Java 测试用 FileChannel 来实现文件锁，filechannel文件锁,本例结合 RandomA

Java 测试用 FileChannel 来实现文件锁，filechannel文件锁,本例结合 RandomA

本例结合 RandomAccessFile 和 FileChannel 实现了文件锁的功能

package com.ronsoft.books.nio.channels;import java.nio.ByteBuffer;import java.nio.IntBuffer;import java.nio.channels.FileChannel;import java.nio.channels.FileLock;import java.io.RandomAccessFile;import java.util.Random;/** * Test locking with FileChannel. * Run one copy of this code with arguments "-w /tmp/locktest.dat" * and one or more copies with "-r /tmp/locktest.dat" to see the * interactions of exclusive and shared locks.  Note how too many * readers can starve out the writer. * Note: The filename you provide will be overwritten.  Substitute * an appropriate temp filename for your favorite operating system. * * Created April, 2002 * @author Ron Hitchens (ron@ronsoft.com) * @version $Id: LockTest.java,v 1.2 2002/05/19 04:55:45 ron Exp $ */public class LockTest{   private static final int SIZEOF_INT = 4;   private static final int INDEX_START = 0;   private static final int INDEX_COUNT = 10;   private static final int INDEX_SIZE = INDEX_COUNT * SIZEOF_INT;   private ByteBuffer buffer = ByteBuffer.allocate (INDEX_SIZE);   private IntBuffer indexBuffer = buffer.asIntBuffer();   private Random rand = new Random();   public static void main (String [] argv)      throws Exception   {      boolean writer = false;      String filename;      if (argv.length != 2) {         System.out.println ("Usage: [ -r | -w ] filename");         return;      }      writer = argv [0].equals ("-w");      filename = argv [1];      RandomAccessFile raf = new RandomAccessFile (filename,         (writer) ? "rw" : "r");      FileChannel fc = raf.getChannel();      LockTest lockTest = new LockTest();      if (writer) {         lockTest.doUpdates (fc);      } else {         lockTest.doQueries (fc);      }   }   // ----------------------------------------------------------------   // Simulate a series of read-only queries while   // holding a shared lock on the index area   void doQueries (FileChannel fc)      throws Exception   {      while (true) {         println ("trying for shared lock...");         FileLock lock = fc.lock (INDEX_START, INDEX_SIZE, true);         int reps = rand.nextInt (60) + 20;         for (int i = 0; i < reps; i++) {            int n = rand.nextInt (INDEX_COUNT);            int position = INDEX_START + (n * SIZEOF_INT);            buffer.clear();            fc.read (buffer, position);            int value = indexBuffer.get (n);            println ("Index entry " + n + "=" + value);            // Pretend to be doing some work            Thread.sleep (100);         }         lock.release();         println ("<sleeping>");         Thread.sleep (rand.nextInt (3000) + 500);      }   }   // Simulate a series of updates to the index area   // while holding an exclusive lock   void doUpdates (FileChannel fc)      throws Exception   {      while (true) {         println ("trying for exclusive lock...");         FileLock lock = fc.lock (INDEX_START,            INDEX_SIZE, false);         updateIndex (fc);         lock.release();         println ("<sleeping>");         Thread.sleep (rand.nextInt (2000) + 500);      }   }   // write new values to the index slots   private int idxval = 1;   private void updateIndex (FileChannel fc)      throws Exception   {      // "indexBuffer" is an int view of "buffer"      indexBuffer.clear();      for (int i = 0; i < INDEX_COUNT; i++) {         idxval++;         println ("Updating index " + i + "=" + idxval);         indexBuffer.put (idxval);         // Pretend to this is really hard work         Thread.sleep (500);      }      // leaves position and limit correct for whole buffer      buffer.clear();      fc.write (buffer, INDEX_START);   }   // ----------------------------------------------------------------   private int lastLineLen = 0;   // Specialized println which repaints the current line   private void println (String msg)   {      System.out.print ("\\r ");      System.out.print (msg);      for (int i = msg.length(); i < lastLineLen; i++) {         System.out.print (" ");      }      System.out.print ("\\r");      System.out.flush();      lastLineLen = msg.length();   }}//该片段来自于http://byrx.net