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	/*
	* Copyright (c) 2001, 2013, Oracle and/or its affiliates. All rights reserved.
	* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
	*
	* This code is free software; you can redistribute it and/or modify it
	* under the terms of the GNU General Public License version 2 only, as
	* published by the Free Software Foundation. Oracle designates this
	* particular file as subject to the "Classpath" exception as provided
	* by Oracle in the LICENSE file that accompanied this code.
	*
	* This code is distributed in the hope that it will be useful, but WITHOUT
	* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
	* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
	* version 2 for more details (a copy is included in the LICENSE file that
	* accompanied this code).
	*
	* You should have received a copy of the GNU General Public License version
	* 2 along with this work; if not, write to the Free Software Foundation,
	* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
	*
	* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
	* or visit www.oracle.com if you need additional information or have any
	* questions.
	*/

	package java.nio.channels;

	import java.io.IOException;

	/**
	* A token representing a lock on a region of a file.
	*
	* <p> 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.
	*
	* <p> 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.
	*
	* <p> A file lock is either <i>exclusive</i> or <i>shared</i>. 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.
	*
	* <p> 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.
	*
	* <p> 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.
	*
	* <p> 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.
	*
	* <p> 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.
	*
	* <p> File-lock objects are safe for use by multiple concurrent threads.
	*
	*
	* <a name="pdep"></a><h2> Platform dependencies </h2>
	*
	* <p> 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.
	*
	* <p> 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
	* <i>advisory</i>, meaning that programs must cooperatively observe a known
	* locking protocol in order to guarantee data integrity. On other systems
	* native file locks are <i>mandatory</i>, 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.
	*
	* <p> On some systems, acquiring a mandatory lock on a region of a file
	* prevents that region from being {@link java.nio.channels.FileChannel#map
	* <i>mapped into memory</i>}, and vice versa. Programs that combine
	* locking and mapping should be prepared for this combination to fail.
	*
	* <p> 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.
	*
	* <p> 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 2<sup>30</sup> or 2<sup>31</sup>. 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
	* <tt>position</tt> + <tt>size</tt> must be non-negative
	*
	* @param shared
	* <tt>true</tt> if this lock is shared,
	* <tt>false</tt> 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
	* <tt>position</tt> + <tt>size</tt> must be non-negative
	*
	* @param shared
	* <tt>true</tt> if this lock is shared,
	* <tt>false</tt> 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.
	*
	* <p> 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.
	*
	* <p> 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. </p>
	*
	* @return The position
	*/
	public final long position() {
	return position;
	}

	/**
	* Returns the size of the locked region in bytes.
	*
	* <p> 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. </p>
	*
	* @return The size of the locked region
	*/
	public final long size() {
	return size;
	}

	/**
	* Tells whether this lock is shared.
	*
	* @return <tt>true</tt> if lock is shared,
	* <tt>false</tt> if it is exclusive
	*/
	public final boolean isShared() {
	return shared;
	}

	/**
	* Tells whether or not this lock overlaps the given lock range.
	*
	* @param position
	* The starting position of the lock range
	* @param size
	* The size of the lock range
	*
	* @return <tt>true</tt> 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.
	*
	* <p> A lock object remains valid until it is released or the associated
	* file channel is closed, whichever comes first. </p>
	*
	* @return <tt>true</tt> if, and only if, this lock is valid
	*/
	public abstract boolean isValid();

	/**
	* Releases this lock.
	*
	* <p> 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. </p>
	*
	* @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")
	+ "]");
	}

	}

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