/* * Copyright (c) 2001, 2010, 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"><h4> Platform dependencies </h4></a> * * <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.  </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;    }     /**     * 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>&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     *     * @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.  </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;     /**     * 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")                + "]");    } }