v4/util/AtomicFile.java

/*
 * 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.support.v4.util;

import android.util.Log;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;

/**
 * Static library support version of the framework's {@link android.util.AtomicFile},
 * a helper class for performing atomic operations on a file by creating a
 * backup file until a write has successfully completed.
 * <p>
 * Atomic file guarantees file integrity by ensuring that a file has
 * been completely written and sync'd to disk before removing its backup.
 * As long as the backup file exists, the original file is considered
 * to be invalid (left over from a previous attempt to write the file).
 * </p><p>
 * Atomic file does not confer any file locking semantics.
 * Do not use this class when the file may be accessed or modified concurrently
 * by multiple threads or processes.  The caller is responsible for ensuring
 * appropriate mutual exclusion invariants whenever it accesses the file.
 * </p>
 */
public class AtomicFile {
    private final File mBaseName;
    private final File mBackupName;

    /**
     * Create a new AtomicFile for a file located at the given File path.
     * The secondary backup file will be the same file path with ".bak" appended.
     */
    public AtomicFile(File baseName) {
        mBaseName = baseName;
        mBackupName = new File(baseName.getPath() + ".bak");
    }

    /**
     * Return the path to the base file.  You should not generally use this,
     * as the data at that path may not be valid.
     */
    public File getBaseFile() {
        return mBaseName;
    }

    /**
     * Delete the atomic file.  This deletes both the base and backup files.
     */
    public void delete() {
        mBaseName.delete();
        mBackupName.delete();
    }

    /**
     * Start a new write operation on the file.  This returns a FileOutputStream
     * to which you can write the new file data.  The existing file is replaced
     * with the new data.  You <em>must not</em> directly close the given
     * FileOutputStream; instead call either {@link #finishWrite(FileOutputStream)}
     * or {@link #failWrite(FileOutputStream)}.
     *
     * <p>Note that if another thread is currently performing
     * a write, this will simply replace whatever that thread is writing
     * with the new file being written by this thread, and when the other
     * thread finishes the write the new write operation will no longer be
     * safe (or will be lost).  You must do your own threading protection for
     * access to AtomicFile.
     */
    public FileOutputStream startWrite() throws IOException {
        // Rename the current file so it may be used as a backup during the next read
        if (mBaseName.exists()) {
            if (!mBackupName.exists()) {
                if (!mBaseName.renameTo(mBackupName)) {
                    Log.w("AtomicFile", "Couldn't rename file " + mBaseName
                            + " to backup file " + mBackupName);
                }
            } else {
                mBaseName.delete();
            }
        }
        FileOutputStream str = null;
        try {
            str = new FileOutputStream(mBaseName);
        } catch (FileNotFoundException e) {
            File parent = mBaseName.getParentFile();
            if (!parent.mkdir()) {
                throw new IOException("Couldn't create directory " + mBaseName);
            }
            try {
                str = new FileOutputStream(mBaseName);
            } catch (FileNotFoundException e2) {
                throw new IOException("Couldn't create " + mBaseName);
            }
        }
        return str;
    }

    /**
     * Call when you have successfully finished writing to the stream
     * returned by {@link #startWrite()}.  This will close, sync, and
     * commit the new data.  The next attempt to read the atomic file
     * will return the new file stream.
     */
    public void finishWrite(FileOutputStream str) {
        if (str != null) {
            sync(str);
            try {
                str.close();
                mBackupName.delete();
            } catch (IOException e) {
                Log.w("AtomicFile", "finishWrite: Got exception:", e);
            }
        }
    }

    /**
     * Call when you have failed for some reason at writing to the stream
     * returned by {@link #startWrite()}.  This will close the current
     * write stream, and roll back to the previous state of the file.
     */
    public void failWrite(FileOutputStream str) {
        if (str != null) {
            sync(str);
            try {
                str.close();
                mBaseName.delete();
                mBackupName.renameTo(mBaseName);
            } catch (IOException e) {
                Log.w("AtomicFile", "failWrite: Got exception:", e);
            }
        }
    }

    /**
     * Open the atomic file for reading.  If there previously was an
     * incomplete write, this will roll back to the last good data before
     * opening for read.  You should call close() on the FileInputStream when
     * you are done reading from it.
     *
     * <p>Note that if another thread is currently performing
     * a write, this will incorrectly consider it to be in the state of a bad
     * write and roll back, causing the new data currently being written to
     * be dropped.  You must do your own threading protection for access to
     * AtomicFile.
     */
    public FileInputStream openRead() throws FileNotFoundException {
        if (mBackupName.exists()) {
            mBaseName.delete();
            mBackupName.renameTo(mBaseName);
        }
        return new FileInputStream(mBaseName);
    }

    /**
     * A convenience for {@link #openRead()} that also reads all of the
     * file contents into a byte array which is returned.
     */
    public byte[] readFully() throws IOException {
        FileInputStream stream = openRead();
        try {
            int pos = 0;
            int avail = stream.available();
            byte[] data = new byte[avail];
            while (true) {
                int amt = stream.read(data, pos, data.length-pos);
                //Log.i("foo", "Read " + amt + " bytes at " + pos
                //        + " of avail " + data.length);
                if (amt <= 0) {
                    //Log.i("foo", "**** FINISHED READING: pos=" + pos
                    //        + " len=" + data.length);
                    return data;
                }
                pos += amt;
                avail = stream.available();
                if (avail > data.length-pos) {
                    byte[] newData = new byte[pos+avail];
                    System.arraycopy(data, 0, newData, 0, pos);
                    data = newData;
                }
            }
        } finally {
            stream.close();
        }
    }

    static boolean sync(FileOutputStream stream) {
        try {
            if (stream != null) {
                stream.getFD().sync();
            }
            return true;
        } catch (IOException e) {
        }
        return false;
    }
}