/*

 * Copyright 2001 Sun Microsystems, Inc.  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.  Sun designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,

 * CA 95054 USA or visit www.sun.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.

 *

 * <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">

 * <h4> Platform dependencies </h4>

 *

 * <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

 * </code>mapped into memory<code>}, 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 {

    private final FileChannel channel;

    private final long position;

    private final long size;

    private final boolean shared;

    /**

     * Initializes a new instance of this class.  </p>

     *

     * @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>&nbsp;+&nbsp;<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;

    }

    /**

     * Returns the file channel upon whose file this lock is held.  </p>

     *

     * @return  The file channel

     */

    public final FileChannel channel() {

        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.  </p>

     *

     * @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.  </p>

     *

     * @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;

    /**

     * 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")

                + "]");

    }

}