FileLockrepresents 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
Locks held by a particular process cannot overlap one another. Applications can determine whether a proposed lock will overlap by using the
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).
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.