PortUtils.java

package com.splout.db.common;

/*
 * #%L
 * Splout SQL commons
 * %%
 * Copyright (C) 2012 - 2014 Datasalt Systems S.L.
 * %%
 * 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.
 * #L%
 */

import java.io.File;
import java.io.IOException;
import java.io.RandomAccessFile;
import java.net.ServerSocket;
import java.nio.channels.FileChannel;
import java.nio.channels.FileLock;

public class PortUtils {


  public static class PortLock {
    final int port;
    final FileLock lock;
    final File file;

    public PortLock(int port, FileLock lock, File file) {
      this.port = port;
      this.lock = lock;
      this.file = file;
    }

    public int getPort() {
      return port;
    }

    public void release() {
      try {
        lock.release();
      } catch (IOException e) {
      }
      file.delete();
    }
  }

  public static PortLock getNextAvailablePort(int port) {
    // Look for next available port
    FileLock lock = null;
    File lockFile = null;
    boolean free = false;
    do {
      try {
        ServerSocket socket = new ServerSocket(port);
        socket.close();
        /*
         * It's actually unsafe to assume the port will still be free when using it after calling this method. And
				 * "mysqld" can't handle well the situation where two daemons are started with the same port concurrently.
				 * Therefore we need to ensure that only ONE PROCESS locks the port at a time. We use NIO FileLock for that.
				 * Because this is fast, we lock on a temporary file and release it afterwards.
				 */
        lockFile = new File("/tmp", "portlock_" + port); // can't use java.io.tmpdir as it is overriden by Hadoop
        if (!lockFile.exists()) {
          lockFile.createNewFile();
          FileChannel channel = new RandomAccessFile(lockFile, "rw").getChannel();
          lock = channel.tryLock();
          if (lock != null) {
            free = true;
          }
        }
        if (!free) {
          port++;
        }
      } catch (Exception e) {
        port++;
      }
    } while (!free);
    return new PortLock(port, lock, lockFile);
  }
}