Android example source code file (BackupDataInput.java)

This example Android source code file (BackupDataInput.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Android by Example" ^TM.

Java - Android tags/keywords

backupdatainput, entity, entityheader, illegalstateexception, io, ioexception, native, nullpointerexception, runtimeexception, string, throwable

The BackupDataInput.java Android example source code

/*
 * Copyright (C) 2009 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package android.app.backup;

import java.io.FileDescriptor;
import java.io.IOException;

/**
 * Provides the structured interface through which a {@link BackupAgent} reads
 * information from the backup data set, via its
 * {@link BackupAgent#onRestore(BackupDataInput, int, android.os.ParcelFileDescriptor) onRestore()}
 * method.  The data is presented as a set of "entities," each
 * representing one named record as previously stored by the agent's
 * {@link BackupAgent#onBackup(ParcelFileDescriptor,BackupDataOutput,ParcelFileDescriptor)
 * onBackup()} implementation.  An entity is composed of a descriptive header plus a
 * byte array that holds the raw data saved in the remote backup.
 * <p>
 * The agent must consume every entity in the data stream, otherwise the
 * restored state of the application will be incomplete.
 * <h3>Example
 * <p>
 * A typical
 * {@link BackupAgent#onRestore(BackupDataInput,int,ParcelFileDescriptor)
 * onRestore()} implementation might be structured something like this:
 * <pre>
 * 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.
 *    }
 * }</pre>
 */
public class BackupDataInput {
    int mBackupReader;

    private EntityHeader mHeader = new EntityHeader();
    private boolean mHeaderReady;

    private static class EntityHeader {
        String key;
        int dataSize;
    }

    /** @hide */
    public BackupDataInput(FileDescriptor fd) {
        if (fd == null) throw new NullPointerException();
        mBackupReader = ctor(fd);
        if (mBackupReader == 0) {
            throw new RuntimeException("Native initialization failed with fd=" + fd);
        }
    }

    /** @hide */
    protected void finalize() throws Throwable {
        try {
            dtor(mBackupReader);
        } finally {
            super.finalize();
        }
    }

    /**
     * Extract the next entity header from the restore stream.  After this method
     * return success, the {@link #getKey()} and {@link #getDataSize()} methods can
     * be used to inspect the entity that is now available for processing.
     *
     * @return <code>true when there is an entity ready for consumption from the
     *    restore stream, <code>false if the restore stream has been fully consumed.
     * @throws IOException if an error occurred while reading the restore stream
     */
    public boolean readNextHeader() throws IOException {
        int result = readNextHeader_native(mBackupReader, mHeader);
        if (result == 0) {
            // read successfully
            mHeaderReady = true;
            return true;
        } else if (result > 0) {
            // done
            mHeaderReady = false;
            return false;
        } else {
            // error
            mHeaderReady = false;
            throw new IOException("failed: 0x" + Integer.toHexString(result));
        }
    }

    /**
     * Report the key associated with the current entity in the restore stream
     * @return the current entity's key string
     * @throws IllegalStateException if the next record header has not yet been read
     */
    public String getKey() {
        if (mHeaderReady) {
            return mHeader.key;
        } else {
            throw new IllegalStateException("Entity header not read");
        }
    }

    /**
     * Report the size in bytes of the data associated with the current entity in the
     * restore stream.
     *
     * @return The size of the record's raw data, in bytes
     * @throws IllegalStateException if the next record header has not yet been read
     */
    public int getDataSize() {
        if (mHeaderReady) {
            return mHeader.dataSize;
        } else {
            throw new IllegalStateException("Entity header not read");
        }
    }

    /**
     * Read a record's raw data from the restore stream.  The record's header must first
     * have been processed by the {@link #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.
     *
     * @param data An allocated byte array of at least 'size' bytes
     * @param offset Offset within the 'data' array at which the data will be placed
     *    when read from the stream
     * @param size The number of bytes to read in this pass
     * @return 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
     */
    public int readEntityData(byte[] data, int offset, int size) throws IOException {
        if (mHeaderReady) {
            int result = readEntityData_native(mBackupReader, data, offset, size);
            if (result >= 0) {
                return result;
            } else {
                throw new IOException("result=0x" + Integer.toHexString(result));
            }
        } else {
            throw new IllegalStateException("Entity header not read");
        }
    }

    /**
     * Consume the current entity's data without extracting it into a buffer
     * for further processing.  This allows a {@link android.app.backup.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
     */
    public void skipEntityData() throws IOException {
        if (mHeaderReady) {
            skipEntityData_native(mBackupReader);
        } else {
            throw new IllegalStateException("Entity header not read");
        }
    }

    private native static int ctor(FileDescriptor fd);
    private native static void dtor(int mBackupReader);

    private native int readNextHeader_native(int mBackupReader, EntityHeader entity);
    private native int readEntityData_native(int mBackupReader, byte[] data, int offset, int size);
    private native int skipEntityData_native(int mBackupReader);
}