Android
java.nio.channels
public abstract class

java.nio.channels.FileLock

java.lang.Object
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)) API. Locks held in other processes may overlap locks held in this process.

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 implicitly 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.

Summary

Protected Constructors

            FileLock(FileChannel channel, long position, long size, boolean shared)
Constructor for a new file lock instance for a given channel.

Public Methods

    final      FileChannel  channel()
Returns the lock's FileChannel.
    final      boolean  isShared()
Returns true if the file lock is shared with other processes and false if it is not.
abstract          boolean  isValid()
Returns whether the receiver is a valid file lock or not.
    final      boolean  overlaps(long start, long length)
Returns true if the receiver's lock region overlapps the region described in the parameter list,and returns false otherwise.
    final      long  position()
Returns the lock's starting position in the file.
abstract          void  release()
Releases this particular lock on the file.
    final      long  size()
Returns the length of the file lock in bytes.
    final      String  toString()
Returns a string that shows the details of the lock suitable for display to an end user.
Methods inherited from class java.lang.Object

Details

Protected Constructors

protected FileLock(FileChannel channel, long position, long size, boolean shared)

Constructor for a new file lock instance for a given channel. The constructor enforces the starting position, stretch, and shared status of the lock.

Parameters

channel underlying file channel that holds the lock.
position starting point for the lock.
size length of lock in number of bytes.
shared shared status of lock (true is shared, false is exclusive).

Public Methods

public final FileChannel channel()

Returns the lock's FileChannel.

Returns

  • the channel.

public final boolean isShared()

Returns true if the file lock is shared with other processes and false if it is not.

Returns

  • true if the lock is a shared lock, and false if it is exclusive.

public abstract boolean isValid()

Returns whether the receiver is a valid file lock or not. The lock is valid unless the underlying channel has been closed or it has been explicitly released.

Returns

  • true if the lock is valid, and false otherwise.

public final boolean overlaps(long start, long length)

Returns true if the receiver's lock region overlapps the region described in the parameter list,and returns false otherwise.

Parameters

start the starting position for the comparative lock.
length the length of the comparative lock.

Returns

  • true if there is an overlap, and false otherwise.

public final long position()

Returns the lock's starting position in the file.

Returns

  • the lock position.

public abstract void release()

Releases this particular lock on the file. If the lock is invalid then this method has no effect. Once released the lock becomes invalid.

Throws

ClosedChannelException if the channel is already closed when an attempt to release the lock is made.
IOException some other IO exception occurred.

public final long size()

Returns the length of the file lock in bytes.

Returns

  • the size of file lock in bytes.

public final String toString()

Returns a string that shows the details of the lock suitable for display to an end user.

Returns

  • the display string.
Copyright 2007 Google Inc. Build 0.9_r1-98467 - 14 Aug 2008 18:56