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Xodus is pure Java transactional schema-less embedded database

There is a newer version: 2.0.1

/**
 * Copyright 2010 - 2018 JetBrains s.r.o.
 *
 * 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 jetbrains.exodus.util;

import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.util.concurrent.atomic.AtomicInteger;

public class SharedRandomAccessFile extends RandomAccessFile {

    private final File file;
    private final AtomicInteger clients;

    public SharedRandomAccessFile(File file, String mode) throws FileNotFoundException {
        super(file, mode);
        this.file = file;
        clients = new AtomicInteger();
    }

    public File getFile() {
        return file;
    }

    /**
     * Employs the file and returns resulting number of its clients.
     *
     * @return number of clients of the file.
     */
    public int employ() {
        return clients.incrementAndGet();
    }

    public void close() throws IOException {
        if (clients.getAndDecrement() == 0) {
            super.close();
        }
    }
}