Joe Onorato | 9bb8fd7 | 2009-07-28 18:24:51 -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 com.android.server; |
| 18 | |
| 19 | import java.io.File; |
| 20 | import java.io.IOException; |
| 21 | |
| 22 | public class JournaledFile { |
| 23 | File mReal; |
| 24 | File mTemp; |
| 25 | boolean mWriting; |
| 26 | |
| 27 | public JournaledFile(File real, File temp) { |
| 28 | mReal = real; |
| 29 | mTemp = temp; |
| 30 | } |
| 31 | |
| 32 | /** Returns the file for you to read. |
| 33 | * @more |
| 34 | * Prefers the real file. If it doesn't exist, uses the temp one, and then copies |
| 35 | * it to the real one. If there is both a real file and a temp one, assumes that the |
| 36 | * temp one isn't fully written and deletes it. |
| 37 | */ |
| 38 | public File chooseForRead() { |
| 39 | File result; |
| 40 | if (mReal.exists()) { |
| 41 | result = mReal; |
| 42 | if (mTemp.exists()) { |
| 43 | mTemp.delete(); |
| 44 | } |
| 45 | } else if (mTemp.exists()) { |
| 46 | result = mTemp; |
| 47 | mTemp.renameTo(mReal); |
| 48 | } else { |
| 49 | return mReal; |
| 50 | } |
| 51 | return result; |
| 52 | } |
| 53 | |
| 54 | /** |
| 55 | * Returns a file for you to write. |
| 56 | * @more |
| 57 | * If a write is already happening, throws. In other words, you must provide your |
| 58 | * own locking. |
| 59 | * <p> |
| 60 | * Call {@link #commit} to commit the changes, or {@link #rollback} to forget the changes. |
| 61 | */ |
| 62 | public File chooseForWrite() { |
| 63 | if (mWriting) { |
| 64 | throw new IllegalStateException("uncommitted write already in progress"); |
| 65 | } |
| 66 | if (!mReal.exists()) { |
| 67 | // If the real one doesn't exist, it's either because this is the first time |
| 68 | // or because something went wrong while copying them. In this case, we can't |
| 69 | // trust anything that's in temp. In order to have the chooseForRead code not |
| 70 | // use the temporary one until it's fully written, create an empty file |
| 71 | // for real, which will we'll shortly delete. |
| 72 | try { |
| 73 | mReal.createNewFile(); |
| 74 | } catch (IOException e) { |
| 75 | // Ignore |
| 76 | } |
| 77 | } |
| 78 | |
| 79 | if (mTemp.exists()) { |
| 80 | mTemp.delete(); |
| 81 | } |
| 82 | mWriting = true; |
| 83 | return mTemp; |
| 84 | } |
| 85 | |
| 86 | /** |
| 87 | * Commit changes. |
| 88 | */ |
| 89 | public void commit() { |
| 90 | if (!mWriting) { |
| 91 | throw new IllegalStateException("no file to commit"); |
| 92 | } |
| 93 | mWriting = false; |
| 94 | mTemp.renameTo(mReal); |
| 95 | } |
| 96 | |
| 97 | /** |
| 98 | * Roll back changes. |
| 99 | */ |
| 100 | public void rollback() { |
| 101 | if (!mWriting) { |
| 102 | throw new IllegalStateException("no file to roll back"); |
| 103 | } |
| 104 | mWriting = false; |
| 105 | mTemp.delete(); |
| 106 | } |
| 107 | } |