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   * 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
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   */
  
  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 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>&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.
       *
       * @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")
                  + "]");
      }
  
  }
  