public class BackupDataInput extends Object
BackupAgent
reads
information from the backup data set, via its
onRestore()
method. The data is presented as a set of "entities," each
representing one named record as previously stored by the agent's
onBackup()
implementation. An entity is composed of a descriptive header plus a
byte array that holds the raw data saved in the remote backup.
The agent must consume every entity in the data stream, otherwise the restored state of the application will be incomplete.
A typical
onRestore()
implementation might be structured something like this:
public void onRestore(BackupDataInput data, int appVersionCode, ParcelFileDescriptor newState) { while (data.readNextHeader()) { String key = data.getKey(); int dataSize = data.getDataSize(); if (key.equals(MY_BACKUP_KEY_ONE)) { // process this kind of record here byte[] buffer = new byte[dataSize]; data.readEntityData(buffer, 0, dataSize); // reads the entire entity at once // now 'buffer' holds the raw data and can be processed however // the agent wishes processBackupKeyOne(buffer); } else if (key.equals(MY_BACKUP_KEY_TO_IGNORE) { // a key we recognize but wish to discard data.skipEntityData(); } // ... etc. } }
Constructor and Description |
---|
BackupDataInput(FileDescriptor fd) |
Modifier and Type | Method and Description |
---|---|
protected void |
finalize()
Invoked when the garbage collector has detected that this instance is no longer reachable.
|
int |
getDataSize()
Report the size in bytes of the data associated with the current entity in the
restore stream.
|
String |
getKey()
Report the key associated with the current entity in the restore stream
|
int |
readEntityData(byte[] data,
int offset,
int size)
Read a record's raw data from the restore stream.
|
boolean |
readNextHeader()
Extract the next entity header from the restore stream.
|
void |
skipEntityData()
Consume the current entity's data without extracting it into a buffer
for further processing.
|
public BackupDataInput(FileDescriptor fd)
protected void finalize() throws Throwable
Object
Note that objects that override finalize
are significantly more expensive than
objects that don't. Finalizers may be run a long time after the object is no longer
reachable, depending on memory pressure, so it's a bad idea to rely on them for cleanup.
Note also that finalizers are run on a single VM-wide finalizer thread,
so doing blocking work in a finalizer is a bad idea. A finalizer is usually only necessary
for a class that has a native peer and needs to call a native method to destroy that peer.
Even then, it's better to provide an explicit close
method (and implement
Closeable
), and insist that callers manually dispose of instances. This
works well for something like files, but less well for something like a BigInteger
where typical calling code would have to deal with lots of temporaries. Unfortunately,
code that creates lots of temporaries is the worst kind of code from the point of view of
the single finalizer thread.
If you must use finalizers, consider at least providing your own
ReferenceQueue
and having your own thread process that queue.
Unlike constructors, finalizers are not automatically chained. You are responsible for
calling super.finalize()
yourself.
Uncaught exceptions thrown by finalizers are ignored and do not terminate the finalizer thread. See Effective Java Item 7, "Avoid finalizers" for more.
public boolean readNextHeader() throws IOException
getKey()
and getDataSize()
methods can
be used to inspect the entity that is now available for processing.true
when there is an entity ready for consumption from the
restore stream, false
if the restore stream has been fully consumed.IOException
- if an error occurred while reading the restore streampublic String getKey()
IllegalStateException
- if the next record header has not yet been readpublic int getDataSize()
IllegalStateException
- if the next record header has not yet been readpublic int readEntityData(byte[] data, int offset, int size) throws IOException
readNextHeader()
method. Multiple calls to
this method may be made in order to process the data in chunks; not all of it
must be read in a single call. Once all of the raw data for the current entity
has been read, further calls to this method will simply return zero.data
- An allocated byte array of at least 'size' bytesoffset
- Offset within the 'data' array at which the data will be placed
when read from the streamsize
- The number of bytes to read in this passIOException
- if an error occurred when trying to read the restore data streampublic void skipEntityData() throws IOException
BackupAgent
to
efficiently discard obsolete or otherwise uninteresting records during the
restore operation.IOException
- if an error occurred when trying to read the restore data stream