Android example source code file (JournaledFile.java)

This example Android source code file (JournaledFile.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Android by Example" ^TM.

Java - Android tags/keywords

file, illegalstateexception, io, ioexception, journaledfile

The JournaledFile.java Android example source code

/*
 * 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.internal.util;

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();
    }
}