Platform dependencies
This file-locking API is intended to map directly to the native locking
facility of the underlying operating system. Thus the locks held on a file
should be visible to all programs that have access to the file, regardless
of the language in which those programs are written.
Whether or not a lock actually prevents another program from accessing
the content of the locked region is system-dependent and therefore
unspecified. The native file-locking facilities of some systems are merely
advisory, meaning that programs must cooperatively observe a known
locking protocol in order to guarantee data integrity. On other systems
native file locks are mandatory, meaning that if one program locks a
region of a file then other programs are actually prevented from accessing
that region in a way that would violate the lock. On yet other systems,
whether native file locks are advisory or mandatory is configurable on a
per-file basis. To ensure consistent and correct behavior across platforms,
it is strongly recommended that the locks provided by this API be used as if
they were advisory locks.
On some systems, acquiring a mandatory lock on a region of a file
prevents that region from being
mapped into memory
, and vice versa. Programs that combine
locking and mapping should be prepared for this combination to fail.
On some systems, closing a channel releases all locks held by the Java
virtual machine on the underlying file regardless of whether the locks were
acquired via that channel or via another channel open on the same file. It
is strongly recommended that, within a program, a unique channel be used to
acquire all locks on any given file.
Some network filesystems permit file locking to be used with
memory-mapped files only when the locked regions are page-aligned and a
whole multiple of the underlying hardware's page size. Some network
filesystems do not implement file locks on regions that extend past a
certain position, often 230 or 231. In general, great
care should be taken when locking files that reside on network filesystems.