Dianne Hackborn | 39606a0 | 2012-07-31 17:54:35 -0700 | [diff] [blame^] | 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 android.util; |
| 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. If you need this |
| 31 | * on older versions of the platform you can use |
| 32 | * {@link android.support.v4.util.AtomicFile} in the v4 support library. |
| 33 | * <p> |
| 34 | * Atomic file guarantees file integrity by ensuring that a file has |
| 35 | * been completely written and sync'd to disk before removing its backup. |
| 36 | * As long as the backup file exists, the original file is considered |
| 37 | * to be invalid (left over from a previous attempt to write the file). |
| 38 | * </p><p> |
| 39 | * Atomic file does not confer any file locking semantics. |
| 40 | * Do not use this class when the file may be accessed or modified concurrently |
| 41 | * by multiple threads or processes. The caller is responsible for ensuring |
| 42 | * appropriate mutual exclusion invariants whenever it accesses the file. |
| 43 | * </p> |
| 44 | */ |
| 45 | public class AtomicFile { |
| 46 | private final File mBaseName; |
| 47 | private final File mBackupName; |
| 48 | |
| 49 | /** |
| 50 | * Create a new AtomicFile for a file located at the given File path. |
| 51 | * The secondary backup file will be the same file path with ".bak" appended. |
| 52 | */ |
| 53 | public AtomicFile(File baseName) { |
| 54 | mBaseName = baseName; |
| 55 | mBackupName = new File(baseName.getPath() + ".bak"); |
| 56 | } |
| 57 | |
| 58 | /** |
| 59 | * Return the path to the base file. You should not generally use this, |
| 60 | * as the data at that path may not be valid. |
| 61 | */ |
| 62 | public File getBaseFile() { |
| 63 | return mBaseName; |
| 64 | } |
| 65 | |
| 66 | /** |
| 67 | * Delete the atomic file. This deletes both the base and backup files. |
| 68 | */ |
| 69 | public void delete() { |
| 70 | mBaseName.delete(); |
| 71 | mBackupName.delete(); |
| 72 | } |
| 73 | |
| 74 | /** |
| 75 | * Start a new write operation on the file. This returns a FileOutputStream |
| 76 | * to which you can write the new file data. The existing file is replaced |
| 77 | * with the new data. You <em>must not</em> directly close the given |
| 78 | * FileOutputStream; instead call either {@link #finishWrite(FileOutputStream)} |
| 79 | * or {@link #failWrite(FileOutputStream)}. |
| 80 | * |
| 81 | * <p>Note that if another thread is currently performing |
| 82 | * a write, this will simply replace whatever that thread is writing |
| 83 | * with the new file being written by this thread, and when the other |
| 84 | * thread finishes the write the new write operation will no longer be |
| 85 | * safe (or will be lost). You must do your own threading protection for |
| 86 | * access to AtomicFile. |
| 87 | */ |
| 88 | public FileOutputStream startWrite() throws IOException { |
| 89 | // Rename the current file so it may be used as a backup during the next read |
| 90 | if (mBaseName.exists()) { |
| 91 | if (!mBackupName.exists()) { |
| 92 | if (!mBaseName.renameTo(mBackupName)) { |
| 93 | Log.w("AtomicFile", "Couldn't rename file " + mBaseName |
| 94 | + " to backup file " + mBackupName); |
| 95 | } |
| 96 | } else { |
| 97 | mBaseName.delete(); |
| 98 | } |
| 99 | } |
| 100 | FileOutputStream str = null; |
| 101 | try { |
| 102 | str = new FileOutputStream(mBaseName); |
| 103 | } catch (FileNotFoundException e) { |
| 104 | File parent = mBaseName.getParentFile(); |
| 105 | if (!parent.mkdir()) { |
| 106 | throw new IOException("Couldn't create directory " + mBaseName); |
| 107 | } |
| 108 | FileUtils.setPermissions( |
| 109 | parent.getPath(), |
| 110 | FileUtils.S_IRWXU|FileUtils.S_IRWXG|FileUtils.S_IXOTH, |
| 111 | -1, -1); |
| 112 | try { |
| 113 | str = new FileOutputStream(mBaseName); |
| 114 | } catch (FileNotFoundException e2) { |
| 115 | throw new IOException("Couldn't create " + mBaseName); |
| 116 | } |
| 117 | } |
| 118 | return str; |
| 119 | } |
| 120 | |
| 121 | /** |
| 122 | * Call when you have successfully finished writing to the stream |
| 123 | * returned by {@link #startWrite()}. This will close, sync, and |
| 124 | * commit the new data. The next attempt to read the atomic file |
| 125 | * will return the new file stream. |
| 126 | */ |
| 127 | public void finishWrite(FileOutputStream str) { |
| 128 | if (str != null) { |
| 129 | FileUtils.sync(str); |
| 130 | try { |
| 131 | str.close(); |
| 132 | mBackupName.delete(); |
| 133 | } catch (IOException e) { |
| 134 | Log.w("AtomicFile", "finishWrite: Got exception:", e); |
| 135 | } |
| 136 | } |
| 137 | } |
| 138 | |
| 139 | /** |
| 140 | * Call when you have failed for some reason at writing to the stream |
| 141 | * returned by {@link #startWrite()}. This will close the current |
| 142 | * write stream, and roll back to the previous state of the file. |
| 143 | */ |
| 144 | public void failWrite(FileOutputStream str) { |
| 145 | if (str != null) { |
| 146 | FileUtils.sync(str); |
| 147 | try { |
| 148 | str.close(); |
| 149 | mBaseName.delete(); |
| 150 | mBackupName.renameTo(mBaseName); |
| 151 | } catch (IOException e) { |
| 152 | Log.w("AtomicFile", "failWrite: Got exception:", e); |
| 153 | } |
| 154 | } |
| 155 | } |
| 156 | |
| 157 | /** @hide |
| 158 | * @deprecated This is not safe. |
| 159 | */ |
| 160 | @Deprecated public void truncate() throws IOException { |
| 161 | try { |
| 162 | FileOutputStream fos = new FileOutputStream(mBaseName); |
| 163 | FileUtils.sync(fos); |
| 164 | fos.close(); |
| 165 | } catch (FileNotFoundException e) { |
| 166 | throw new IOException("Couldn't append " + mBaseName); |
| 167 | } catch (IOException e) { |
| 168 | } |
| 169 | } |
| 170 | |
| 171 | /** @hide |
| 172 | * @deprecated This is not safe. |
| 173 | */ |
| 174 | @Deprecated public FileOutputStream openAppend() throws IOException { |
| 175 | try { |
| 176 | return new FileOutputStream(mBaseName, true); |
| 177 | } catch (FileNotFoundException e) { |
| 178 | throw new IOException("Couldn't append " + mBaseName); |
| 179 | } |
| 180 | } |
| 181 | |
| 182 | /** |
| 183 | * Open the atomic file for reading. If there previously was an |
| 184 | * incomplete write, this will roll back to the last good data before |
| 185 | * opening for read. You should call close() on the FileInputStream when |
| 186 | * you are done reading from it. |
| 187 | * |
| 188 | * <p>Note that if another thread is currently performing |
| 189 | * a write, this will incorrectly consider it to be in the state of a bad |
| 190 | * write and roll back, causing the new data currently being written to |
| 191 | * be dropped. You must do your own threading protection for access to |
| 192 | * AtomicFile. |
| 193 | */ |
| 194 | public FileInputStream openRead() throws FileNotFoundException { |
| 195 | if (mBackupName.exists()) { |
| 196 | mBaseName.delete(); |
| 197 | mBackupName.renameTo(mBaseName); |
| 198 | } |
| 199 | return new FileInputStream(mBaseName); |
| 200 | } |
| 201 | |
| 202 | /** |
| 203 | * A convenience for {@link #openRead()} that also reads all of the |
| 204 | * file contents into a byte array which is returned. |
| 205 | */ |
| 206 | public byte[] readFully() throws IOException { |
| 207 | FileInputStream stream = openRead(); |
| 208 | try { |
| 209 | int pos = 0; |
| 210 | int avail = stream.available(); |
| 211 | byte[] data = new byte[avail]; |
| 212 | while (true) { |
| 213 | int amt = stream.read(data, pos, data.length-pos); |
| 214 | //Log.i("foo", "Read " + amt + " bytes at " + pos |
| 215 | // + " of avail " + data.length); |
| 216 | if (amt <= 0) { |
| 217 | //Log.i("foo", "**** FINISHED READING: pos=" + pos |
| 218 | // + " len=" + data.length); |
| 219 | return data; |
| 220 | } |
| 221 | pos += amt; |
| 222 | avail = stream.available(); |
| 223 | if (avail > data.length-pos) { |
| 224 | byte[] newData = new byte[pos+avail]; |
| 225 | System.arraycopy(data, 0, newData, 0, pos); |
| 226 | data = newData; |
| 227 | } |
| 228 | } |
| 229 | } finally { |
| 230 | stream.close(); |
| 231 | } |
| 232 | } |
| 233 | } |