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.
FileLock(FileChannel channel, long position, long size, boolean shared) | ||||||
Constructor for a new file lock instance for a given channel. |
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. |
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). |
start | the starting position for the comparative lock. |
---|---|
length | the length of the comparative lock. |
ClosedChannelException | if the channel is already closed when an attempt to release the lock is made. |
---|---|
IOException | some other IO exception occurred. |
Copyright 2007 Google Inc. | Build 0.9_r1-98467 - 14 Aug 2008 18:56 |