A file-lock object is created each time a lock is acquired on a file via * one of the {@link FileChannel#lock(long,long,boolean) lock} or {@link * FileChannel#tryLock(long,long,boolean) tryLock} methods of the * {@link FileChannel} class, or the {@link * AsynchronousFileChannel#lock(long,long,boolean,Object,CompletionHandler) lock} * or {@link AsynchronousFileChannel#tryLock(long,long,boolean) tryLock} * methods of the {@link AsynchronousFileChannel} class. * *
A file-lock object is initially valid. It remains valid until the lock * is released by invoking the {@link #release release} method, by closing the * channel that was used to acquire it, or by the termination of the Java * virtual machine, whichever comes first. The validity of a lock may be * tested by invoking its {@link #isValid isValid} method. * *
A file lock is either exclusive or shared. A shared lock * prevents other concurrently-running programs from acquiring an overlapping * exclusive lock, but does allow them to acquire overlapping shared locks. An * exclusive lock prevents other programs from acquiring an overlapping lock of * either type. Once it is released, a lock has no further effect on the locks * that may be acquired by other programs. * *
Whether a lock is exclusive or shared may be determined by invoking its * {@link #isShared isShared} method. Some platforms do not support shared * locks, in which case a request for a shared lock is automatically converted * into a request for an exclusive lock. * *
The locks held on a particular file by a single Java virtual machine do * not overlap. The {@link #overlaps overlaps} method may be used to test * whether a candidate lock range overlaps an existing lock. * *
A file-lock object records the file channel upon whose file the lock is * held, the type and validity of the lock, and the position and size of the * locked region. Only the validity of a lock is subject to change over time; * all other aspects of a lock's state are immutable. * *
File locks are held on behalf of the entire Java virtual machine. * They are not suitable for controlling access to a file by multiple * threads within the same virtual machine. * *
File-lock objects are safe for use by multiple concurrent threads.
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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 {@link java.nio.channels.FileChannel#map * 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. * * * @author Mark Reinhold * @author JSR-51 Expert Group * @since 1.4 */ public abstract class FileLock implements AutoCloseable { private final Channel channel; private final long position; private final long size; private final boolean shared; /** * Initializes a new instance of this class.
* * @param channel * The file channel upon whose file this lock is held * * @param position * The position within the file at which the locked region starts; * must be non-negative * * @param size * The size of the locked region; must be non-negative, and the sum * position + size must be non-negative * * @param shared * true if this lock is shared, * false if it is exclusive * * @throws IllegalArgumentException * If the preconditions on the parameters do not hold */ protected FileLock(FileChannel channel, long position, long size, boolean shared) { if (position < 0) throw new IllegalArgumentException("Negative position"); if (size < 0) throw new IllegalArgumentException("Negative size"); if (position + size < 0) throw new IllegalArgumentException("Negative position + size"); this.channel = channel; this.position = position; this.size = size; this.shared = shared; } /** * Initializes a new instance of this class. * * @param channel * The channel upon whose file this lock is held * * @param position * The position within the file at which the locked region starts; * must be non-negative * * @param size * The size of the locked region; must be non-negative, and the sum * position + size must be non-negative * * @param shared * true if this lock is shared, * false if it is exclusive * * @throws IllegalArgumentException * If the preconditions on the parameters do not hold * * @since 1.7 */ protected FileLock(AsynchronousFileChannel channel, long position, long size, boolean shared) { if (position < 0) throw new IllegalArgumentException("Negative position"); if (size < 0) throw new IllegalArgumentException("Negative size"); if (position + size < 0) throw new IllegalArgumentException("Negative position + size"); this.channel = channel; this.position = position; this.size = size; this.shared = shared; } /** * Returns the file channel upon whose file this lock was acquired. * *This method has been superseded by the {@link #acquiredBy acquiredBy} * method. * * @return The file channel, or {@code null} if the file lock was not * acquired by a file channel. */ public final FileChannel channel() { return (channel instanceof FileChannel) ? (FileChannel)channel : null; } /** * Returns the channel upon whose file this lock was acquired. * * @return The channel upon whose file this lock was acquired. * * @since 1.7 */ public Channel acquiredBy() { return channel; } /** * Returns the position within the file of the first byte of the locked * region. * *
A locked region need not be contained within, or even overlap, the * actual underlying file, so the value returned by this method may exceed * the file's current size.
* * @return The position */ public final long position() { return position; } /** * Returns the size of the locked region in bytes. * *A locked region need not be contained within, or even overlap, the * actual underlying file, so the value returned by this method may exceed * the file's current size.
* * @return The size of the locked region */ public final long size() { return size; } /** * Tells whether this lock is shared. * * @return true if lock is shared, * false if it is exclusive */ public final boolean isShared() { return shared; } /** * Tells whether or not this lock overlaps the given lock range. * * @return true if, and only if, this lock and the given lock * range overlap by at least one byte */ public final boolean overlaps(long position, long size) { if (position + size <= this.position) return false; // That is below this if (this.position + this.size <= position) return false; // This is below that return true; } /** * Tells whether or not this lock is valid. * *A lock object remains valid until it is released or the associated * file channel is closed, whichever comes first.
* * @return true if, and only if, this lock is valid */ public abstract boolean isValid(); /** * Releases this lock. * *If this lock object is valid then invoking this method releases the * lock and renders the object invalid. If this lock object is invalid * then invoking this method has no effect.
* * @throws ClosedChannelException * If the channel that was used to acquire this lock * is no longer open * * @throws IOException * If an I/O error occurs */ public abstract void release() throws IOException; /** * This method invokes the {@link #release} method. It was added * to the class so that it could be used in conjunction with the * automatic resource management block construct. * * @since 1.7 */ public final void close() throws IOException { release(); } /** * Returns a string describing the range, type, and validity of this lock. * * @return A descriptive string */ public final String toString() { return (this.getClass().getName() + "[" + position + ":" + size + " " + (shared ? "shared" : "exclusive") + " " + (isValid() ? "valid" : "invalid") + "]"); } }