FileLockSimulator.java

## FileLockSimulator.java
/**
 * Logback: the reliable, generic, fast and flexible logging framework.
 * Copyright (C) 1999-2011, QOS.ch. All rights reserved.
 *
 * This program and the accompanying materials are dual-licensed under
 * either the terms of the Eclipse Public License v1.0 as published by
 * the Eclipse Foundation
 *
 *   or (per the licensee's choosing)
 *
 * under the terms of the GNU Lesser General Public License version 2.1
 * as published by the Free Software Foundation.
 */

import java.io.FileOutputStream;
import java.io.IOException;
import java.nio.channels.FileChannel;
import java.nio.channels.FileLock;


/**
 * FileLockSimulator is a small application intended to simulate FileAppender in prudent mode.
 * In this mode, the application obtains an exclusive lock on the file, writes to the file and
 * then releases the lock.
 *
 * <pre>  Usage:
 *   javac FileLockSimulator.java
 *   java  FileLockSimulator instanceName pathToLogFile delay
 * where
 * "instanceName" is the name given to the current instance of the application
 * "pathToLogFile" is the path to the log file
 * "delay" is the number of milliseconds of sleep observed every 128 writes
 * </pre>
 *
 * <b>This small application requires only the JDK to compile and to execute.</b>
 *
 * <p>FileLockSimulator should be launched as many times and from as many hosts as there will be
 * JVMs writing to a log file in prudent mode. Performance should be quite good if
 * "pathToLogFile" is on a local file system. On networked file systems such as NFS, performance
 * depends on the speed of the network and NFS implementation. It has been observed that file
 * locking over NFS is biased so that the current owner of the lock is favored over other
 * processes. Thus, while one process hogs the lock for the log file, other processes starve
 * waiting for the lock to the point of appearing deadlocked.
 *
 */
public class FileLockSimulator  {

  static String LINE_SEPARATOR = System.getProperty("line.separator");
  static final int DOT_FREQ = 128;
  static final int DOT_WITH_NEW_LINE_FREQ = DOT_FREQ * 80;

  static String instanceName;
  static int delay;
  static FileOutputStream fos;
  static FileChannel fileChannel;

  public static void main(String[] args) throws IOException, InterruptedException {

    String instanceName = args[0];
    System.out.println("Instance named as [" + instanceName + "]");

    String fileStr = args[1];
    System.out.println("Output target specified as [" + fileStr + "]");

    int delay = Integer.parseInt(args[2]);
    System.out.println("Sleep delay specified as [" + delay + "] milliseconds");

    fos = new FileOutputStream(fileStr, true);
    fileChannel = fos.getChannel();

    for (int i = 1; ; i++) {
      printDotAndSleep(i);
      lockAndWrite(i);
    }
  }

  static void lockAndWrite(int i) throws InterruptedException, IOException {
    FileLock fileLock = null;
    try {
      fileLock = fileChannel.lock();
      long position = fileChannel.position();
      long size = fileChannel.size();
      if (size != position) {
        fileChannel.position(size);
      }
      String msg = "hello from" + instanceName + " " + i + LINE_SEPARATOR;
      fos.write(msg.getBytes());
    } finally {
      if (fileLock != null) {
        fileLock.release();
      }
    }
  }

  static void printDotAndSleep(int i) throws InterruptedException {
    if (i % DOT_FREQ == 0) {
      System.out.print(".");
      Thread.sleep(delay);
    }
    if (i % DOT_WITH_NEW_LINE_FREQ == 0) System.out.println("");
  }
}