blob: 3d1f52db34b3e76496b3b1f2dfb9d671bab9cefb [file] [log] [blame]
/*
* 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 com.android.server;
import java.io.File;
import java.io.IOException;
public class JournaledFile {
File mReal;
File mTemp;
boolean mWriting;
public JournaledFile(File real, File temp) {
mReal = real;
mTemp = temp;
}
/** Returns the file for you to read.
* @more
* Prefers the real file. If it doesn't exist, uses the temp one, and then copies
* it to the real one. If there is both a real file and a temp one, assumes that the
* temp one isn't fully written and deletes it.
*/
public File chooseForRead() {
File result;
if (mReal.exists()) {
result = mReal;
if (mTemp.exists()) {
mTemp.delete();
}
} else if (mTemp.exists()) {
result = mTemp;
mTemp.renameTo(mReal);
} else {
return mReal;
}
return result;
}
/**
* Returns a file for you to write.
* @more
* If a write is already happening, throws. In other words, you must provide your
* own locking.
* <p>
* Call {@link #commit} to commit the changes, or {@link #rollback} to forget the changes.
*/
public File chooseForWrite() {
if (mWriting) {
throw new IllegalStateException("uncommitted write already in progress");
}
if (!mReal.exists()) {
// If the real one doesn't exist, it's either because this is the first time
// or because something went wrong while copying them. In this case, we can't
// trust anything that's in temp. In order to have the chooseForRead code not
// use the temporary one until it's fully written, create an empty file
// for real, which will we'll shortly delete.
try {
mReal.createNewFile();
} catch (IOException e) {
// Ignore
}
}
if (mTemp.exists()) {
mTemp.delete();
}
mWriting = true;
return mTemp;
}
/**
* Commit changes.
*/
public void commit() {
if (!mWriting) {
throw new IllegalStateException("no file to commit");
}
mWriting = false;
mTemp.renameTo(mReal);
}
/**
* Roll back changes.
*/
public void rollback() {
if (!mWriting) {
throw new IllegalStateException("no file to roll back");
}
mWriting = false;
mTemp.delete();
}
}