vincent4vx / SingleInstance

 * This file is part of SingleInstance.

 *

 * SingleInstance is free software: you can redistribute it and/or modify

 * it under the terms of the GNU Lesser General Public License as published by

 * the Free Software Foundation, either version 3 of the License, or

 * (at your option) any later version.

 *

 * SingleInstance is distributed in the hope that it will be useful,

 * but WITHOUT ANY WARRANTY; without even the implied warranty of

 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the

 * GNU Lesser General Public License for more details.

 *

 * You should have received a copy of the GNU Lesser General Public License

 * along with SingleInstance.  If not, see <https://www.gnu.org/licenses/>.

 *

 * Copyright (c) 2020 Vincent Quatrevieux

 */

package fr.quatrevieux.singleinstance;

import java.io.*;

import java.nio.channels.FileLock;

import java.util.HashMap;

import java.util.Map;

import java.util.Objects;

/**

 * Handle lock file

 * A lock file can be acquire to block all other process, until it's released

 *

 * <pre>{@code

 *     LockFile lock = new LockFile("app.lock");

 *

 *     if (!lock.acquire()) {

 *         System.out.println("An other instance is running");

 *         System.exit(1);

 *     }

 *

 *     Runtime.getRuntime().addShutdownHook(new Thread(lock::release));

 * }</pre>

 */

final public class LockFile {

    @FunctionalInterface

    public interface Writer {

        public void write(DataOutput output) throws IOException;

    }

    @FunctionalInterface

    public interface Reader<R> {

        public R read(DataInput input) throws IOException;

    }

    /**

     * Handle sharing the FileLock instances

     */

    static private class SharedLock {

        @FunctionalInterface

        private interface FileLockFactory {

            public FileLock create() throws IOException;

        }

        /**

         * Store all lock instances

         */

        final static private Map<File, SharedLock> instances = new HashMap<>();

        final private File file;

        final private FileLock fileLock;

        private int count = 1;

        public SharedLock(File file, FileLock fileLock) {

            this.file = file;

            this.fileLock = fileLock;

        }

        /**

         * Try to remove the shared lock

         *

         * @return true if the lock is removed

         */

        private boolean remove() throws IOException {

            synchronized (instances) {

                --count;

                if (count <= 0) {

                    fileLock.release();

                    instances.remove(file);

                    return true;

                }

                return false;

            }

        }

        /**

         * Create the SharedLock instance

         *

         * @return The lock instance, or null if cannot get the lock

         */

        static private SharedLock create(File file, FileLockFactory factory) throws IOException {

            synchronized (instances) {

                SharedLock sharedLock = instances.get(file);

                if (sharedLock != null) {

                    ++sharedLock.count;

                    return sharedLock;

                }

                FileLock lock = factory.create();

                if (lock == null) {

                    return null;

                }

                sharedLock = new SharedLock(file, lock);

                instances.put(file, sharedLock);

                return sharedLock;

            }

        }

    }

    final static public String DEFAULT_FILENAME = ".lock";

    final private File file;

    private RandomAccessFile randomAccessFile;

    private SharedLock lock;

    /**

     * Create the lock file using the default file name

     */

    public LockFile() {

        this(DEFAULT_FILENAME);

    }

    public LockFile(String filename) {

        this(new File(filename));

    }

    public LockFile(File file) {

        this.file = file.getAbsoluteFile();

    }

    /**

     * Acquire the lock file

     * This method is not blocking : if the lock file is already acquired, false is immediately returned

     *

     * @return true if the lock is acquired, or false if the lock is already acquired

     * @throws IOException When lock file creation failed

     */

    public boolean acquire() throws IOException {

        if (lock == null) {

            lock = SharedLock.create(file, () -> file().getChannel().tryLock());

        }

        return lock != null;

    }

    /**

     * Wait until lock is release, and then acquire the lock file

     * This method is blocking

     *

     * @throws IOException When lock file creation failed

     */

    public void lock() throws IOException {

        if (lock == null) {

            lock = SharedLock.create(file, () -> file().getChannel().lock());

        }

    }

    /**

     * Release the lock file

     * This method will never fail

     */

    public void release() {

        try {

            boolean deleteFile = false;

            if (lock != null) {

                deleteFile = lock.remove();

                lock = null;

            }

            if (randomAccessFile != null) {

                randomAccessFile.close();

                randomAccessFile = null;

            }

            if (deleteFile) {

                file.delete();

            }

        } catch (IOException e) {

            // ignore

        }

    }

    /**

     * Write data to the lock file

     * The file is truncated before write

     *

     * @param writer Write operation

     * @throws IOException When write failed

     */

    public void write(Writer writer) throws IOException {

        if (!acquire()) {

            throw new IllegalStateException("Cannot acquire write access to lock file");

        }

        randomAccessFile.getChannel().truncate(0);

        writer.write(randomAccessFile);

    }

    /**

     * Read data from lock file

     * The cursor is set to start of the file before read

     *

     * @param reader The data reader

     * @param <R> The return type

     *

     * @return The read data

     * @throws IOException When read failed

     */

    public <R> R read(Reader<R> reader) throws IOException {

        final RandomAccessFile file = file();

        file.seek(0);

        return reader.read(file());

    }

    @Override

    protected void finalize() throws Throwable {

        super.finalize();

        release();

    }

    @Override

    public int hashCode() {

        return Objects.hash(file);

    }

    private RandomAccessFile file() throws FileNotFoundException {

        if (randomAccessFile == null) {

            randomAccessFile = new RandomAccessFile(file, "rw");

        }

        return randomAccessFile;

    }

}