JAVA NIO – FileLock

Locking on File introduced with java.nio package that implemented by FileChannel. A FileLock is two types exclusive and shared. A shared lock allows other concurrently running program to acquire overlapping shared Lock. An Exclusive lock doesn’t allow other program to acquire overlapping lock.

Lock ideally supports shared Lock but it depends OS if OS doesn’t support share lock then it will act as exclusive locking. Locking applied on File not per channel or thread means if channel obtained the exclusive lock then other JVM’ channel will be block till first thread channel resume the lock.

 

Locks are associated with a file, not with individual file handles or channels Locking applied on File not per channel or thread means if channel obtained the lock before processing the file then other JVM’ channel will be block till first thread channel resume the lock. Since Lock is on File therefore it is not advisable to use lock on a single JVM.

Below is FileLock API in FileChannel class

 

 

 

 

 

 

 

 

 

 

public abstract class FileChannel extends AbstractInterruptibleChannel implements SeekableByteChannel,
      GatheringByteChannel, ScatteringByteChannel {
public abstract FileLock lock(long position, long size, boolean shared) throws IOException;
   public final FileLock lock() throws IOException {
       return lock(0L, Long.MAX_VALUE, false);
   }
   public abstract FileLock tryLock(long position, long size, boolean shared) throws IOException;
   public final FileLock tryLock() throws IOException {
       return tryLock(0L, Long.MAX_VALUE, false);
   }
}

 

The FileLock (long position, long size, boolean shared) acquire lock on specified part of a file. The region of file specified by beginning position and size whereas last argument specified if lock is shared lock (value True) of exclusive (value False). To obtain the shared lock, you need to open file with read permission whereas write permission require exclusive lock.

We can use lock () method to acquire lock on whole file not the specified area on file. File can grow therefore we need to specify the size while obtaining the lock.

Method tryLock on specified area or on full file doesn’t block/hold while acquiring a lock means it immediately return value without going to wait to acquire lock. If other process already acquired lock for specific file then it will immediate return null.

 

 

 

 

FileLock API specified as below.

public abstract class FileLock {
  public final FileChannel channel( )
   public final long position( )
   public final long size( )
   public final boolean isShared( )
   public final boolean overlaps (long position, long size)
   public abstract boolean isValid( );
   public abstract void release( ) throws IOException;
}

The FileLock encapsulates specific region, which is acquired by FileChannel. FileLock associate with specific FileChannel instance and FileLock keep the reference of FileChannel that could be determining by channel () method in FileLock.

FileLock lifecycle will start when lock method or tryLock invoke from FileChannel and end when release () method called. It FileLock also invalid if JVM shutdown or associated channel closed.

isShared() method return whether lock is shared or exclusive. If shared lock is not supported by operating system then this method always return false.

FileLock objects are thread-safe; multiple threads may access a lock object concurrently.

Finally, overlaps(long position, long size) return if lock overlap the given block range or not.

FileLock object associated with an underlying file which occurred deadlock if you don’t release the lock hence it is advisable to always release lock as mentioned below

 

 

FileLock lock = fileChannel.lock( )
try {
       ..............
} catch (IOException) [
       ..................
} finally {
   lock.release( )
}
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