108 lines
3.1 KiB
Java
108 lines
3.1 KiB
Java
/*
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* Copyright (C) 2009 The Android Open Source Project
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*/
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package com.android.server;
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import java.io.File;
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import java.io.IOException;
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public class JournaledFile {
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File mReal;
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File mTemp;
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boolean mWriting;
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public JournaledFile(File real, File temp) {
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mReal = real;
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mTemp = temp;
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}
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/** Returns the file for you to read.
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* @more
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* Prefers the real file. If it doesn't exist, uses the temp one, and then copies
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* it to the real one. If there is both a real file and a temp one, assumes that the
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* temp one isn't fully written and deletes it.
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*/
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public File chooseForRead() {
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File result;
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if (mReal.exists()) {
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result = mReal;
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if (mTemp.exists()) {
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mTemp.delete();
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}
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} else if (mTemp.exists()) {
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result = mTemp;
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mTemp.renameTo(mReal);
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} else {
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return mReal;
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}
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return result;
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}
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/**
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* Returns a file for you to write.
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* @more
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* If a write is already happening, throws. In other words, you must provide your
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* own locking.
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* <p>
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* Call {@link #commit} to commit the changes, or {@link #rollback} to forget the changes.
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*/
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public File chooseForWrite() {
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if (mWriting) {
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throw new IllegalStateException("uncommitted write already in progress");
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}
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if (!mReal.exists()) {
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// If the real one doesn't exist, it's either because this is the first time
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// or because something went wrong while copying them. In this case, we can't
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// trust anything that's in temp. In order to have the chooseForRead code not
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// use the temporary one until it's fully written, create an empty file
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// for real, which will we'll shortly delete.
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try {
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mReal.createNewFile();
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} catch (IOException e) {
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// Ignore
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}
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}
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if (mTemp.exists()) {
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mTemp.delete();
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}
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mWriting = true;
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return mTemp;
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}
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/**
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* Commit changes.
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*/
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public void commit() {
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if (!mWriting) {
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throw new IllegalStateException("no file to commit");
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}
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mWriting = false;
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mTemp.renameTo(mReal);
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}
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/**
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* Roll back changes.
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*/
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public void rollback() {
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if (!mWriting) {
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throw new IllegalStateException("no file to roll back");
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}
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mWriting = false;
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mTemp.delete();
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}
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}
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