1 /*
   2  * Copyright (c) 2007, 2011, Oracle and/or its affiliates. All rights reserved.
   3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   4  *
   5  * This code is free software; you can redistribute it and/or modify it
   6  * under the terms of the GNU General Public License version 2 only, as
   7  * published by the Free Software Foundation.  Oracle designates this
   8  * particular file as subject to the "Classpath" exception as provided
   9  * by Oracle in the LICENSE file that accompanied this code.
  10  *
  11  * This code is distributed in the hope that it will be useful, but WITHOUT
  12  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  13  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  14  * version 2 for more details (a copy is included in the LICENSE file that
  15  * accompanied this code).
  16  *
  17  * You should have received a copy of the GNU General Public License version
  18  * 2 along with this work; if not, write to the Free Software Foundation,
  19  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  20  *
  21  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  22  * or visit www.oracle.com if you need additional information or have any
  23  * questions.
  24  */
  25 
  26 package java.nio.channels;
  27 
  28 import java.nio.file.*;
  29 import java.nio.file.attribute.FileAttribute;
  30 import java.nio.file.spi.*;
  31 import java.nio.ByteBuffer;
  32 import java.io.IOException;
  33 import java.util.concurrent.Future;
  34 import java.util.concurrent.ExecutorService;
  35 import java.util.Set;
  36 import java.util.HashSet;
  37 import java.util.Collections;
  38 
  39 /**
  40  * An asynchronous channel for reading, writing, and manipulating a file.
  41  *
  42  * <p> An asynchronous file channel is created when a file is opened by invoking
  43  * one of the {@link #open open} methods defined by this class. The file contains
  44  * a variable-length sequence of bytes that can be read and written and whose
  45  * current size can be {@link #size() queried}. The size of the file increases
  46  * when bytes are written beyond its  current size; the size of the file decreases
  47  * when it is {@link #truncate truncated}.
  48  *
  49  * <p> An asynchronous file channel does not have a <i>current position</i>
  50  * within the file. Instead, the file position is specified to each read and
  51  * write method that initiates asynchronous operations. A {@link CompletionHandler}
  52  * is specified as a parameter and is invoked to consume the result of the I/O
  53  * operation. This class also defines read and write methods that initiate
  54  * asynchronous operations, returning a {@link Future} to represent the pending
  55  * result of the operation. The {@code Future} may be used to check if the
  56  * operation has completed, wait for its completion, and retrieve the result.
  57  *
  58  * <p> In addition to read and write operations, this class defines the
  59  * following operations: </p>
  60  *
  61  * <ul>
  62  *
  63  *   <li><p> Updates made to a file may be {@link #force <i>forced
  64  *   out</i>} to the underlying storage device, ensuring that data are not
  65  *   lost in the event of a system crash.  </p></li>
  66  *
  67  *   <li><p> A region of a file may be {@link #lock <i>locked</i>} against
  68  *   access by other programs.  </p></li>
  69  *
  70  * </ul>
  71  *
  72  * <p> An {@code AsynchronousFileChannel} is associated with a thread pool to
  73  * which tasks are submitted to handle I/O events and dispatch to completion
  74  * handlers that consume the results of I/O operations on the channel. The
  75  * completion handler for an I/O operation initiated on a channel is guaranteed
  76  * to be invoked by one of the threads in the thread pool (This ensures that the
  77  * completion handler is run by a thread with the expected <em>identity</em>).
  78  * Where an I/O operation completes immediately, and the initiating thread is
  79  * itself a thread in the thread pool, then the completion handler may be invoked
  80  * directly by the initiating thread. When an {@code AsynchronousFileChannel} is
  81  * created without specifying a thread pool then the channel is associated with
  82  * a system-dependent default thread pool that may be shared with other
  83  * channels. The default thread pool is configured by the system properties
  84  * defined by the {@link AsynchronousChannelGroup} class.
  85  *
  86  * <p> Channels of this type are safe for use by multiple concurrent threads. The
  87  * {@link Channel#close close} method may be invoked at any time, as specified
  88  * by the {@link Channel} interface. This causes all outstanding asynchronous
  89  * operations on the channel to complete with the exception {@link
  90  * AsynchronousCloseException}. Multiple read and write operations may be
  91  * outstanding at the same time. When multiple read and write operations are
  92  * outstanding then the ordering of the I/O operations, and the order that the
  93  * completion handlers are invoked, is not specified; they are not, in particular,
  94  * guaranteed to execute in the order that the operations were initiated. The
  95  * {@link java.nio.ByteBuffer ByteBuffers} used when reading or writing are not
  96  * safe for use by multiple concurrent I/O operations. Furthermore, after an I/O
  97  * operation is initiated then care should be taken to ensure that the buffer is
  98  * not accessed until after the operation has completed.
  99  *
 100  * <p> As with {@link FileChannel}, the view of a file provided by an instance of
 101  * this class is guaranteed to be consistent with other views of the same file
 102  * provided by other instances in the same program.  The view provided by an
 103  * instance of this class may or may not, however, be consistent with the views
 104  * seen by other concurrently-running programs due to caching performed by the
 105  * underlying operating system and delays induced by network-filesystem protocols.
 106  * This is true regardless of the language in which these other programs are
 107  * written, and whether they are running on the same machine or on some other
 108  * machine.  The exact nature of any such inconsistencies are system-dependent
 109  * and are therefore unspecified.
 110  *
 111  * @since 1.7
 112  */
 113 
 114 public abstract class AsynchronousFileChannel
 115     implements AsynchronousChannel
 116 {
 117     /**
 118      * Initializes a new instance of this class.
 119      */
 120     protected AsynchronousFileChannel() {
 121     }
 122 
 123     /**
 124      * Opens or creates a file for reading and/or writing, returning an
 125      * asynchronous file channel to access the file.
 126      *
 127      * <p> The {@code options} parameter determines how the file is opened.
 128      * The {@link StandardOpenOption#READ READ} and {@link StandardOpenOption#WRITE
 129      * WRITE} options determines if the file should be opened for reading and/or
 130      * writing. If neither option is contained in the array then an existing file
 131      * is opened for  reading.
 132      *
 133      * <p> In addition to {@code READ} and {@code WRITE}, the following options
 134      * may be present:
 135      *
 136      * <table border=1 cellpadding=5 summary="">
 137      * <tr> <th>Option</th> <th>Description</th> </tr>
 138      * <tr>
 139      *   <td> {@link StandardOpenOption#TRUNCATE_EXISTING TRUNCATE_EXISTING} </td>
 140      *   <td> When opening an existing file, the file is first truncated to a
 141      *   size of 0 bytes. This option is ignored when the file is opened only
 142      *   for reading.</td>
 143      * </tr>
 144      * <tr>
 145      *   <td> {@link StandardOpenOption#CREATE_NEW CREATE_NEW} </td>
 146      *   <td> If this option is present then a new file is created, failing if
 147      *   the file already exists. When creating a file the check for the
 148      *   existence of the file and the creation of the file if it does not exist
 149      *   is atomic with respect to other file system operations. This option is
 150      *   ignored when the file is opened only for reading. </td>
 151      * </tr>
 152      * <tr>
 153      *   <td > {@link StandardOpenOption#CREATE CREATE} </td>
 154      *   <td> If this option is present then an existing file is opened if it
 155      *   exists, otherwise a new file is created. When creating a file the check
 156      *   for the existence of the file and the creation of the file if it does
 157      *   not exist is atomic with respect to other file system operations. This
 158      *   option is ignored if the {@code CREATE_NEW} option is also present or
 159      *   the file is opened only for reading. </td>
 160      * </tr>
 161      * <tr>
 162      *   <td > {@link StandardOpenOption#DELETE_ON_CLOSE DELETE_ON_CLOSE} </td>
 163      *   <td> When this option is present then the implementation makes a
 164      *   <em>best effort</em> attempt to delete the file when closed by the
 165      *   the {@link #close close} method. If the {@code close} method is not
 166      *   invoked then a <em>best effort</em> attempt is made to delete the file
 167      *   when the Java virtual machine terminates. </td>
 168      * </tr>
 169      * <tr>
 170      *   <td>{@link StandardOpenOption#SPARSE SPARSE} </td>
 171      *   <td> When creating a new file this option is a <em>hint</em> that the
 172      *   new file will be sparse. This option is ignored when not creating
 173      *   a new file. </td>
 174      * </tr>
 175      * <tr>
 176      *   <td> {@link StandardOpenOption#SYNC SYNC} </td>
 177      *   <td> Requires that every update to the file's content or metadata be
 178      *   written synchronously to the underlying storage device. (see <a
 179      *   href="../file/package-summary.html#integrity"> Synchronized I/O file
 180      *   integrity</a>). </td>
 181      * <tr>
 182      * <tr>
 183      *   <td> {@link StandardOpenOption#DSYNC DSYNC} </td>
 184      *   <td> Requires that every update to the file's content be written
 185      *   synchronously to the underlying storage device. (see <a
 186      *   href="../file/package-summary.html#integrity"> Synchronized I/O file
 187      *   integrity</a>). </td>
 188      * </tr>
 189      * </table>
 190      *
 191      * <p> An implementation may also support additional options.
 192      *
 193      * <p> The {@code executor} parameter is the {@link ExecutorService} to
 194      * which tasks are submitted to handle I/O events and dispatch completion
 195      * results for operations initiated on resulting channel.
 196      * The nature of these tasks is highly implementation specific and so care
 197      * should be taken when configuring the {@code Executor}. Minimally it
 198      * should support an unbounded work queue and should not run tasks on the
 199      * caller thread of the {@link ExecutorService#execute execute} method.
 200      * Shutting down the executor service while the channel is open results in
 201      * unspecified behavior.
 202      *
 203      * <p> The {@code attrs} parameter is an optional array of file {@link
 204      * FileAttribute file-attributes} to set atomically when creating the file.
 205      *
 206      * <p> The new channel is created by invoking the {@link
 207      * FileSystemProvider#newFileChannel newFileChannel} method on the
 208      * provider that created the {@code Path}.
 209      *
 210      * @param   file
 211      *          The path of the file to open or create
 212      * @param   options
 213      *          Options specifying how the file is opened
 214      * @param   executor
 215      *          The thread pool or {@code null} to associate the channel with
 216      *          the default thread pool
 217      * @param   attrs
 218      *          An optional list of file attributes to set atomically when
 219      *          creating the file
 220      *
 221      * @return  A new asynchronous file channel
 222      *
 223      * @throws  IllegalArgumentException
 224      *          If the set contains an invalid combination of options
 225      * @throws  UnsupportedOperationException
 226      *          If the {@code file} is associated with a provider that does not
 227      *          support creating asynchronous file channels, or an unsupported
 228      *          open option is specified, or the array contains an attribute that
 229      *          cannot be set atomically when creating the file
 230      * @throws  IOException
 231      *          If an I/O error occurs
 232      * @throws  SecurityException
 233      *          If a security manager is installed and it denies an
 234      *          unspecified permission required by the implementation.
 235      *          In the case of the default provider, the {@link
 236      *          SecurityManager#checkRead(String)} method is invoked to check
 237      *          read access if the file is opened for reading. The {@link
 238      *          SecurityManager#checkWrite(String)} method is invoked to check
 239      *          write access if the file is opened for writing
 240      */
 241     public static AsynchronousFileChannel open(Path file,
 242                                                Set<? extends OpenOption> options,
 243                                                ExecutorService executor,
 244                                                FileAttribute<?>... attrs)
 245         throws IOException
 246     {
 247         FileSystemProvider provider = file.getFileSystem().provider();
 248         return provider.newAsynchronousFileChannel(file, options, executor, attrs);
 249     }
 250 
 251     @SuppressWarnings({"unchecked", "rawtypes"}) // generic array construction
 252     private static final FileAttribute<?>[] NO_ATTRIBUTES = new FileAttribute[0];
 253 
 254     /**
 255      * Opens or creates a file for reading and/or writing, returning an
 256      * asynchronous file channel to access the file.
 257      *
 258      * <p> An invocation of this method behaves in exactly the same way as the
 259      * invocation
 260      * <pre>
 261      *     ch.{@link #open(Path,Set,ExecutorService,FileAttribute[])
 262      *       open}(file, opts, null, new FileAttribute&lt;?&gt;[0]);
 263      * </pre>
 264      * where {@code opts} is a {@code Set} containing the options specified to
 265      * this method.
 266      *
 267      * <p> The resulting channel is associated with default thread pool to which
 268      * tasks are submitted to handle I/O events and dispatch to completion
 269      * handlers that consume the result of asynchronous operations performed on
 270      * the resulting channel.
 271      *
 272      * @param   file
 273      *          The path of the file to open or create
 274      * @param   options
 275      *          Options specifying how the file is opened
 276      *
 277      * @return  A new asynchronous file channel
 278      *
 279      * @throws  IllegalArgumentException
 280      *          If the set contains an invalid combination of options
 281      * @throws  UnsupportedOperationException
 282      *          If the {@code file} is associated with a provider that does not
 283      *          support creating file channels, or an unsupported open option is
 284      *          specified
 285      * @throws  IOException
 286      *          If an I/O error occurs
 287      * @throws  SecurityException
 288      *          If a security manager is installed and it denies an
 289      *          unspecified permission required by the implementation.
 290      *          In the case of the default provider, the {@link
 291      *          SecurityManager#checkRead(String)} method is invoked to check
 292      *          read access if the file is opened for reading. The {@link
 293      *          SecurityManager#checkWrite(String)} method is invoked to check
 294      *          write access if the file is opened for writing
 295      */
 296     public static AsynchronousFileChannel open(Path file, OpenOption... options)
 297         throws IOException
 298     {
 299         Set<OpenOption> set = new HashSet<OpenOption>(options.length);
 300         Collections.addAll(set, options);
 301         return open(file, set, null, NO_ATTRIBUTES);
 302     }
 303 
 304     /**
 305      * Returns the current size of this channel's file.
 306      *
 307      * @return  The current size of this channel's file, measured in bytes
 308      *
 309      * @throws  ClosedChannelException
 310      *          If this channel is closed
 311      * @throws  IOException
 312      *          If some other I/O error occurs
 313      */
 314     public abstract long size() throws IOException;
 315 
 316     /**
 317      * Truncates this channel's file to the given size.
 318      *
 319      * <p> If the given size is less than the file's current size then the file
 320      * is truncated, discarding any bytes beyond the new end of the file.  If
 321      * the given size is greater than or equal to the file's current size then
 322      * the file is not modified. </p>
 323      *
 324      * @param  size
 325      *         The new size, a non-negative byte count
 326      *
 327      * @return  This file channel
 328      *
 329      * @throws  NonWritableChannelException
 330      *          If this channel was not opened for writing
 331      *
 332      * @throws  ClosedChannelException
 333      *          If this channel is closed
 334      *
 335      * @throws  IllegalArgumentException
 336      *          If the new size is negative
 337      *
 338      * @throws  IOException
 339      *          If some other I/O error occurs
 340      */
 341     public abstract AsynchronousFileChannel truncate(long size) throws IOException;
 342 
 343     /**
 344      * Forces any updates to this channel's file to be written to the storage
 345      * device that contains it.
 346      *
 347      * <p> If this channel's file resides on a local storage device then when
 348      * this method returns it is guaranteed that all changes made to the file
 349      * since this channel was created, or since this method was last invoked,
 350      * will have been written to that device.  This is useful for ensuring that
 351      * critical information is not lost in the event of a system crash.
 352      *
 353      * <p> If the file does not reside on a local device then no such guarantee
 354      * is made.
 355      *
 356      * <p> The {@code metaData} parameter can be used to limit the number of
 357      * I/O operations that this method is required to perform.  Passing
 358      * {@code false} for this parameter indicates that only updates to the
 359      * file's content need be written to storage; passing {@code true}
 360      * indicates that updates to both the file's content and metadata must be
 361      * written, which generally requires at least one more I/O operation.
 362      * Whether this parameter actually has any effect is dependent upon the
 363      * underlying operating system and is therefore unspecified.
 364      *
 365      * <p> Invoking this method may cause an I/O operation to occur even if the
 366      * channel was only opened for reading.  Some operating systems, for
 367      * example, maintain a last-access time as part of a file's metadata, and
 368      * this time is updated whenever the file is read.  Whether or not this is
 369      * actually done is system-dependent and is therefore unspecified.
 370      *
 371      * <p> This method is only guaranteed to force changes that were made to
 372      * this channel's file via the methods defined in this class.
 373      *
 374      * @param   metaData
 375      *          If {@code true} then this method is required to force changes
 376      *          to both the file's content and metadata to be written to
 377      *          storage; otherwise, it need only force content changes to be
 378      *          written
 379      *
 380      * @throws  ClosedChannelException
 381      *          If this channel is closed
 382      *
 383      * @throws  IOException
 384      *          If some other I/O error occurs
 385      */
 386     public abstract void force(boolean metaData) throws IOException;
 387 
 388     /**
 389      * Acquires a lock on the given region of this channel's file.
 390      *
 391      * <p> This method initiates an operation to acquire a lock on the given
 392      * region of this channel's file. The {@code handler} parameter is a
 393      * completion handler that is invoked when the lock is acquired (or the
 394      * operation fails). The result passed to the completion handler is the
 395      * resulting {@code FileLock}.
 396      *
 397      * <p> The region specified by the {@code position} and {@code size}
 398      * parameters need not be contained within, or even overlap, the actual
 399      * underlying file.  Lock regions are fixed in size; if a locked region
 400      * initially contains the end of the file and the file grows beyond the
 401      * region then the new portion of the file will not be covered by the lock.
 402      * If a file is expected to grow in size and a lock on the entire file is
 403      * required then a region starting at zero, and no smaller than the
 404      * expected maximum size of the file, should be locked.  The two-argument
 405      * {@link #lock(Object,CompletionHandler)} method simply locks a region
 406      * of size {@link Long#MAX_VALUE}. If a lock that overlaps the requested
 407      * region is already held by this Java virtual machine, or this method has
 408      * been invoked to lock an overlapping region and that operation has not
 409      * completed, then this method throws {@link OverlappingFileLockException}.
 410      *
 411      * <p> Some operating systems do not support a mechanism to acquire a file
 412      * lock in an asynchronous manner. Consequently an implementation may
 413      * acquire the file lock in a background thread or from a task executed by
 414      * a thread in the associated thread pool. If there are many lock operations
 415      * outstanding then it may consume threads in the Java virtual machine for
 416      * indefinite periods.
 417      *
 418      * <p> Some operating systems do not support shared locks, in which case a
 419      * request for a shared lock is automatically converted into a request for
 420      * an exclusive lock.  Whether the newly-acquired lock is shared or
 421      * exclusive may be tested by invoking the resulting lock object's {@link
 422      * FileLock#isShared() isShared} method.
 423      *
 424      * <p> File locks are held on behalf of the entire Java virtual machine.
 425      * They are not suitable for controlling access to a file by multiple
 426      * threads within the same virtual machine.
 427      *
 428      * @param   position
 429      *          The position at which the locked region is to start; must be
 430      *          non-negative
 431      * @param   size
 432      *          The size of the locked region; must be non-negative, and the sum
 433      *          {@code position}&nbsp;+&nbsp;{@code size} must be non-negative
 434      * @param   shared
 435      *          {@code true} to request a shared lock, in which case this
 436      *          channel must be open for reading (and possibly writing);
 437      *          {@code false} to request an exclusive lock, in which case this
 438      *          channel must be open for writing (and possibly reading)
 439      * @param   attachment
 440      *          The object to attach to the I/O operation; can be {@code null}
 441      * @param   handler
 442      *          The handler for consuming the result
 443      *
 444      * @throws  OverlappingFileLockException
 445      *          If a lock that overlaps the requested region is already held by
 446      *          this Java virtual machine, or there is already a pending attempt
 447      *          to lock an overlapping region
 448      * @throws  IllegalArgumentException
 449      *          If the preconditions on the parameters do not hold
 450      * @throws  NonReadableChannelException
 451      *          If {@code shared} is true but this channel was not opened for reading
 452      * @throws  NonWritableChannelException
 453      *          If {@code shared} is false but this channel was not opened for writing
 454      */
 455     public abstract <A> void lock(long position,
 456                                   long size,
 457                                   boolean shared,
 458                                   A attachment,
 459                                   CompletionHandler<FileLock,? super A> handler);
 460 
 461     /**
 462      * Acquires an exclusive lock on this channel's file.
 463      *
 464      * <p> This method initiates an operation to acquire a lock on the given
 465      * region of this channel's file. The {@code handler} parameter is a
 466      * completion handler that is invoked when the lock is acquired (or the
 467      * operation fails). The result passed to the completion handler is the
 468      * resulting {@code FileLock}.
 469      *
 470      * <p> An invocation of this method of the form {@code ch.lock(att,handler)}
 471      * behaves in exactly the same way as the invocation
 472      * <pre>
 473      *     ch.{@link #lock(long,long,boolean,Object,CompletionHandler) lock}(0L, Long.MAX_VALUE, false, att, handler)
 474      * </pre>
 475      *
 476      * @param   attachment
 477      *          The object to attach to the I/O operation; can be {@code null}
 478      * @param   handler
 479      *          The handler for consuming the result
 480      *
 481      * @throws  OverlappingFileLockException
 482      *          If a lock is already held by this Java virtual machine, or there
 483      *          is already a pending attempt to lock a region
 484      * @throws  NonWritableChannelException
 485      *          If this channel was not opened for writing
 486      */
 487     public final <A> void lock(A attachment,
 488                                CompletionHandler<FileLock,? super A> handler)
 489     {
 490         lock(0L, Long.MAX_VALUE, false, attachment, handler);
 491     }
 492 
 493     /**
 494      * Acquires a lock on the given region of this channel's file.
 495      *
 496      * <p> This method initiates an operation to acquire a lock on the given
 497      * region of this channel's file.  The method behaves in exactly the same
 498      * manner as the {@link #lock(long, long, boolean, Object, CompletionHandler)}
 499      * method except that instead of specifying a completion handler, this
 500      * method returns a {@code Future} representing the pending result. The
 501      * {@code Future}'s {@link Future#get() get} method returns the {@link
 502      * FileLock} on successful completion.
 503      *
 504      * @param   position
 505      *          The position at which the locked region is to start; must be
 506      *          non-negative
 507      * @param   size
 508      *          The size of the locked region; must be non-negative, and the sum
 509      *          {@code position}&nbsp;+&nbsp;{@code size} must be non-negative
 510      * @param   shared
 511      *          {@code true} to request a shared lock, in which case this
 512      *          channel must be open for reading (and possibly writing);
 513      *          {@code false} to request an exclusive lock, in which case this
 514      *          channel must be open for writing (and possibly reading)
 515      *
 516      * @return  a {@code Future} object representing the pending result
 517      *
 518      * @throws  OverlappingFileLockException
 519      *          If a lock is already held by this Java virtual machine, or there
 520      *          is already a pending attempt to lock a region
 521      * @throws  IllegalArgumentException
 522      *          If the preconditions on the parameters do not hold
 523      * @throws  NonReadableChannelException
 524      *          If {@code shared} is true but this channel was not opened for reading
 525      * @throws  NonWritableChannelException
 526      *          If {@code shared} is false but this channel was not opened for writing
 527      */
 528     public abstract Future<FileLock> lock(long position, long size, boolean shared);
 529 
 530     /**
 531      * Acquires an exclusive lock on this channel's file.
 532      *
 533      * <p> This method initiates an operation to acquire an exclusive lock on this
 534      * channel's file. The method returns a {@code Future} representing the
 535      * pending result of the operation. The {@code Future}'s {@link Future#get()
 536      * get} method returns the {@link FileLock} on successful completion.
 537      *
 538      * <p> An invocation of this method behaves in exactly the same way as the
 539      * invocation
 540      * <pre>
 541      *     ch.{@link #lock(long,long,boolean) lock}(0L, Long.MAX_VALUE, false)
 542      * </pre>
 543      *
 544      * @return  a {@code Future} object representing the pending result
 545      *
 546      * @throws  OverlappingFileLockException
 547      *          If a lock is already held by this Java virtual machine, or there
 548      *          is already a pending attempt to lock a region
 549      * @throws  NonWritableChannelException
 550      *          If this channel was not opened for writing
 551      */
 552     public final Future<FileLock> lock() {
 553         return lock(0L, Long.MAX_VALUE, false);
 554     }
 555 
 556     /**
 557      * Attempts to acquire a lock on the given region of this channel's file.
 558      *
 559      * <p> This method does not block. An invocation always returns immediately,
 560      * either having acquired a lock on the requested region or having failed to
 561      * do so.  If it fails to acquire a lock because an overlapping lock is held
 562      * by another program then it returns {@code null}.  If it fails to acquire
 563      * a lock for any other reason then an appropriate exception is thrown.
 564      *
 565      * @param  position
 566      *         The position at which the locked region is to start; must be
 567      *         non-negative
 568      *
 569      * @param  size
 570      *         The size of the locked region; must be non-negative, and the sum
 571      *         {@code position}&nbsp;+&nbsp;{@code size} must be non-negative
 572      *
 573      * @param  shared
 574      *         {@code true} to request a shared lock,
 575      *         {@code false} to request an exclusive lock
 576      *
 577      * @return  A lock object representing the newly-acquired lock,
 578      *          or {@code null} if the lock could not be acquired
 579      *          because another program holds an overlapping lock
 580      *
 581      * @throws  IllegalArgumentException
 582      *          If the preconditions on the parameters do not hold
 583      * @throws  ClosedChannelException
 584      *          If this channel is closed
 585      * @throws  OverlappingFileLockException
 586      *          If a lock that overlaps the requested region is already held by
 587      *          this Java virtual machine, or if another thread is already
 588      *          blocked in this method and is attempting to lock an overlapping
 589      *          region of the same file
 590      * @throws  NonReadableChannelException
 591      *          If {@code shared} is true but this channel was not opened for reading
 592      * @throws  NonWritableChannelException
 593      *          If {@code shared} is false but this channel was not opened for writing
 594      *
 595      * @throws  IOException
 596      *          If some other I/O error occurs
 597      *
 598      * @see     #lock(Object,CompletionHandler)
 599      * @see     #lock(long,long,boolean,Object,CompletionHandler)
 600      * @see     #tryLock()
 601      */
 602     public abstract FileLock tryLock(long position, long size, boolean shared)
 603         throws IOException;
 604 
 605     /**
 606      * Attempts to acquire an exclusive lock on this channel's file.
 607      *
 608      * <p> An invocation of this method of the form {@code ch.tryLock()}
 609      * behaves in exactly the same way as the invocation
 610      *
 611      * <pre>
 612      *     ch.{@link #tryLock(long,long,boolean) tryLock}(0L, Long.MAX_VALUE, false) </pre>
 613      *
 614      * @return  A lock object representing the newly-acquired lock,
 615      *          or {@code null} if the lock could not be acquired
 616      *          because another program holds an overlapping lock
 617      *
 618      * @throws  ClosedChannelException
 619      *          If this channel is closed
 620      * @throws  OverlappingFileLockException
 621      *          If a lock that overlaps the requested region is already held by
 622      *          this Java virtual machine, or if another thread is already
 623      *          blocked in this method and is attempting to lock an overlapping
 624      *          region
 625      * @throws  NonWritableChannelException
 626      *          If {@code shared} is false but this channel was not opened for writing
 627      *
 628      * @throws  IOException
 629      *          If some other I/O error occurs
 630      *
 631      * @see     #lock(Object,CompletionHandler)
 632      * @see     #lock(long,long,boolean,Object,CompletionHandler)
 633      * @see     #tryLock(long,long,boolean)
 634      */
 635     public final FileLock tryLock() throws IOException {
 636         return tryLock(0L, Long.MAX_VALUE, false);
 637     }
 638 
 639     /**
 640      * Reads a sequence of bytes from this channel into the given buffer,
 641      * starting at the given file position.
 642      *
 643      * <p> This method initiates the reading of a sequence of bytes from this
 644      * channel into the given buffer, starting at the given file position. The
 645      * result of the read is the number of bytes read or {@code -1} if the given
 646      * position is greater than or equal to the file's size at the time that the
 647      * read is attempted.
 648      *
 649      * <p> This method works in the same manner as the {@link
 650      * AsynchronousByteChannel#read(ByteBuffer,Object,CompletionHandler)}
 651      * method, except that bytes are read starting at the given file position.
 652      * If the given file position is greater than the file's size at the time
 653      * that the read is attempted then no bytes are read.
 654      *
 655      * @param   dst
 656      *          The buffer into which bytes are to be transferred
 657      * @param   position
 658      *          The file position at which the transfer is to begin;
 659      *          must be non-negative
 660      * @param   attachment
 661      *          The object to attach to the I/O operation; can be {@code null}
 662      * @param   handler
 663      *          The handler for consuming the result
 664      *
 665      * @throws  IllegalArgumentException
 666      *          If the position is negative or the buffer is read-only
 667      * @throws  NonReadableChannelException
 668      *          If this channel was not opened for reading
 669      */
 670     public abstract <A> void read(ByteBuffer dst,
 671                                   long position,
 672                                   A attachment,
 673                                   CompletionHandler<Integer,? super A> handler);
 674 
 675     /**
 676      * Reads a sequence of bytes from this channel into the given buffer,
 677      * starting at the given file position.
 678      *
 679      * <p> This method initiates the reading of a sequence of bytes from this
 680      * channel into the given buffer, starting at the given file position. This
 681      * method returns a {@code Future} representing the pending result of the
 682      * operation. The {@code Future}'s {@link Future#get() get} method returns
 683      * the number of bytes read or {@code -1} if the given position is greater
 684      * than or equal to the file's size at the time that the read is attempted.
 685      *
 686      * <p> This method works in the same manner as the {@link
 687      * AsynchronousByteChannel#read(ByteBuffer)} method, except that bytes are
 688      * read starting at the given file position. If the given file position is
 689      * greater than the file's size at the time that the read is attempted then
 690      * no bytes are read.
 691      *
 692      * @param   dst
 693      *          The buffer into which bytes are to be transferred
 694      * @param   position
 695      *          The file position at which the transfer is to begin;
 696      *          must be non-negative
 697      *
 698      * @return  A {@code Future} object representing the pending result
 699      *
 700      * @throws  IllegalArgumentException
 701      *          If the position is negative or the buffer is read-only
 702      * @throws  NonReadableChannelException
 703      *          If this channel was not opened for reading
 704      */
 705     public abstract Future<Integer> read(ByteBuffer dst, long position);
 706 
 707     /**
 708      * Writes a sequence of bytes to this channel from the given buffer, starting
 709      * at the given file position.
 710      *
 711      * <p> This method works in the same manner as the {@link
 712      * AsynchronousByteChannel#write(ByteBuffer,Object,CompletionHandler)}
 713      * method, except that bytes are written starting at the given file position.
 714      * If the given position is greater than the file's size, at the time that
 715      * the write is attempted, then the file will be grown to accommodate the new
 716      * bytes; the values of any bytes between the previous end-of-file and the
 717      * newly-written bytes are unspecified.
 718      *
 719      * @param   src
 720      *          The buffer from which bytes are to be transferred
 721      * @param   position
 722      *          The file position at which the transfer is to begin;
 723      *          must be non-negative
 724      * @param   attachment
 725      *          The object to attach to the I/O operation; can be {@code null}
 726      * @param   handler
 727      *          The handler for consuming the result
 728      *
 729      * @throws  IllegalArgumentException
 730      *          If the position is negative
 731      * @throws  NonWritableChannelException
 732      *          If this channel was not opened for writing
 733      */
 734     public abstract <A> void write(ByteBuffer src,
 735                                    long position,
 736                                    A attachment,
 737                                    CompletionHandler<Integer,? super A> handler);
 738 
 739     /**
 740      * Writes a sequence of bytes to this channel from the given buffer, starting
 741      * at the given file position.
 742      *
 743      * <p> This method initiates the writing of a sequence of bytes to this
 744      * channel from the given buffer, starting at the given file position. The
 745      * method returns a {@code Future} representing the pending result of the
 746      * write operation. The {@code Future}'s {@link Future#get() get} method
 747      * returns the number of bytes written.
 748      *
 749      * <p> This method works in the same manner as the {@link
 750      * AsynchronousByteChannel#write(ByteBuffer)} method, except that bytes are
 751      * written starting at the given file position. If the given position is
 752      * greater than the file's size, at the time that the write is attempted,
 753      * then the file will be grown to accommodate the new bytes; the values of
 754      * any bytes between the previous end-of-file and the newly-written bytes
 755      * are unspecified.
 756      *
 757      * @param   src
 758      *          The buffer from which bytes are to be transferred
 759      * @param   position
 760      *          The file position at which the transfer is to begin;
 761      *          must be non-negative
 762      *
 763      * @return  A {@code Future} object representing the pending result
 764      *
 765      * @throws  IllegalArgumentException
 766      *          If the position is negative
 767      * @throws  NonWritableChannelException
 768      *          If this channel was not opened for writing
 769      */
 770     public abstract Future<Integer> write(ByteBuffer src, long position);
 771 }
--- EOF ---