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Android Reference

BackupDataInput


android.app.backup

Class BackupDataInput



  • public class BackupDataInput
    extends Object
    Provides the structured interface through which a 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.

    Example

    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.
        }
     }
    • Method Summary

      Methods
      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.
    • Method Detail

      • finalize

        protected void finalize()
                         throws Throwable
        Description copied from class: Object
        Invoked when the garbage collector has detected that this instance is no longer reachable. The default implementation does nothing, but this method can be overridden to free resources.

        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.

        Overrides:
        finalize in class Object
        Throws:
        Throwable
      • readNextHeader

        public boolean readNextHeader()
                               throws IOException
        Extract the next entity header from the restore stream. After this method return success, the getKey() and getDataSize() methods can be used to inspect the entity that is now available for processing.
        Returns:
        true when there is an entity ready for consumption from the restore stream, false if the restore stream has been fully consumed.
        Throws:
        IOException - if an error occurred while reading the restore stream
      • getKey

        public String getKey()
        Report the key associated with the current entity in the restore stream
        Returns:
        the current entity's key string
        Throws:
        IllegalStateException - if the next record header has not yet been read
      • getDataSize

        public int getDataSize()
        Report the size in bytes of the data associated with the current entity in the restore stream.
        Returns:
        The size of the record's raw data, in bytes
        Throws:
        IllegalStateException - if the next record header has not yet been read
      • readEntityData

        public int readEntityData(byte[] data,
                         int offset,
                         int size)
                           throws IOException
        Read a record's raw data from the restore stream. The record's header must first have been processed by the 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.
        Parameters:
        data - An allocated byte array of at least 'size' bytes
        offset - Offset within the 'data' array at which the data will be placed when read from the stream
        size - The number of bytes to read in this pass
        Returns:
        The number of bytes of data read. Once all of the data for this entity has been read, further calls to this method will return zero.
        Throws:
        IOException - if an error occurred when trying to read the restore data stream
      • skipEntityData

        public void skipEntityData()
                            throws IOException
        Consume the current entity's data without extracting it into a buffer for further processing. This allows a BackupAgent to efficiently discard obsolete or otherwise uninteresting records during the restore operation.
        Throws:
        IOException - if an error occurred when trying to read the restore data stream


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