that wishes
to participate in the backup and restore mechanism will declare a subclass of
BackupAgent
, implement the
onBackup()
and onRestore()
methods,
and provide the name of its backup agent class in its AndroidManifest.xml
file via
the
<application>
tag's android:backupAgent
attribute.
Developer Guides
For more information about using BackupAgent, read the Data Backup developer guide.
Basic Operation
When the application makes changes to data that it wishes to keep backed up,
it should call the
BackupManager.dataChanged()
method.
This notifies the Android Backup Manager that the application needs an opportunity
to update its backup image. The Backup Manager, in turn, schedules a
backup pass to be performed at an opportune time.
Restore operations are typically performed only when applications are first installed on a device. At that time, the operating system checks to see whether there is a previously-saved data set available for the application being installed, and if so, begins an immediate restore pass to deliver the backup data as part of the installation process.
When a backup or restore pass is run, the application's process is launched
(if not already running), the manifest-declared backup agent class (in the android:backupAgent
attribute) is instantiated within
that process, and the agent's onCreate()
method is invoked. This prepares the
agent instance to run the actual backup or restore logic. At this point the
agent's
onBackup()
or
onRestore()
method will be
invoked as appropriate for the operation being performed.
A backup data set consists of one or more "entities," flattened binary data records that are each identified with a key string unique within the data set. Adding a record to the active data set or updating an existing record is done by simply writing new entity data under the desired key. Deleting an entity from the data set is done by writing an entity under that key with header specifying a negative data size, and no actual entity data.
Helper Classes
An extensible agent based on convenient helper classes is available in
BackupAgentHelper
. That class is particularly
suited to handling of simple file or SharedPreferences
backup and restore.
Threading
The constructor, as well as onCreate()
and onDestroy()
lifecycle callbacks run
on the main thread (UI thread) of the application that implements the BackupAgent.
The data-handling callbacks:
onBackup()
,
onFullBackup(android.app.backup.FullBackupDataOutput)
,
onRestore()
,
onRestoreFile()
,
onRestoreFinished()
, and onQuotaExceeded()
run on binder pool threads.
Summary
Constants | |
---|---|
int |
FLAG_CLIENT_SIDE_ENCRYPTION_ENABLED
Flag for |
int |
FLAG_DEVICE_TO_DEVICE_TRANSFER
Flag for |
int |
TYPE_DIRECTORY
During a full restore, indicates that the file system object being restored is a directory. |
int |
TYPE_FILE
During a full restore, indicates that the file system object being restored is an ordinary file. |
Inherited constants |
---|
Public constructors | |
---|---|
BackupAgent()
|
Public methods | |
---|---|
final
void
|
fullBackupFile(File file, FullBackupDataOutput output)
Write an entire file as part of a full-backup operation. |
abstract
void
|
onBackup(ParcelFileDescriptor oldState, BackupDataOutput data, ParcelFileDescriptor newState)
The application is being asked to write any data changed since the last time it performed a backup operation. |
void
|
onCreate()
Provided as a convenience for agent implementations that need an opportunity to do one-time initialization before the actual backup or restore operation is begun. |
void
|
onDestroy()
Provided as a convenience for agent implementations that need to do some sort of shutdown process after backup or restore is completed. |
void
|
onFullBackup(FullBackupDataOutput data)
The application is having its entire file system contents backed up. |
void
|
onQuotaExceeded(long backupDataBytes, long quotaBytes)
Notification that the application's current backup operation causes it to exceed the maximum size permitted by the transport. |
abstract
void
|
onRestore(BackupDataInput data, int appVersionCode, ParcelFileDescriptor newState)
The application is being restored from backup and should replace any existing data with the contents of the backup. |
void
|
onRestore(BackupDataInput data, long appVersionCode, ParcelFileDescriptor newState)
New version of |
void
|
onRestoreFile(ParcelFileDescriptor data, long size, File destination, int type, long mode, long mtime)
Handle the data delivered via the given file descriptor during a full restore operation. |
void
|
onRestoreFinished()
The application's restore operation has completed. |
Inherited methods | |
---|---|
Constants
FLAG_CLIENT_SIDE_ENCRYPTION_ENABLED
public static final int FLAG_CLIENT_SIDE_ENCRYPTION_ENABLED
Flag for BackupDataOutput.getTransportFlags()
and
FullBackupDataOutput.getTransportFlags()
only.
The transport has client-side encryption enabled. i.e., the user's backup has been encrypted with a key known only to the device, and not to the remote storage solution. Even if an attacker had root access to the remote storage provider they should not be able to decrypt the user's backup data.
Constant Value: 1 (0x00000001)
FLAG_DEVICE_TO_DEVICE_TRANSFER
public static final int FLAG_DEVICE_TO_DEVICE_TRANSFER
Flag for BackupDataOutput.getTransportFlags()
and
FullBackupDataOutput.getTransportFlags()
only.
The transport is for a device-to-device transfer. There is no third party or intermediate storage. The user's backup data is sent directly to another device over e.g., USB or WiFi.
Constant Value: 2 (0x00000002)
TYPE_DIRECTORY
public static final int TYPE_DIRECTORY
During a full restore, indicates that the file system object being restored is a directory.
Constant Value: 2 (0x00000002)
TYPE_FILE
public static final int TYPE_FILE
During a full restore, indicates that the file system object being restored is an ordinary file.
Constant Value: 1 (0x00000001)
Public constructors
Public methods
fullBackupFile
public final void fullBackupFile (File file, FullBackupDataOutput output)
Write an entire file as part of a full-backup operation. The file's contents will be delivered to the backup destination along with the metadata necessary to place it with the proper location and permissions on the device where the data is restored.
Attempting to back up files in directories that are ignored by
the backup system will have no effect. For example, if the app calls this method
with a file inside the ContextWrapper.getNoBackupFilesDir()
directory, it will be ignored.
See onFullBackup(android.app.backup.FullBackupDataOutput)
for details on what directories
are excluded from backups.
Parameters | |
---|---|
file |
File : The file to be backed up. The file must exist and be readable by
the caller. |
output |
FullBackupDataOutput : The destination to which the backed-up file data will be sent. |
onBackup
public abstract void onBackup (ParcelFileDescriptor oldState, BackupDataOutput data, ParcelFileDescriptor newState)
The application is being asked to write any data changed since the last
time it performed a backup operation. The state data recorded during the
last backup pass is provided in the oldState
file
descriptor. If oldState
is null
, no old state
is available and the application should perform a full backup. In both
cases, a representation of the final backup state after this pass should
be written to the file pointed to by the file descriptor wrapped in
newState
.
Each entity written to the BackupDataOutput
data
stream will be transmitted
over the current backup transport and stored in the remote data set under
the key supplied as part of the entity. Writing an entity with a negative
data size instructs the transport to delete whatever entity currently exists
under that key from the remote data set.
Parameters | |
---|---|
oldState |
ParcelFileDescriptor : An open, read-only ParcelFileDescriptor pointing to the
last backup state provided by the application. May be
null , in which case no prior state is being
provided and the application should perform a full backup. |
data |
BackupDataOutput : A structured wrapper around an open, read/write
file descriptor pointing to the backup data destination.
Typically the application will use backup helper classes to
write to this file. |
newState |
ParcelFileDescriptor : An open, read/write ParcelFileDescriptor pointing to an
empty file. The application should record the final backup
state here after writing the requested data to the data
output stream. |
Throws | |
---|---|
IOException |
onCreate
public void onCreate ()
Provided as a convenience for agent implementations that need an opportunity to do one-time initialization before the actual backup or restore operation is begun.
onDestroy
public void onDestroy ()
Provided as a convenience for agent implementations that need to do some sort of shutdown process after backup or restore is completed.
Agents do not need to override this method.
onFullBackup
public void onFullBackup (FullBackupDataOutput data)
The application is having its entire file system contents backed up. data
points to the backup destination, and the app has the opportunity to choose which
files are to be stored. To commit a file as part of the backup, call the
fullBackupFile(java.io.File, android.app.backup.FullBackupDataOutput)
helper method. After all file
data is written to the output, the agent returns from this method and the backup
operation concludes.
Certain parts of the app's data are never backed up even if the app explicitly sends them to the output:
- The contents of the
ContextWrapper.getCacheDir()
directory - The contents of the
ContextWrapper.getCodeCacheDir()
directory - The contents of the
ContextWrapper.getNoBackupFilesDir()
directory - The contents of the app's shared library directory
The default implementation of this method backs up the entirety of the
application's "owned" file system trees to the output other than the few exceptions
listed above. Apps only need to override this method if they need to impose special
limitations on which files are being stored beyond the control that
ContextWrapper.getNoBackupFilesDir()
offers.
Alternatively they can provide an xml resource to specify what data to include or exclude.
Parameters | |
---|---|
data |
FullBackupDataOutput : A structured wrapper pointing to the backup destination. |
Throws | |
---|---|
|
java.io.IOException |
IOException |
onQuotaExceeded
public void onQuotaExceeded (long backupDataBytes, long quotaBytes)
Notification that the application's current backup operation causes it to exceed the maximum size permitted by the transport. The ongoing backup operation is halted and rolled back: any data that had been stored by a previous backup operation is still intact. Typically the quota-exceeded state will be detected before any data is actually transmitted over the network.
The quotaBytes
value is the total data size currently permitted for this
application. If desired, the application can use this as a hint for determining
how much data to store. For example, a messaging application might choose to
store only the newest messages, dropping enough older content to stay under
the quota.
Note that the maximum quota for the application can change over time. In particular, in the future the quota may grow. Applications that adapt to the quota when deciding what data to store should be aware of this and implement their data storage mechanisms in a way that can take advantage of additional quota.
Parameters | |
---|---|
backupDataBytes |
long : The amount of data measured while initializing the backup
operation, if the total exceeds the app's alloted quota. If initial measurement
suggested that the data would fit but then too much data was actually submitted
as part of the operation, then this value is the amount of data that had been
streamed into the transport at the time the quota was reached. |
quotaBytes |
long : The maximum data size that the transport currently permits
this application to store as a backup. |
onRestore
public abstract void onRestore (BackupDataInput data, int appVersionCode, ParcelFileDescriptor newState)
The application is being restored from backup and should replace any
existing data with the contents of the backup. The backup data is
provided through the data
parameter. Once
the restore is finished, the application should write a representation of
the final state to the newState
file descriptor.
The application is responsible for properly erasing its old data and replacing it with the data supplied to this method. No "clear user data" operation will be performed automatically by the operating system. The exception to this is in the case of a failed restore attempt: if onRestore() throws an exception, the OS will assume that the application's data may now be in an incoherent state, and will clear it before proceeding.
Parameters | |
---|---|
data |
BackupDataInput : A structured wrapper around an open, read-only
file descriptor pointing to a full snapshot of the
application's data. The application should consume every
entity represented in this data stream. |
appVersionCode |
int : The value of the android:versionCode manifest attribute,
from the application that backed up this particular data set. This
makes it possible for an application's agent to distinguish among any
possible older data versions when asked to perform the restore
operation. |
newState |
ParcelFileDescriptor : An open, read/write ParcelFileDescriptor pointing to an
empty file. The application should record the final backup
state here after restoring its data from the data stream.
When a full-backup dataset is being restored, this will be null . |
Throws | |
---|---|
IOException |
onRestore
public void onRestore (BackupDataInput data, long appVersionCode, ParcelFileDescriptor newState)
New version of onRestore(android.app.backup.BackupDataInput, int, android.os.ParcelFileDescriptor)
that handles a long app version code. Default implementation casts the version code to
an int and calls onRestore(android.app.backup.BackupDataInput, int, android.os.ParcelFileDescriptor)
.
Parameters | |
---|---|
data |
BackupDataInput |
appVersionCode |
long |
newState |
ParcelFileDescriptor |
Throws | |
---|---|
IOException |
onRestoreFile
public void onRestoreFile (ParcelFileDescriptor data, long size, File destination, int type, long mode, long mtime)
Handle the data delivered via the given file descriptor during a full restore operation. The agent is given the path to the file's original location as well as its size and metadata.
The file descriptor can only be read for size
bytes; attempting to read
more data has undefined behavior.
The default implementation creates the destination file/directory and populates it with the data from the file descriptor, then sets the file's access mode and modification time to match the restore arguments.
Parameters | |
---|---|
data |
ParcelFileDescriptor : A read-only file descriptor from which the agent can read size
bytes of file data. |
size |
long : The number of bytes of file content to be restored to the given
destination. If the file system object being restored is a directory, size
will be zero. |
destination |
File : The File on disk to be restored with the given data. |
type |
int : The kind of file system object being restored. This will be either
BackupAgent.TYPE_FILE or BackupAgent.TYPE_DIRECTORY . |
mode |
long : The access mode to be assigned to the destination after its data is
written. This is in the standard format used by chmod() . |
mtime |
long : The modification time of the file when it was backed up, suitable to
be assigned to the file after its data is written. |
Throws | |
---|---|
|
java.io.IOException |
IOException |
onRestoreFinished
public void onRestoreFinished ()
The application's restore operation has completed. This method is called after
all available data has been delivered to the application for restore (via either
the onRestore()
or
onRestoreFile()
callbacks). This provides the app with a stable end-of-restore opportunity to
perform any appropriate post-processing on the data that was just delivered.