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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183     // check the layout version inside the storage file
    // Lock and Read old storage file
    RandomAccessFile oldFile = new RandomAccessFile(oldF, "rws");
    if (oldFile == null)
      throw new IOException("Cannot read file: " + oldF);
    FileLock oldLock = oldFile.getChannel().tryLock();
    try {
      oldFile.seek(0);
      int odlVersion = oldFile.readInt();
      if (odlVersion < LAST_PRE_UPGRADE_LAYOUT_VERSION)
        return false;
    } finally {
      oldLock.release();
      oldFile.close();
    }
    // check consistency of the old storage
    File oldDataDir = new File(sd.root, "data");
    if (!oldDataDir.exists()) 

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248     
    // Lock and Read old storage file
    RandomAccessFile oldFile = new RandomAccessFile(oldF, "rws");
    if (oldFile == null)
      throw new IOException("Cannot read file: " + oldF);
    FileLock oldLock = oldFile.getChannel().tryLock();
    if (oldLock == null)
      throw new IOException("Cannot lock file: " + oldF);
    String odlStorageID = "";
    try {
      oldFile.seek(0);
      int odlVersion = oldFile.readInt();
      if (odlVersion < LAST_PRE_UPGRADE_LAYOUT_VERSION)
        throw new IncorrectVersionException(odlVersion, "file " + oldF,
                                            LAST_PRE_UPGRADE_LAYOUT_VERSION);
      odlStorageID = org.apache.hadoop.io.UTF8.readString(oldFile);
  
      // check new storage
      File newDataDir = sd.getCurrentDir();
      File versionF = sd.getVersionFile();
      if (versionF.exists())
        throw new IOException("Version file already exists: " + versionF);
      if (newDataDir.exists()) // somebody created current dir manually
        deleteDir(newDataDir);
      // move "data" to "current"
      rename(oldDataDir, newDataDir);
      // close and unlock old file
    } finally {
      oldLock.release();
      oldFile.close();
    }
    // move old storage file into current dir
    rename(oldF, new File(sd.getCurrentDir(), "storage"));


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302     File f = pathToFile(p);
    f.createNewFile();
    
    if (shared) {
      FileInputStream lockData = new FileInputStream(f);
      FileLock lockObj =
        lockData.getChannel().lock(0L, Long.MAX_VALUE, shared);
      synchronized (this) {
        sharedLockDataSet.put(f, lockData);
        lockObjSet.put(f, lockObj);
      }
    } else {
      FileOutputStream lockData = new FileOutputStream(f);
      FileLock lockObj = lockData.getChannel().lock(0L, Long.MAX_VALUE, shared);
      synchronized (this) {
        nonsharedLockDataSet.put(f, lockData);
        lockObjSet.put(f, lockObj);
      }
    }

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330   
  /** @deprecated */ @Deprecated
    public void release(Path p) throws IOException {
    File f = pathToFile(p);
    
    FileLock lockObj;
    FileInputStream sharedLockData;
    FileOutputStream nonsharedLockData;
    synchronized (this) {
      lockObj = lockObjSet.remove(f);
      sharedLockData = sharedLockDataSet.remove(f);
      nonsharedLockData = nonsharedLockDataSet.remove(f);
    }
    
    if (lockObj == null) {
      throw new IOException("Given target not held as lock");
    }
    if (sharedLockData == null && nonsharedLockData == null) {
      throw new IOException("Given target not held as lock");
    }
    
    lockObj.release();
    
    if (sharedLockData != null) {
      sharedLockData.close();
    } else {
      nonsharedLockData.close();
 

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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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819    * storage directory. Otherwise, no guarantee is given.
   *
   * @throws IOException if locking fails
   */
  private void lock(File dir) throws IOException {
    FileLock lock = tryLock(dir);
    if (lock == null) {
      String msg = "Cannot lock " + dir
          + ". The directory is already locked. "
          + channelNameDescriptor;
      LOGGER.info(msg);
      throw new IOException(msg);
    }
    FileLock secondLock = tryLock(dir);
    if(secondLock != null) {
      LOGGER.warn("Directory "+dir+" does not support locking");
      secondLock.release();
      secondLock.channel().close();
    }
    locks.put(dir.getAbsolutePath(), lock);
  }
 

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837    */
  private FileLock tryLock(File dir) throws IOException {
    File lockF = new File(dir, 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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859    * Unlock directory.
   *
   * @throws IOException
   */
  private void unlock(File dir) throws IOException {
    FileLock lock = locks.remove(dir.getAbsolutePath());
    if(lock == null) {
      return;
    }
    lock.release();
    lock.channel().close();
    lock = null;
  }
 

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216         }


        // try to acquire rw lock on the file before we can consume it
        FileChannel channel = new RandomAccessFile(file, "rw").getChannel();
        try {
            FileLock lock = channel.lock();
            if (LOG.isTraceEnabled()) {
                LOG.trace("Acquired exclusive read lock: " + lock + " to file: " + file);
            }
            // just release it now we dont want to hold it during the rest of the processing
            lock.release();
        } finally {
            // must close channel
            ObjectHelper.close(channel, "FileConsumer during acquiring of exclusive read lock", LOG);
        }
    }

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261     File f = pathToFile(p);
    f.createNewFile();
    
    if (shared) {
      FileInputStream lockData = new FileInputStream(f);
      FileLock lockObj =
        lockData.getChannel().lock(0L, Long.MAX_VALUE, shared);
      synchronized (this) {
        sharedLockDataSet.put(f, lockData);
        lockObjSet.put(f, lockObj);
      }
    } else {
      FileOutputStream lockData = new FileOutputStream(f);
      FileLock lockObj = lockData.getChannel().lock(0L, Long.MAX_VALUE, shared);
      synchronized (this) {
        nonsharedLockDataSet.put(f, lockData);
        lockObjSet.put(f, lockObj);
      }
    }

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