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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810    * @throws IOException
   * @see StorageDirectory#lock()
   */
  public boolean isLockSupported(int idx) throws IOException {
    StorageDirectory sd = storageDirs.get(idx);
    FileLock firstLock = null;
    FileLock secondLock = null;
    try {
      firstLock = sd.lock;
      if(firstLock == null) {
        firstLock = sd.tryLock();
        if(firstLock == null)
          return true;
      }
      secondLock = sd.tryLock();
      if(secondLock == null)
        return true;
    } finally {
      if(firstLock != null && firstLock != sd.lock) {
        firstLock.release();
        firstLock.channel().close();
      }
      if(secondLock != null) {
        secondLock.release();
        secondLock.channel().close();
      }
    }
    return false;
  }
 

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186     for (String root : rootDirs) {
      File rootFile = new File(root);
      rootFile.mkdirs();
      @SuppressWarnings("resource")
      FileOutputStream lockOutputStream = new FileOutputStream(root + "/.lock");
      FileLock fileLock = lockOutputStream.getChannel().tryLock();
      if (fileLock == null)
        try {
          throw new IOException("Failed to acquire lock file");
        } finally {
          if (lockOutputStream != null)

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674     public void lock() throws IOException {
      if (isShared()) {
        LOG.info("Locking is disabled");
        return;
      }
      FileLock newLock = tryLock();
      if (newLock == null) {
        String msg = "Cannot lock storage " + this.root 
          + ". The directory is already locked";
        LOG.info(msg);
        throw new IOException(msg);

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704         lockF.deleteOnExit();
        deletionHookAdded = true;
      }
      RandomAccessFile file = new RandomAccessFile(lockF, "rws");
      String jvmName = ManagementFactory.getRuntimeMXBean().getName();
      FileLock res = null;
      try {
        res = file.getChannel().tryLock();
        if (null == res) {
          throw new OverlappingFileLockException();
        }

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779      *         <code>false</code> otherwise.
     * @throws IOException
     * @see StorageDirectory#lock()
     */
    public boolean isLockSupported() throws IOException {
      FileLock firstLock = null;
      FileLock secondLock = null;
      try {
        firstLock = lock;
        if(firstLock == null) {
          firstLock = tryLock();
          if(firstLock == null)
            return true;
        }
        secondLock = tryLock();
        if(secondLock == null)
          return true;
      } finally {
        if(firstLock != null && firstLock != lock) {
          firstLock.release();
          firstLock.channel().close();
        }
        if(secondLock != null) {
          secondLock.release();
          secondLock.channel().close();
        }
      }
      return false;
    }
 

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94         assertFalse(tree.isEmpty());
    }


    // @Test(expected = IOException.class)
    public void testSaveFileException() throws IOException {
        FileLock lock = null;
        RandomAccessFile file = null;
        try {
            String filename = "target/locked.png";
            File f = new File(filename);
            f.createNewFile();
            file = new RandomAccessFile(f, "rw");
            lock = file.getChannel().lock();
            RTree.create().visualize(600, 600).save(filename, "PNG");
        } finally {
            try {
                lock.release();
                file.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

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195                 log.warn("File: " + file.getName() + " is not writable");
                return false;
            }


            FileOutputStream fos = null;
            FileLock lock = null;
            boolean writable;


            try {
                fos = new FileOutputStream(file, true);
                FileChannel channel = fos.getChannel();
                lock = channel.tryLock();
            } catch (IOException e) {
                log.warn("Error while attempting to lock the file: " + file.getName(), e);
                writable = false;
            } finally {
                if (lock != null) {
                    writable = true;
                    try {
                        lock.release();
                    } catch (IOException e) {
                        log.warn("Error while releasing the lock on file: " + file.getName(), e);
                        writable = false;
                    }
                } else {

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75     @Override
    public boolean writeExifGpsInfo(File jpeg, Wpt wpt, boolean overwrite){
        boolean result = false;
        try{
            FileChannel input = (new RandomAccessFile(jpeg, "rw")).getChannel();
            FileLock lock = input.tryLock();
            if(lock == null){
                throw new IOException("Cannot lock input file.");
            }
            File tmpFile = null;
            try{
                XMPJpeg xmpjpg = new XMPJpeg(input, true);
                ByteBuffer xmp = xmpjpg.getXMP();
                int previousSize =  (xmp == null)
                ? 0
                : xmp.remaining();
                XMPTree xmpEditor = (xmp == null)
                ? new XMPTree()
                : new XMPTree(xmp);
                //System.out.println(new String(xmpEditor.toUTF8Bytes(null)));
                xmpEditor.setXMPProperties(XMPTree.NS_EXIF, asXMPExifProperties(wpt));
                tmpFile = xmpjpg.setXMP(ByteBuffer.wrap(xmpEditor.toXPacket(previousSize)));
            } finally {
                lock.release();
                input.close();
            }
            result = (tmpFile == null) || (jpeg.delete() && tmpFile.renameTo(jpeg));
            if(!result){
                throw new IOException("Cannot overwrite " + jpeg + " with temporary file " + tmpFile);

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    @Override
    protected ByteBuffer getXMPBytes(File jpeg) throws IOException, XMPReadException{
        ByteBuffer result = null;
        FileChannel input = (new FileInputStream(jpeg)).getChannel();
        FileLock lock = input.tryLock(0L, Long.MAX_VALUE, true);
        if(lock == null){
            input.close();
            throw new IOException("Cannot lock input file.");
        }
        try{
            result = new XMPJpeg(input, false).getXMP();
        } finally {
            //Do not release lock AFTER closing
            lock.release();
            input.close();
        }
        return result;
    }

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361       FileChannel destination) throws IOException {
    
    inputNotNull(source,destination);
    if(!source.isOpen()||!destination.isOpen())
      throw new IllegalStateException("Source and destination channels must be open.");    
    FileLock lock = null;
    try {


      lock = destination.lock();
      final long sourceSize = source.size();
      long pos = 0;
      while (pos < sourceSize) {
        // read and flip
        final long remaining = (sourceSize - pos);
        final int mappedZoneSize = remaining >= bufferSize ? bufferSize
            : (int) remaining;
        destination.transferFrom(source, pos, mappedZoneSize);
        // update zone
        pos += mappedZoneSize;


      }
    } finally {
      if (lock != null) {
        try {
          lock.release();
        }catch (Throwable t) {
              if(LOGGER.isLoggable(Level.INFO))
                LOGGER.log(Level.INFO,t.getLocalizedMessage(),t);
        }
      }

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