Home | History | Annotate | Download | only in os 
      1 /*
      2  * Copyright (C) 2009 The Android Open Source Project
      3  *
      4  * Licensed under the Apache License, Version 2.0 (the "License");
      5  * you may not use this file except in compliance with the License.
      6  * You may obtain a copy of the License at
      7  *
      8  *      http://www.apache.org/licenses/LICENSE-2.0
      9  *
     10  * Unless required by applicable law or agreed to in writing, software
     11  * distributed under the License is distributed on an "AS IS" BASIS,
     12  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
     13  * See the License for the specific language governing permissions and
     14  * limitations under the License.
     15  */
     16 
     17 package com.android.internal.os;
     18 
     19 import android.os.FileUtils;
     20 import android.util.Log;
     21 
     22 import java.io.File;
     23 import java.io.FileInputStream;
     24 import java.io.FileNotFoundException;
     25 import java.io.FileOutputStream;
     26 import java.io.IOException;
     27 
     28 /**
     29  * Helper class for performing atomic operations on a file, by creating a
     30  * backup file until a write has successfully completed.
     31  * <p>
     32  * Atomic file guarantees file integrity by ensuring that a file has
     33  * been completely written and sync'd to disk before removing its backup.
     34  * As long as the backup file exists, the original file is considered
     35  * to be invalid (left over from a previous attempt to write the file).
     36  * </p><p>
     37  * Atomic file does not confer any file locking semantics.
     38  * Do not use this class when the file may be accessed or modified concurrently
     39  * by multiple threads or processes.  The caller is responsible for ensuring
     40  * appropriate mutual exclusion invariants whenever it accesses the file.
     41  * </p>
     42  */
     43 public class AtomicFile {
     44     private final File mBaseName;
     45     private final File mBackupName;
     46 
     47     public AtomicFile(File baseName) {
     48         mBaseName = baseName;
     49         mBackupName = new File(baseName.getPath() + ".bak");
     50     }
     51 
     52     public File getBaseFile() {
     53         return mBaseName;
     54     }
     55 
     56     public FileOutputStream startWrite() throws IOException {
     57         // Rename the current file so it may be used as a backup during the next read
     58         if (mBaseName.exists()) {
     59             if (!mBackupName.exists()) {
     60                 if (!mBaseName.renameTo(mBackupName)) {
     61                     Log.w("AtomicFile", "Couldn't rename file " + mBaseName
     62                             + " to backup file " + mBackupName);
     63                 }
     64             } else {
     65                 mBaseName.delete();
     66             }
     67         }
     68         FileOutputStream str = null;
     69         try {
     70             str = new FileOutputStream(mBaseName);
     71         } catch (FileNotFoundException e) {
     72             File parent = mBaseName.getParentFile();
     73             if (!parent.mkdir()) {
     74                 throw new IOException("Couldn't create directory " + mBaseName);
     75             }
     76             FileUtils.setPermissions(
     77                 parent.getPath(),
     78                 FileUtils.S_IRWXU|FileUtils.S_IRWXG|FileUtils.S_IXOTH,
     79                 -1, -1);
     80             try {
     81                 str = new FileOutputStream(mBaseName);
     82             } catch (FileNotFoundException e2) {
     83                 throw new IOException("Couldn't create " + mBaseName);
     84             }
     85         }
     86         return str;
     87     }
     88 
     89     public void finishWrite(FileOutputStream str) {
     90         if (str != null) {
     91             FileUtils.sync(str);
     92             try {
     93                 str.close();
     94                 mBackupName.delete();
     95             } catch (IOException e) {
     96                 Log.w("AtomicFile", "finishWrite: Got exception:", e);
     97             }
     98         }
     99     }
    100 
    101     public void failWrite(FileOutputStream str) {
    102         if (str != null) {
    103             FileUtils.sync(str);
    104             try {
    105                 str.close();
    106                 mBaseName.delete();
    107                 mBackupName.renameTo(mBaseName);
    108             } catch (IOException e) {
    109                 Log.w("AtomicFile", "failWrite: Got exception:", e);
    110             }
    111         }
    112     }
    113 
    114     public FileOutputStream openAppend() throws IOException {
    115         try {
    116             return new FileOutputStream(mBaseName, true);
    117         } catch (FileNotFoundException e) {
    118             throw new IOException("Couldn't append " + mBaseName);
    119         }
    120     }
    121 
    122     public void truncate() throws IOException {
    123         try {
    124             FileOutputStream fos = new FileOutputStream(mBaseName);
    125             FileUtils.sync(fos);
    126             fos.close();
    127         } catch (FileNotFoundException e) {
    128             throw new IOException("Couldn't append " + mBaseName);
    129         } catch (IOException e) {
    130         }
    131     }
    132 
    133     public FileInputStream openRead() throws FileNotFoundException {
    134         if (mBackupName.exists()) {
    135             mBaseName.delete();
    136             mBackupName.renameTo(mBaseName);
    137         }
    138         return new FileInputStream(mBaseName);
    139     }
    140 
    141     public byte[] readFully() throws IOException {
    142         FileInputStream stream = openRead();
    143         try {
    144             int pos = 0;
    145             int avail = stream.available();
    146             byte[] data = new byte[avail];
    147             while (true) {
    148                 int amt = stream.read(data, pos, data.length-pos);
    149                 //Log.i("foo", "Read " + amt + " bytes at " + pos
    150                 //        + " of avail " + data.length);
    151                 if (amt <= 0) {
    152                     //Log.i("foo", "**** FINISHED READING: pos=" + pos
    153                     //        + " len=" + data.length);
    154                     return data;
    155                 }
    156                 pos += amt;
    157                 avail = stream.available();
    158                 if (avail > data.length-pos) {
    159                     byte[] newData = new byte[pos+avail];
    160                     System.arraycopy(data, 0, newData, 0, pos);
    161                     data = newData;
    162                 }
    163             }
    164         } finally {
    165             stream.close();
    166         }
    167     }
    168 }
    169