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/*
* Copyright @ 2015 Atlassian Pty Ltd
*
* 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 org.jitsi.impl.fileaccess;
import java.io.*;
import java.nio.file.*;
import org.jitsi.service.fileaccess.*;
/**
* A failsafe transaction class. By failsafe we mean here that the file
* concerned always stays in a coherent state. This class use the transactional
* model.
*
* @author Benoit Pradelle
*/
public class FailSafeTransactionImpl
implements FailSafeTransaction
{
/**
* Original file used by the transaction
*/
private File file;
/**
* Backup file used by the transaction
*/
private File backup;
/**
* Extension of a partial file
*/
private static final String PART_EXT = ".part";
/**
* Extension of a backup copy
*/
private static final String BAK_EXT = ".bak";
/**
* Creates a new transaction.
*
* @param file The file associated with this transaction
*
* @throws NullPointerException if the file is null
*/
protected FailSafeTransactionImpl(File file)
throws NullPointerException
{
if (file == null)
{
throw new NullPointerException("null file provided");
}
this.file = file;
this.backup = null;
}
/**
* Ensure that the file accessed is in a coherent state. This function is
* useful to do a failsafe read without starting a transaction.
*
* @throws IllegalStateException if the file doesn't exists anymore
* @throws IOException if an IOException occurs during the file restoration
*/
public void restoreFile() throws IllegalStateException, IOException
{
File back = new File(this.file.getAbsolutePath() + BAK_EXT);
// if a backup copy is still present, simply restore it
if (back.exists())
{
failsafeCopy(back, this.file);
back.delete();
}
}
/**
* Begins a new transaction. If a transaction is already active, commits the
* changes and begin a new transaction.
* A transaction can be closed by a commit or rollback operation.
* When the transaction begins, the file is restored to a coherent state if
* needed.
*
* @throws IllegalStateException if the file doesn't exists anymore
* @throws IOException if an IOException occurs during the transaction
* creation
*/
public void beginTransaction()
throws IllegalStateException, IOException
{
// if the last transaction hasn't been closed, commit it
if (this.backup != null)
{
this.commit();
}
// if needed, restore the file in its previous state
restoreFile();
this.backup = new File(this.file.getAbsolutePath() + BAK_EXT);
// else backup the current file
failsafeCopy(this.file, this.backup);
}
/**
* Closes the transaction and commit the changes. Everything written in the
* file during the transaction is saved.
*
* @throws IllegalStateException if the file doesn't exists anymore
*/
public void commit() throws IllegalStateException
{
if (this.backup == null)
{
return;
}
// simply delete the backup file
this.backup.delete();
this.backup = null;
}
/**
* Closes the transaction and cancels the changes. Everything written in the
* file during the transaction is NOT saved.
* @throws IllegalStateException if the file doesn't exists anymore
* @throws IOException if an IOException occurs during the operation
*/
public void rollback() throws IllegalStateException, IOException
{
if (this.backup == null)
{
return;
}
// restore the backup and delete it
failsafeCopy(this.backup, this.file);
this.backup.delete();
this.backup = null;
}
/**
* Copy a file in a fail-safe way. The destination is created in an atomic
* way.
*
* @param from The file to copy
* @param to The copy to create
*
* @throws IllegalStateException if the file doesn't exists anymore
* @throws IOException if an IOException occurs during the operation
*/
private void failsafeCopy(File from, File to)
throws IllegalStateException, IOException
{
// to ensure a perfect copy, delete the destination if it exists
Files.deleteIfExists(to.toPath());
File ptoF = new File(to.getAbsolutePath() + PART_EXT);
Files.deleteIfExists(ptoF.toPath());
Files.copy(from.toPath(), ptoF.toPath());
// once done, rename the partial file to the final copy
Files.move(ptoF.toPath(), to.toPath(), StandardCopyOption.ATOMIC_MOVE);
}
}
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