Examples of java.nio.channels.FileLock

java.nio.channels.FileLock
A FileLock represents a locked region of a file.
Locks have certain properties that enable collaborating processes to avoid the lost update problem, or reading inconsistent data.

logically, a file lock can be 'exclusive' or 'shared'. Multiple processes can hold shared locks on the same region of a file, but only a single process can hold an exclusive lock on a given region of a file and no other process can simultaneously hold a shared lock overlapping the exclusive lock. An application can determine whether a FileLock is shared or exclusive via the isShared() API.

Locks held by a particular process cannot overlap one another. Applications can determine whether a proposed lock will overlap by using the overlaps(long, long)
Locks are shared amongst all threads in the acquiring process, and are therefore unsuitable for intra-process synchronization.
Once a lock is acquired it is immutable in all its state except isValid(). The lock will initially be valid, but may be rendered invalid by explicit removal of the lock, using release(), or implictly by closing the channel or exiting the process (terminating the JVM). Platform dependencies Locks are intended to be true platform operating system file locks, and therefore locks held by the JVM process will be visible to other OS processes. The characteristics of the underlying OS locks will show through in the Java implementation. For example, some platforms' locks are 'mandatory' -- meaning the operating system enforces the locks on processes that attempt to access locked regions of file; whereas other platforms' locks are only 'advisory' -- meaning that processes are required to collaborate on ensuring locks are acquired and there is a potential for processes not to play well. The only safe answer is to assume that the platform is adopting advisory locks an always acquire shared locks when reading a region of file. On some platforms the presence of a lock will prevent the file being memory mapped. On some platforms closing a channel on a given file handle will release all the locks held on that file -- even if there are other channels open on the same file (their locks will be released). The safe option here is to ensure that you only acquire locks on a single channel for a particular file, and that becomes the synchronization point. Further care should be exercised when locking files maintained on network file systems since they often have further limitations.




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487         writer.doWrite(q);


        File file = new File(String.format("%s/%s-%s.json", outputPath, isQueue ? "queue": "topic", exchangeName));
        FileChannel channel = new RandomAccessFile(file, "rw").getChannel();
        try {
            FileLock lock = channel.lock();
            try {
                mapper.writeValue(file, metric);
            }
            catch(Exception e) {
                logger.error("write metric error: ", e);
            }
            lock.release();
        }
        catch(Exception e) {
            logger.error("Cannot lock file {}", file.getAbsolutePath());
        }
        finally {

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120     private synchronized void writeToFile(String action) {
        Date date = new Date();
        try {
            FileOutputStream out = new FileOutputStream(logFile, true);
            FileChannel channel = out.getChannel();
            FileLock lock = channel.lock(0, Long.MAX_VALUE, false);
            PrintWriter writer = new PrintWriter(out, false);
            writer.println(DATE_FORMAT.format(date)+" - "+action+" - "+username);
            writer.flush();
            writer.close();
            if(lock.isValid()) {
                lock.release();
            }
        } catch (IOException e) {
            throw new RuntimeException("Unable to write to authentication log file", e);
        }
    }

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331     }


    public synchronized FileLock tryLock(long position, long size, boolean shared) throws IOException {
        if (shared) {
            // cast to FileChannel to avoid JDK 1.7 ambiguity
            return new FileLock((FileChannel) null, position, size, shared) {


                @Override
                public boolean isValid() {
                    return true;
                }

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302                 return null;
            }
        }


        // cast to FileChannel to avoid JDK 1.7 ambiguity
        FileLock lock = new FileLock((FileChannel) null, position, size, shared) {


            @Override
            public boolean isValid() {
                return true;
            }

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101     private synchronized void writeToFile(String action) {
        Date date = new Date();
        try {
            FileOutputStream out = new FileOutputStream(logFile, true);
            FileChannel channel = out.getChannel();
            FileLock lock = channel.lock(0, Long.MAX_VALUE, false);
            PrintWriter writer = new PrintWriter(out, false);
            writer.println(DATE_FORMAT.format(date)+" - "+action+" - "+username);
            writer.flush();
            writer.close();
            if(lock.isValid()) {
                lock.release();
            }
        } catch (IOException e) {
            throw new RuntimeException("Unable to write to authentication log file", e);
        }
    }

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163     return val;
  }


  private String getFromCache(ZooKeeperCall call) throws IOException,
          InterruptedException, KeeperException {
    FileLock lock = tryLock(true, call);
    String result = null;
    BufferedReader reader = null;
    try {
      File cache = call.getDataFile(cacheDir);
      if (!cache.exists()) {
        return retrieveAndPopulateCache(cache, call);
      }
      updateReadTime(cache.getName(), cache.lastModified());
      reader = new BufferedReader(new FileReader(cache));
      result = reader.readLine();
    } finally {
      if (reader != null) {
        reader.close();
      }
      lock.release();
    }
    return result;
  }

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196           Thread.sleep(250);
        }
      }
    }


    FileLock lock = null;
    do {
      try {
        lock = file.getChannel().tryLock();
      } catch (OverlappingFileLockException ex) {
        // A thread inside of this JVM has the lock on the file

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223     return lock;
  }


  private String populateCache(ZooKeeperCall call)
      throws IOException, InterruptedException, KeeperException {
    FileLock lock = tryLock(true, call);
    String val = null;
    try {
      File cache = call.getDataFile(cacheDir);
      long fileModTime = cache.lastModified();
      Long lastFileReadTime = getReadTime(cache.getName());
      if (lastFileReadTime != null && lastFileReadTime >= fileModTime) {
        // Cache has not been updated and we need to populate it
        val = retrieveAndPopulateCache(cache, call);
      }
    } finally {
      if (lock != null) {
        lock.release();
      }
    }
    if (val == null) {
      val = getFromCache(call);
    }

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58       CachingAvatarZooKeeperClient cazkc = new CachingAvatarZooKeeperClient(conf, null);
      Method m = CachingAvatarZooKeeperClient.class.getDeclaredMethod(
          "tryLock", Boolean.TYPE, ZooKeeperCall.class);
      ZooKeeperCall call = cazkc.new GetStat(null);
      m.setAccessible(true);
      FileLock fl = (FileLock) m.invoke(cazkc, true, call);
      fl.release();
      TestCase.assertNotNull(fl);
      fl = (FileLock) m.invoke(cazkc, true, call);
      TestCase.assertNotNull(fl);
      fl.release();
      new File(directoryName, ".avatar_zk_cache_lock").delete();
      m.invoke(cazkc, true, call);
    } finally {
      new File(directoryName, ".avatar_zk_cache_lock").delete();
      new File(directoryName).delete();

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600      */
    FileLock tryLock() throws IOException {
      File lockF = new File(root, STORAGE_FILE_LOCK);
      lockF.deleteOnExit();
      RandomAccessFile file = new RandomAccessFile(lockF, "rws");
      FileLock res = null;
      try {
        res = file.getChannel().tryLock();
      } catch(OverlappingFileLockException oe) {
        file.close();
        return null;
 

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