1 /*
   2  * Copyright (c) 2000, 2013, 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.io.*;
  29 import java.nio.ByteBuffer;
  30 import java.nio.MappedByteBuffer;
  31 import java.nio.channels.spi.AbstractInterruptibleChannel;
  32 import java.nio.file.*;
  33 import java.nio.file.attribute.FileAttribute;
  34 import java.nio.file.spi.*;
  35 import java.util.Set;
  36 import java.util.HashSet;
  37 import java.util.Collections;
  38 
  39 /**
  40  * A channel for reading, writing, mapping, and manipulating a file.
  41  *
  42  * <p> A file channel is a {@link SeekableByteChannel} that is connected to
  43  * a file. It has a current <i>position</i> within its file which can
  44  * be both {@link #position() <i>queried</i>} and {@link #position(long)
  45  * <i>modified</i>}.  The file itself contains a variable-length sequence
  46  * of bytes that can be read and written and whose current {@link #size
  47  * <i>size</i>} can be queried.  The size of the file increases
  48  * when bytes are written beyond its current size; the size of the file
  49  * decreases when it is {@link #truncate <i>truncated</i>}.  The
  50  * file may also have some associated <i>metadata</i> such as access
  51  * permissions, content type, and last-modification time; this class does not
  52  * define methods for metadata access.
  53  *
  54  * <p> In addition to the familiar read, write, and close operations of byte
  55  * channels, this class defines the following file-specific operations: </p>
  56  *
  57  * <ul>
  58  *
  59  *   <li><p> Bytes may be {@link #read(ByteBuffer, long) read} or
  60  *   {@link #write(ByteBuffer, long) <i>written</i>} at an absolute
  61  *   position in a file in a way that does not affect the channel's current
  62  *   position.  </p></li>
  63  *
  64  *   <li><p> A region of a file may be {@link #map <i>mapped</i>}
  65  *   directly into memory; for large files this is often much more efficient
  66  *   than invoking the usual {@code read} or {@code write} methods.
  67  *   </p></li>
  68  *
  69  *   <li><p> Updates made to a file may be {@link #force <i>forced
  70  *   out</i>} to the underlying storage device, ensuring that data are not
  71  *   lost in the event of a system crash.  </p></li>
  72  *
  73  *   <li><p> Bytes can be transferred from a file {@link #transferTo <i>to
  74  *   some other channel</i>}, and {@link #transferFrom <i>vice
  75  *   versa</i>}, in a way that can be optimized by many operating systems
  76  *   into a very fast transfer directly to or from the filesystem cache.
  77  *   </p></li>
  78  *
  79  *   <li><p> A region of a file may be {@link FileLock <i>locked</i>}
  80  *   against access by other programs.  </p></li>
  81  *
  82  * </ul>
  83  *
  84  * <p> File channels are safe for use by multiple concurrent threads.  The
  85  * {@link Channel#close close} method may be invoked at any time, as specified
  86  * by the {@link Channel} interface.  Only one operation that involves the
  87  * channel's position or can change its file's size may be in progress at any
  88  * given time; attempts to initiate a second such operation while the first is
  89  * still in progress will block until the first operation completes.  Other
  90  * operations, in particular those that take an explicit position, may proceed
  91  * concurrently; whether they in fact do so is dependent upon the underlying
  92  * implementation and is therefore unspecified.
  93  *
  94  * <p> The view of a file provided by an instance of this class is guaranteed
  95  * to be consistent with other views of the same file provided by other
  96  * instances in the same program.  The view provided by an instance of this
  97  * class may or may not, however, be consistent with the views seen by other
  98  * concurrently-running programs due to caching performed by the underlying
  99  * operating system and delays induced by network-filesystem protocols.  This
 100  * is true regardless of the language in which these other programs are
 101  * written, and whether they are running on the same machine or on some other
 102  * machine.  The exact nature of any such inconsistencies are system-dependent
 103  * and are therefore unspecified.
 104  *
 105  * <p> A file channel is created by invoking one of the {@link #open open}
 106  * methods defined by this class. A file channel can also be obtained from an
 107  * existing {@link java.io.FileInputStream#getChannel FileInputStream}, {@link
 108  * java.io.FileOutputStream#getChannel FileOutputStream}, or {@link
 109  * java.io.RandomAccessFile#getChannel RandomAccessFile} object by invoking
 110  * that object's {@code getChannel} method, which returns a file channel that
 111  * is connected to the same underlying file. Where the file channel is obtained
 112  * from an existing stream or random access file then the state of the file
 113  * channel is intimately connected to that of the object whose {@code getChannel}
 114  * method returned the channel.  Changing the channel's position, whether
 115  * explicitly or by reading or writing bytes, will change the file position of
 116  * the originating object, and vice versa. Changing the file's length via the
 117  * file channel will change the length seen via the originating object, and vice
 118  * versa.  Changing the file's content by writing bytes will change the content
 119  * seen by the originating object, and vice versa.
 120  *
 121  * <a name="open-mode"></a> <p> At various points this class specifies that an
 122  * instance that is "open for reading," "open for writing," or "open for
 123  * reading and writing" is required.  A channel obtained via the {@link
 124  * java.io.FileInputStream#getChannel getChannel} method of a {@link
 125  * java.io.FileInputStream} instance will be open for reading.  A channel
 126  * obtained via the {@link java.io.FileOutputStream#getChannel getChannel}
 127  * method of a {@link java.io.FileOutputStream} instance will be open for
 128  * writing.  Finally, a channel obtained via the {@link
 129  * java.io.RandomAccessFile#getChannel getChannel} method of a {@link
 130  * java.io.RandomAccessFile} instance will be open for reading if the instance
 131  * was created with mode {@code "r"} and will be open for reading and writing
 132  * if the instance was created with mode {@code "rw"}.
 133  *
 134  * <a name="append-mode"></a><p> A file channel that is open for writing may be in
 135  * <i>append mode</i>, for example if it was obtained from a file-output stream
 136  * that was created by invoking the {@link
 137  * java.io.FileOutputStream#FileOutputStream(java.io.File,boolean)
 138  * FileOutputStream(File,boolean)} constructor and passing {@code true} for
 139  * the second parameter.  In this mode each invocation of a relative write
 140  * operation first advances the position to the end of the file and then writes
 141  * the requested data.  Whether the advancement of the position and the writing
 142  * of the data are done in a single atomic operation is system-dependent and
 143  * therefore unspecified.
 144  *
 145  * @see java.io.FileInputStream#getChannel()
 146  * @see java.io.FileOutputStream#getChannel()
 147  * @see java.io.RandomAccessFile#getChannel()
 148  *
 149  * @author Mark Reinhold
 150  * @author Mike McCloskey
 151  * @author JSR-51 Expert Group
 152  * @since 1.4
 153  */
 154 
 155 public abstract class FileChannel
 156     extends AbstractInterruptibleChannel
 157     implements SeekableByteChannel, GatheringByteChannel, ScatteringByteChannel
 158 {
 159     /**
 160      * Initializes a new instance of this class.
 161      */
 162     protected FileChannel() { }
 163 
 164     /**
 165      * Opens or creates a file, returning a file channel to access the file.
 166      *
 167      * <p> The {@code options} parameter determines how the file is opened.
 168      * The {@link StandardOpenOption#READ READ} and {@link StandardOpenOption#WRITE
 169      * WRITE} options determine if the file should be opened for reading and/or
 170      * writing. If neither option (or the {@link StandardOpenOption#APPEND APPEND}
 171      * option) is contained in the array then the file is opened for reading.
 172      * By default reading or writing commences at the beginning of the file.
 173      *
 174      * <p> In the addition to {@code READ} and {@code WRITE}, the following
 175      * options may be present:
 176      *
 177      * <table border=1 cellpadding=5 summary="">
 178      * <tr> <th>Option</th> <th>Description</th> </tr>
 179      * <tr>
 180      *   <td> {@link StandardOpenOption#APPEND APPEND} </td>
 181      *   <td> If this option is present then the file is opened for writing and
 182      *     each invocation of the channel's {@code write} method first advances
 183      *     the position to the end of the file and then writes the requested
 184      *     data. Whether the advancement of the position and the writing of the
 185      *     data are done in a single atomic operation is system-dependent and
 186      *     therefore unspecified. This option may not be used in conjunction
 187      *     with the {@code READ} or {@code TRUNCATE_EXISTING} options. </td>
 188      * </tr>
 189      * <tr>
 190      *   <td> {@link StandardOpenOption#TRUNCATE_EXISTING TRUNCATE_EXISTING} </td>
 191      *   <td> If this option is present then the existing file is truncated to
 192      *   a size of 0 bytes. This option is ignored when the file is opened only
 193      *   for reading. </td>
 194      * </tr>
 195      * <tr>
 196      *   <td> {@link StandardOpenOption#CREATE_NEW CREATE_NEW} </td>
 197      *   <td> If this option is present then a new file is created, failing if
 198      *   the file already exists. When creating a file the check for the
 199      *   existence of the file and the creation of the file if it does not exist
 200      *   is atomic with respect to other file system operations. This option is
 201      *   ignored when the file is opened only for reading. </td>
 202      * </tr>
 203      * <tr>
 204      *   <td > {@link StandardOpenOption#CREATE CREATE} </td>
 205      *   <td> If this option is present then an existing file is opened if it
 206      *   exists, otherwise a new file is created. When creating a file the check
 207      *   for the existence of the file and the creation of the file if it does
 208      *   not exist is atomic with respect to other file system operations. This
 209      *   option is ignored if the {@code CREATE_NEW} option is also present or
 210      *   the file is opened only for reading. </td>
 211      * </tr>
 212      * <tr>
 213      *   <td > {@link StandardOpenOption#DELETE_ON_CLOSE DELETE_ON_CLOSE} </td>
 214      *   <td> When this option is present then the implementation makes a
 215      *   <em>best effort</em> attempt to delete the file when closed by the
 216      *   the {@link #close close} method. If the {@code close} method is not
 217      *   invoked then a <em>best effort</em> attempt is made to delete the file
 218      *   when the Java virtual machine terminates. </td>
 219      * </tr>
 220      * <tr>
 221      *   <td>{@link StandardOpenOption#SPARSE SPARSE} </td>
 222      *   <td> When creating a new file this option is a <em>hint</em> that the
 223      *   new file will be sparse. This option is ignored when not creating
 224      *   a new file. </td>
 225      * </tr>
 226      * <tr>
 227      *   <td> {@link StandardOpenOption#SYNC SYNC} </td>
 228      *   <td> Requires that every update to the file's content or metadata be
 229      *   written synchronously to the underlying storage device. (see <a
 230      *   href="../file/package-summary.html#integrity"> Synchronized I/O file
 231      *   integrity</a>). </td>
 232      * </tr>
 233      * <tr>
 234      *   <td> {@link StandardOpenOption#DSYNC DSYNC} </td>
 235      *   <td> Requires that every update to the file's content be written
 236      *   synchronously to the underlying storage device. (see <a
 237      *   href="../file/package-summary.html#integrity"> Synchronized I/O file
 238      *   integrity</a>). </td>
 239      * </tr>
 240      * </table>
 241      *
 242      * <p> An implementation may also support additional options.
 243      *
 244      * <p> The {@code attrs} parameter is an optional array of file {@link
 245      * FileAttribute file-attributes} to set atomically when creating the file.
 246      *
 247      * <p> The new channel is created by invoking the {@link
 248      * FileSystemProvider#newFileChannel newFileChannel} method on the
 249      * provider that created the {@code Path}.
 250      *
 251      * @param   path
 252      *          The path of the file to open or create
 253      * @param   options
 254      *          Options specifying how the file is opened
 255      * @param   attrs
 256      *          An optional list of file attributes to set atomically when
 257      *          creating the file
 258      *
 259      * @return  A new file channel
 260      *
 261      * @throws  IllegalArgumentException
 262      *          If the set contains an invalid combination of options
 263      * @throws  UnsupportedOperationException
 264      *          If the {@code path} is associated with a provider that does not
 265      *          support creating file channels, or an unsupported open option is
 266      *          specified, or the array contains an attribute that cannot be set
 267      *          atomically when creating the file
 268      * @throws  IOException
 269      *          If an I/O error occurs
 270      * @throws  SecurityException
 271      *          If a security manager is installed and it denies an
 272      *          unspecified permission required by the implementation.
 273      *          In the case of the default provider, the {@link
 274      *          SecurityManager#checkRead(String)} method is invoked to check
 275      *          read access if the file is opened for reading. The {@link
 276      *          SecurityManager#checkWrite(String)} method is invoked to check
 277      *          write access if the file is opened for writing
 278      *
 279      * @since   1.7
 280      */
 281     public static FileChannel open(Path path,
 282                                    Set<? extends OpenOption> options,
 283                                    FileAttribute<?>... attrs)
 284         throws IOException
 285     {
 286         FileSystemProvider provider = path.getFileSystem().provider();
 287         return provider.newFileChannel(path, options, attrs);
 288     }
 289 
 290     @SuppressWarnings({"unchecked", "rawtypes"}) // generic array construction
 291     private static final FileAttribute<?>[] NO_ATTRIBUTES = new FileAttribute[0];
 292 
 293     /**
 294      * Opens or creates a file, returning a file channel to access the file.
 295      *
 296      * <p> An invocation of this method behaves in exactly the same way as the
 297      * invocation
 298      * <pre>
 299      *     fc.{@link #open(Path,Set,FileAttribute[]) open}(file, opts, new FileAttribute&lt;?&gt;[0]);
 300      * </pre>
 301      * where {@code opts} is a set of the options specified in the {@code
 302      * options} array.
 303      *
 304      * @param   path
 305      *          The path of the file to open or create
 306      * @param   options
 307      *          Options specifying how the file is opened
 308      *
 309      * @return  A new file channel
 310      *
 311      * @throws  IllegalArgumentException
 312      *          If the set contains an invalid combination of options
 313      * @throws  UnsupportedOperationException
 314      *          If the {@code path} is associated with a provider that does not
 315      *          support creating file channels, or an unsupported open option is
 316      *          specified
 317      * @throws  IOException
 318      *          If an I/O error occurs
 319      * @throws  SecurityException
 320      *          If a security manager is installed and it denies an
 321      *          unspecified permission required by the implementation.
 322      *          In the case of the default provider, the {@link
 323      *          SecurityManager#checkRead(String)} method is invoked to check
 324      *          read access if the file is opened for reading. The {@link
 325      *          SecurityManager#checkWrite(String)} method is invoked to check
 326      *          write access if the file is opened for writing
 327      *
 328      * @since   1.7
 329      */
 330     public static FileChannel open(Path path, OpenOption... options)
 331         throws IOException
 332     {
 333         Set<OpenOption> set = new HashSet<>(options.length);
 334         Collections.addAll(set, options);
 335         return open(path, set, NO_ATTRIBUTES);
 336     }
 337 
 338     // -- Channel operations --
 339 
 340     /**
 341      * Reads a sequence of bytes from this channel into the given buffer.
 342      *
 343      * <p> Bytes are read starting at this channel's current file position, and
 344      * then the file position is updated with the number of bytes actually
 345      * read.  Otherwise this method behaves exactly as specified in the {@link
 346      * ReadableByteChannel} interface. </p>
 347      */
 348     public abstract int read(ByteBuffer dst) throws IOException;
 349 
 350     /**
 351      * Reads a sequence of bytes from this channel into a subsequence of the
 352      * given buffers.
 353      *
 354      * <p> Bytes are read starting at this channel's current file position, and
 355      * then the file position is updated with the number of bytes actually
 356      * read.  Otherwise this method behaves exactly as specified in the {@link
 357      * ScatteringByteChannel} interface.  </p>
 358      */
 359     public abstract long read(ByteBuffer[] dsts, int offset, int length)
 360         throws IOException;
 361 
 362     /**
 363      * Reads a sequence of bytes from this channel into the given buffers.
 364      *
 365      * <p> Bytes are read starting at this channel's current file position, and
 366      * then the file position is updated with the number of bytes actually
 367      * read.  Otherwise this method behaves exactly as specified in the {@link
 368      * ScatteringByteChannel} interface.  </p>
 369      */
 370     public final long read(ByteBuffer[] dsts) throws IOException {
 371         return read(dsts, 0, dsts.length);
 372     }
 373 
 374     /**
 375      * Writes a sequence of bytes to this channel from the given buffer.
 376      *
 377      * <p> Bytes are written starting at this channel's current file position
 378      * unless the channel is in append mode, in which case the position is
 379      * first advanced to the end of the file.  The file is grown, if necessary,
 380      * to accommodate the written bytes, and then the file position is updated
 381      * with the number of bytes actually written.  Otherwise this method
 382      * behaves exactly as specified by the {@link WritableByteChannel}
 383      * interface. </p>
 384      */
 385     public abstract int write(ByteBuffer src) throws IOException;
 386 
 387     /**
 388      * Writes a sequence of bytes to this channel from a subsequence of the
 389      * given buffers.
 390      *
 391      * <p> Bytes are written starting at this channel's current file position
 392      * unless the channel is in append mode, in which case the position is
 393      * first advanced to the end of the file.  The file is grown, if necessary,
 394      * to accommodate the written bytes, and then the file position is updated
 395      * with the number of bytes actually written.  Otherwise this method
 396      * behaves exactly as specified in the {@link GatheringByteChannel}
 397      * interface.  </p>
 398      */
 399     public abstract long write(ByteBuffer[] srcs, int offset, int length)
 400         throws IOException;
 401 
 402     /**
 403      * Writes a sequence of bytes to this channel from the given buffers.
 404      *
 405      * <p> Bytes are written starting at this channel's current file position
 406      * unless the channel is in append mode, in which case the position is
 407      * first advanced to the end of the file.  The file is grown, if necessary,
 408      * to accommodate the written bytes, and then the file position is updated
 409      * with the number of bytes actually written.  Otherwise this method
 410      * behaves exactly as specified in the {@link GatheringByteChannel}
 411      * interface.  </p>
 412      */
 413     public final long write(ByteBuffer[] srcs) throws IOException {
 414         return write(srcs, 0, srcs.length);
 415     }
 416 
 417 
 418     // -- Other operations --
 419 
 420     /**
 421      * Returns this channel's file position.
 422      *
 423      * @return  This channel's file position,
 424      *          a non-negative integer counting the number of bytes
 425      *          from the beginning of the file to the current position
 426      *
 427      * @throws  ClosedChannelException
 428      *          If this channel is closed
 429      *
 430      * @throws  IOException
 431      *          If some other I/O error occurs
 432      */
 433     public abstract long position() throws IOException;
 434 
 435     /**
 436      * Sets this channel's file position.
 437      *
 438      * <p> Setting the position to a value that is greater than the file's
 439      * current size is legal but does not change the size of the file.  A later
 440      * attempt to read bytes at such a position will immediately return an
 441      * end-of-file indication.  A later attempt to write bytes at such a
 442      * position will cause the file to be grown to accommodate the new bytes;
 443      * the values of any bytes between the previous end-of-file and the
 444      * newly-written bytes are unspecified.  </p>
 445      *
 446      * @param  newPosition
 447      *         The new position, a non-negative integer counting
 448      *         the number of bytes from the beginning of the file
 449      *
 450      * @return  This file channel
 451      *
 452      * @throws  ClosedChannelException
 453      *          If this channel is closed
 454      *
 455      * @throws  IllegalArgumentException
 456      *          If the new position is negative
 457      *
 458      * @throws  IOException
 459      *          If some other I/O error occurs
 460      */
 461     public abstract FileChannel position(long newPosition) throws IOException;
 462 
 463     /**
 464      * Returns the current size of this channel's file.
 465      *
 466      * @return  The current size of this channel's file,
 467      *          measured in bytes
 468      *
 469      * @throws  ClosedChannelException
 470      *          If this channel is closed
 471      *
 472      * @throws  IOException
 473      *          If some other I/O error occurs
 474      */
 475     public abstract long size() throws IOException;
 476 
 477     /**
 478      * Truncates this channel's file to the given size.
 479      *
 480      * <p> If the given size is less than the file's current size then the file
 481      * is truncated, discarding any bytes beyond the new end of the file.  If
 482      * the given size is greater than or equal to the file's current size then
 483      * the file is not modified.  In either case, if this channel's file
 484      * position is greater than the given size then it is set to that size.
 485      * </p>
 486      *
 487      * @param  size
 488      *         The new size, a non-negative byte count
 489      *
 490      * @return  This file channel
 491      *
 492      * @throws  NonWritableChannelException
 493      *          If this channel was not opened for writing
 494      *
 495      * @throws  ClosedChannelException
 496      *          If this channel is closed
 497      *
 498      * @throws  IllegalArgumentException
 499      *          If the new size is negative
 500      *
 501      * @throws  IOException
 502      *          If some other I/O error occurs
 503      */
 504     public abstract FileChannel truncate(long size) throws IOException;
 505 
 506     /**
 507      * Forces any updates to this channel's file to be written to the storage
 508      * device that contains it.
 509      *
 510      * <p> If this channel's file resides on a local storage device then when
 511      * this method returns it is guaranteed that all changes made to the file
 512      * since this channel was created, or since this method was last invoked,
 513      * will have been written to that device.  This is useful for ensuring that
 514      * critical information is not lost in the event of a system crash.
 515      *
 516      * <p> If the file does not reside on a local device then no such guarantee
 517      * is made.
 518      *
 519      * <p> The {@code metaData} parameter can be used to limit the number of
 520      * I/O operations that this method is required to perform.  Passing
 521      * {@code false} for this parameter indicates that only updates to the
 522      * file's content need be written to storage; passing {@code true}
 523      * indicates that updates to both the file's content and metadata must be
 524      * written, which generally requires at least one more I/O operation.
 525      * Whether this parameter actually has any effect is dependent upon the
 526      * underlying operating system and is therefore unspecified.
 527      *
 528      * <p> Invoking this method may cause an I/O operation to occur even if the
 529      * channel was only opened for reading.  Some operating systems, for
 530      * example, maintain a last-access time as part of a file's metadata, and
 531      * this time is updated whenever the file is read.  Whether or not this is
 532      * actually done is system-dependent and is therefore unspecified.
 533      *
 534      * <p> This method is only guaranteed to force changes that were made to
 535      * this channel's file via the methods defined in this class.  It may or
 536      * may not force changes that were made by modifying the content of a
 537      * {@link MappedByteBuffer <i>mapped byte buffer</i>} obtained by
 538      * invoking the {@link #map map} method.  Invoking the {@link
 539      * MappedByteBuffer#force force} method of the mapped byte buffer will
 540      * force changes made to the buffer's content to be written.  </p>
 541      *
 542      * @param   metaData
 543      *          If {@code true} then this method is required to force changes
 544      *          to both the file's content and metadata to be written to
 545      *          storage; otherwise, it need only force content changes to be
 546      *          written
 547      *
 548      * @throws  ClosedChannelException
 549      *          If this channel is closed
 550      *
 551      * @throws  IOException
 552      *          If some other I/O error occurs
 553      */
 554     public abstract void force(boolean metaData) throws IOException;
 555 
 556     /**
 557      * Transfers bytes from this channel's file to the given writable byte
 558      * channel.
 559      *
 560      * <p> An attempt is made to read up to {@code count} bytes starting at
 561      * the given {@code position} in this channel's file and write them to the
 562      * target channel.  An invocation of this method may or may not transfer
 563      * all of the requested bytes; whether or not it does so depends upon the
 564      * natures and states of the channels.  Fewer than the requested number of
 565      * bytes are transferred if this channel's file contains fewer than
 566      * {@code count} bytes starting at the given {@code position}, or if the
 567      * target channel is non-blocking and it has fewer than {@code count}
 568      * bytes free in its output buffer.
 569      *
 570      * <p> This method does not modify this channel's position.  If the given
 571      * position is greater than the file's current size then no bytes are
 572      * transferred.  If the target channel has a position then bytes are
 573      * written starting at that position and then the position is incremented
 574      * by the number of bytes written.
 575      *
 576      * <p> This method is potentially much more efficient than a simple loop
 577      * that reads from this channel and writes to the target channel.  Many
 578      * operating systems can transfer bytes directly from the filesystem cache
 579      * to the target channel without actually copying them.  </p>
 580      *
 581      * @param  position
 582      *         The position within the file at which the transfer is to begin;
 583      *         must be non-negative
 584      *
 585      * @param  count
 586      *         The maximum number of bytes to be transferred; must be
 587      *         non-negative
 588      *
 589      * @param  target
 590      *         The target channel
 591      *
 592      * @return  The number of bytes, possibly zero,
 593      *          that were actually transferred
 594      *
 595      * @throws IllegalArgumentException
 596      *         If the preconditions on the parameters do not hold
 597      *
 598      * @throws  NonReadableChannelException
 599      *          If this channel was not opened for reading
 600      *
 601      * @throws  NonWritableChannelException
 602      *          If the target channel was not opened for writing
 603      *
 604      * @throws  ClosedChannelException
 605      *          If either this channel or the target channel is closed
 606      *
 607      * @throws  AsynchronousCloseException
 608      *          If another thread closes either channel
 609      *          while the transfer is in progress
 610      *
 611      * @throws  ClosedByInterruptException
 612      *          If another thread interrupts the current thread while the
 613      *          transfer is in progress, thereby closing both channels and
 614      *          setting the current thread's interrupt status
 615      *
 616      * @throws  IOException
 617      *          If some other I/O error occurs
 618      */
 619     public abstract long transferTo(long position, long count,
 620                                     WritableByteChannel target)
 621         throws IOException;
 622 
 623     /**
 624      * Transfers bytes into this channel's file from the given readable byte
 625      * channel.
 626      *
 627      * <p> An attempt is made to read up to {@code count} bytes from the
 628      * source channel and write them to this channel's file starting at the
 629      * given {@code position}.  An invocation of this method may or may not
 630      * transfer all of the requested bytes; whether or not it does so depends
 631      * upon the natures and states of the channels.  Fewer than the requested
 632      * number of bytes will be transferred if the source channel has fewer than
 633      * {@code count} bytes remaining, or if the source channel is non-blocking
 634      * and has fewer than {@code count} bytes immediately available in its
 635      * input buffer.
 636      *
 637      * <p> This method does not modify this channel's position.  If the given
 638      * position is greater than the file's current size then no bytes are
 639      * transferred.  If the source channel has a position then bytes are read
 640      * starting at that position and then the position is incremented by the
 641      * number of bytes read.
 642      *
 643      * <p> This method is potentially much more efficient than a simple loop
 644      * that reads from the source channel and writes to this channel.  Many
 645      * operating systems can transfer bytes directly from the source channel
 646      * into the filesystem cache without actually copying them.  </p>
 647      *
 648      * @param  src
 649      *         The source channel
 650      *
 651      * @param  position
 652      *         The position within the file at which the transfer is to begin;
 653      *         must be non-negative
 654      *
 655      * @param  count
 656      *         The maximum number of bytes to be transferred; must be
 657      *         non-negative
 658      *
 659      * @return  The number of bytes, possibly zero,
 660      *          that were actually transferred
 661      *
 662      * @throws IllegalArgumentException
 663      *         If the preconditions on the parameters do not hold
 664      *
 665      * @throws  NonReadableChannelException
 666      *          If the source channel was not opened for reading
 667      *
 668      * @throws  NonWritableChannelException
 669      *          If this channel was not opened for writing
 670      *
 671      * @throws  ClosedChannelException
 672      *          If either this channel or the source channel is closed
 673      *
 674      * @throws  AsynchronousCloseException
 675      *          If another thread closes either channel
 676      *          while the transfer is in progress
 677      *
 678      * @throws  ClosedByInterruptException
 679      *          If another thread interrupts the current thread while the
 680      *          transfer is in progress, thereby closing both channels and
 681      *          setting the current thread's interrupt status
 682      *
 683      * @throws  IOException
 684      *          If some other I/O error occurs
 685      */
 686     public abstract long transferFrom(ReadableByteChannel src,
 687                                       long position, long count)
 688         throws IOException;
 689 
 690     /**
 691      * Reads a sequence of bytes from this channel into the given buffer,
 692      * starting at the given file position.
 693      *
 694      * <p> This method works in the same manner as the {@link
 695      * #read(ByteBuffer)} method, except that bytes are read starting at the
 696      * given file position rather than at the channel's current position.  This
 697      * method does not modify this channel's position.  If the given position
 698      * is greater than the file's current size then no bytes are read.  </p>
 699      *
 700      * @param  dst
 701      *         The buffer into which bytes are to be transferred
 702      *
 703      * @param  position
 704      *         The file position at which the transfer is to begin;
 705      *         must be non-negative
 706      *
 707      * @return  The number of bytes read, possibly zero, or {@code -1} if the
 708      *          given position is greater than or equal to the file's current
 709      *          size
 710      *
 711      * @throws  IllegalArgumentException
 712      *          If the position is negative
 713      *
 714      * @throws  NonReadableChannelException
 715      *          If this channel was not opened for reading
 716      *
 717      * @throws  ClosedChannelException
 718      *          If this channel is closed
 719      *
 720      * @throws  AsynchronousCloseException
 721      *          If another thread closes this channel
 722      *          while the read operation is in progress
 723      *
 724      * @throws  ClosedByInterruptException
 725      *          If another thread interrupts the current thread
 726      *          while the read operation is in progress, thereby
 727      *          closing the channel and setting the current thread's
 728      *          interrupt status
 729      *
 730      * @throws  IOException
 731      *          If some other I/O error occurs
 732      */
 733     public abstract int read(ByteBuffer dst, long position) throws IOException;
 734 
 735     /**
 736      * Writes a sequence of bytes to this channel from the given buffer,
 737      * starting at the given file position.
 738      *
 739      * <p> This method works in the same manner as the {@link
 740      * #write(ByteBuffer)} method, except that bytes are written starting at
 741      * the given file position rather than at the channel's current position.
 742      * This method does not modify this channel's position.  If the given
 743      * position is greater than the file's current size then the file will be
 744      * grown to accommodate the new bytes; the values of any bytes between the
 745      * previous end-of-file and the newly-written bytes are unspecified.  </p>
 746      *
 747      * @param  src
 748      *         The buffer from which bytes are to be transferred
 749      *
 750      * @param  position
 751      *         The file position at which the transfer is to begin;
 752      *         must be non-negative
 753      *
 754      * @return  The number of bytes written, possibly zero
 755      *
 756      * @throws  IllegalArgumentException
 757      *          If the position is negative
 758      *
 759      * @throws  NonWritableChannelException
 760      *          If this channel was not opened for writing
 761      *
 762      * @throws  ClosedChannelException
 763      *          If this channel is closed
 764      *
 765      * @throws  AsynchronousCloseException
 766      *          If another thread closes this channel
 767      *          while the write operation is in progress
 768      *
 769      * @throws  ClosedByInterruptException
 770      *          If another thread interrupts the current thread
 771      *          while the write operation is in progress, thereby
 772      *          closing the channel and setting the current thread's
 773      *          interrupt status
 774      *
 775      * @throws  IOException
 776      *          If some other I/O error occurs
 777      */
 778     public abstract int write(ByteBuffer src, long position) throws IOException;
 779 
 780 
 781     // -- Memory-mapped buffers --
 782 
 783     /**
 784      * A typesafe enumeration for file-mapping modes.
 785      *
 786      * @since 1.4
 787      *
 788      * @see java.nio.channels.FileChannel#map
 789      */
 790     public static class MapMode {
 791 
 792         /**
 793          * Mode for a read-only mapping.
 794          */
 795         public static final MapMode READ_ONLY
 796             = new MapMode("READ_ONLY");
 797 
 798         /**
 799          * Mode for a read/write mapping.
 800          */
 801         public static final MapMode READ_WRITE
 802             = new MapMode("READ_WRITE");
 803 
 804         /**
 805          * Mode for a private (copy-on-write) mapping.
 806          */
 807         public static final MapMode PRIVATE
 808             = new MapMode("PRIVATE");
 809 
 810         private final String name;
 811 
 812         private MapMode(String name) {
 813             this.name = name;
 814         }
 815 
 816         /**
 817          * Returns a string describing this file-mapping mode.
 818          *
 819          * @return  A descriptive string
 820          */
 821         public String toString() {
 822             return name;
 823         }
 824 
 825     }
 826 
 827     /**
 828      * Maps a region of this channel's file directly into memory.
 829      *
 830      * <p> A region of a file may be mapped into memory in one of three modes:
 831      * </p>
 832      *
 833      * <ul>
 834      *
 835      *   <li><p> <i>Read-only:</i> Any attempt to modify the resulting buffer
 836      *   will cause a {@link java.nio.ReadOnlyBufferException} to be thrown.
 837      *   ({@link MapMode#READ_ONLY MapMode.READ_ONLY}) </p></li>
 838      *
 839      *   <li><p> <i>Read/write:</i> Changes made to the resulting buffer will
 840      *   eventually be propagated to the file; they may or may not be made
 841      *   visible to other programs that have mapped the same file.  ({@link
 842      *   MapMode#READ_WRITE MapMode.READ_WRITE}) </p></li>
 843      *
 844      *   <li><p> <i>Private:</i> Changes made to the resulting buffer will not
 845      *   be propagated to the file and will not be visible to other programs
 846      *   that have mapped the same file; instead, they will cause private
 847      *   copies of the modified portions of the buffer to be created.  ({@link
 848      *   MapMode#PRIVATE MapMode.PRIVATE}) </p></li>
 849      *
 850      * </ul>
 851      *
 852      * <p> For a read-only mapping, this channel must have been opened for
 853      * reading; for a read/write or private mapping, this channel must have
 854      * been opened for both reading and writing.
 855      *
 856      * <p> The {@link MappedByteBuffer <i>mapped byte buffer</i>}
 857      * returned by this method will have a position of zero and a limit and
 858      * capacity of {@code size}; its mark will be undefined.  The buffer and
 859      * the mapping that it represents will remain valid until the buffer itself
 860      * is garbage-collected.
 861      *
 862      * <p> A mapping, once established, is not dependent upon the file channel
 863      * that was used to create it.  Closing the channel, in particular, has no
 864      * effect upon the validity of the mapping.
 865      *
 866      * <p> Many of the details of memory-mapped files are inherently dependent
 867      * upon the underlying operating system and are therefore unspecified.  The
 868      * behavior of this method when the requested region is not completely
 869      * contained within this channel's file is unspecified.  Whether changes
 870      * made to the content or size of the underlying file, by this program or
 871      * another, are propagated to the buffer is unspecified.  The rate at which
 872      * changes to the buffer are propagated to the file is unspecified.
 873      *
 874      * <p> For most operating systems, mapping a file into memory is more
 875      * expensive than reading or writing a few tens of kilobytes of data via
 876      * the usual {@link #read read} and {@link #write write} methods.  From the
 877      * standpoint of performance it is generally only worth mapping relatively
 878      * large files into memory.  </p>
 879      *
 880      * @param  mode
 881      *         One of the constants {@link MapMode#READ_ONLY READ_ONLY}, {@link
 882      *         MapMode#READ_WRITE READ_WRITE}, or {@link MapMode#PRIVATE
 883      *         PRIVATE} defined in the {@link MapMode} class, according to
 884      *         whether the file is to be mapped read-only, read/write, or
 885      *         privately (copy-on-write), respectively
 886      *
 887      * @param  position
 888      *         The position within the file at which the mapped region
 889      *         is to start; must be non-negative
 890      *
 891      * @param  size
 892      *         The size of the region to be mapped; must be non-negative and
 893      *         no greater than {@link java.lang.Integer#MAX_VALUE}
 894      *
 895      * @return  The mapped byte buffer
 896      *
 897      * @throws NonReadableChannelException
 898      *         If the {@code mode} is {@link MapMode#READ_ONLY READ_ONLY} but
 899      *         this channel was not opened for reading
 900      *
 901      * @throws NonWritableChannelException
 902      *         If the {@code mode} is {@link MapMode#READ_WRITE READ_WRITE} or
 903      *         {@link MapMode#PRIVATE PRIVATE} but this channel was not opened
 904      *         for both reading and writing
 905      *
 906      * @throws IllegalArgumentException
 907      *         If the preconditions on the parameters do not hold
 908      *
 909      * @throws IOException
 910      *         If some other I/O error occurs
 911      *
 912      * @see java.nio.channels.FileChannel.MapMode
 913      * @see java.nio.MappedByteBuffer
 914      */
 915     public abstract MappedByteBuffer map(MapMode mode,
 916                                          long position, long size)
 917         throws IOException;
 918 
 919 
 920     // -- Locks --
 921 
 922     /**
 923      * Acquires a lock on the given region of this channel's file.
 924      *
 925      * <p> An invocation of this method will block until the region can be
 926      * locked, this channel is closed, or the invoking thread is interrupted,
 927      * whichever comes first.
 928      *
 929      * <p> If this channel is closed by another thread during an invocation of
 930      * this method then an {@link AsynchronousCloseException} will be thrown.
 931      *
 932      * <p> If the invoking thread is interrupted while waiting to acquire the
 933      * lock then its interrupt status will be set and a {@link
 934      * FileLockInterruptionException} will be thrown.  If the invoker's
 935      * interrupt status is set when this method is invoked then that exception
 936      * will be thrown immediately; the thread's interrupt status will not be
 937      * changed.
 938      *
 939      * <p> The region specified by the {@code position} and {@code size}
 940      * parameters need not be contained within, or even overlap, the actual
 941      * underlying file.  Lock regions are fixed in size; if a locked region
 942      * initially contains the end of the file and the file grows beyond the
 943      * region then the new portion of the file will not be covered by the lock.
 944      * If a file is expected to grow in size and a lock on the entire file is
 945      * required then a region starting at zero, and no smaller than the
 946      * expected maximum size of the file, should be locked.  The zero-argument
 947      * {@link #lock()} method simply locks a region of size {@link
 948      * Long#MAX_VALUE}.
 949      *
 950      * <p> Some operating systems do not support shared locks, in which case a
 951      * request for a shared lock is automatically converted into a request for
 952      * an exclusive lock.  Whether the newly-acquired lock is shared or
 953      * exclusive may be tested by invoking the resulting lock object's {@link
 954      * FileLock#isShared() isShared} method.
 955      *
 956      * <p> File locks are held on behalf of the entire Java virtual machine.
 957      * They are not suitable for controlling access to a file by multiple
 958      * threads within the same virtual machine.  </p>
 959      *
 960      * @param  position
 961      *         The position at which the locked region is to start; must be
 962      *         non-negative
 963      *
 964      * @param  size
 965      *         The size of the locked region; must be non-negative, and the sum
 966      *         {@code position}&nbsp;+&nbsp;{@code size} must be non-negative
 967      *
 968      * @param  shared
 969      *         {@code true} to request a shared lock, in which case this
 970      *         channel must be open for reading (and possibly writing);
 971      *         {@code false} to request an exclusive lock, in which case this
 972      *         channel must be open for writing (and possibly reading)
 973      *
 974      * @return  A lock object representing the newly-acquired lock
 975      *
 976      * @throws  IllegalArgumentException
 977      *          If the preconditions on the parameters do not hold
 978      *
 979      * @throws  ClosedChannelException
 980      *          If this channel is closed
 981      *
 982      * @throws  AsynchronousCloseException
 983      *          If another thread closes this channel while the invoking
 984      *          thread is blocked in this method
 985      *
 986      * @throws  FileLockInterruptionException
 987      *          If the invoking thread is interrupted while blocked in this
 988      *          method
 989      *
 990      * @throws  OverlappingFileLockException
 991      *          If a lock that overlaps the requested region is already held by
 992      *          this Java virtual machine, or if another thread is already
 993      *          blocked in this method and is attempting to lock an overlapping
 994      *          region
 995      *
 996      * @throws  NonReadableChannelException
 997      *          If {@code shared} is {@code true} this channel was not
 998      *          opened for reading
 999      *
1000      * @throws  NonWritableChannelException
1001      *          If {@code shared} is {@code false} but this channel was not
1002      *          opened for writing
1003      *
1004      * @throws  IOException
1005      *          If some other I/O error occurs
1006      *
1007      * @see     #lock()
1008      * @see     #tryLock()
1009      * @see     #tryLock(long,long,boolean)
1010      */
1011     public abstract FileLock lock(long position, long size, boolean shared)
1012         throws IOException;
1013 
1014     /**
1015      * Acquires an exclusive lock on this channel's file.
1016      *
1017      * <p> An invocation of this method of the form {@code fc.lock()} behaves
1018      * in exactly the same way as the invocation
1019      *
1020      * <pre>
1021      *     fc.{@link #lock(long,long,boolean) lock}(0L, Long.MAX_VALUE, false) </pre>
1022      *
1023      * @return  A lock object representing the newly-acquired lock
1024      *
1025      * @throws  ClosedChannelException
1026      *          If this channel is closed
1027      *
1028      * @throws  AsynchronousCloseException
1029      *          If another thread closes this channel while the invoking
1030      *          thread is blocked in this method
1031      *
1032      * @throws  FileLockInterruptionException
1033      *          If the invoking thread is interrupted while blocked in this
1034      *          method
1035      *
1036      * @throws  OverlappingFileLockException
1037      *          If a lock that overlaps the requested region is already held by
1038      *          this Java virtual machine, or if another thread is already
1039      *          blocked in this method and is attempting to lock an overlapping
1040      *          region of the same file
1041      *
1042      * @throws  NonWritableChannelException
1043      *          If this channel was not opened for writing
1044      *
1045      * @throws  IOException
1046      *          If some other I/O error occurs
1047      *
1048      * @see     #lock(long,long,boolean)
1049      * @see     #tryLock()
1050      * @see     #tryLock(long,long,boolean)
1051      */
1052     public final FileLock lock() throws IOException {
1053         return lock(0L, Long.MAX_VALUE, false);
1054     }
1055 
1056     /**
1057      * Attempts to acquire a lock on the given region of this channel's file.
1058      *
1059      * <p> This method does not block.  An invocation always returns
1060      * immediately, either having acquired a lock on the requested region or
1061      * having failed to do so.  If it fails to acquire a lock because an
1062      * overlapping lock is held by another program then it returns
1063      * {@code null}.  If it fails to acquire a lock for any other reason then
1064      * an appropriate exception is thrown.
1065      *
1066      * <p> The region specified by the {@code position} and {@code size}
1067      * parameters need not be contained within, or even overlap, the actual
1068      * underlying file.  Lock regions are fixed in size; if a locked region
1069      * initially contains the end of the file and the file grows beyond the
1070      * region then the new portion of the file will not be covered by the lock.
1071      * If a file is expected to grow in size and a lock on the entire file is
1072      * required then a region starting at zero, and no smaller than the
1073      * expected maximum size of the file, should be locked.  The zero-argument
1074      * {@link #tryLock()} method simply locks a region of size {@link
1075      * Long#MAX_VALUE}.
1076      *
1077      * <p> Some operating systems do not support shared locks, in which case a
1078      * request for a shared lock is automatically converted into a request for
1079      * an exclusive lock.  Whether the newly-acquired lock is shared or
1080      * exclusive may be tested by invoking the resulting lock object's {@link
1081      * FileLock#isShared() isShared} method.
1082      *
1083      * <p> File locks are held on behalf of the entire Java virtual machine.
1084      * They are not suitable for controlling access to a file by multiple
1085      * threads within the same virtual machine.  </p>
1086      *
1087      * @param  position
1088      *         The position at which the locked region is to start; must be
1089      *         non-negative
1090      *
1091      * @param  size
1092      *         The size of the locked region; must be non-negative, and the sum
1093      *         {@code position}&nbsp;+&nbsp;{@code size} must be non-negative
1094      *
1095      * @param  shared
1096      *         {@code true} to request a shared lock,
1097      *         {@code false} to request an exclusive lock
1098      *
1099      * @return  A lock object representing the newly-acquired lock,
1100      *          or {@code null} if the lock could not be acquired
1101      *          because another program holds an overlapping lock
1102      *
1103      * @throws  IllegalArgumentException
1104      *          If the preconditions on the parameters do not hold
1105      *
1106      * @throws  ClosedChannelException
1107      *          If this channel is closed
1108      *
1109      * @throws  OverlappingFileLockException
1110      *          If a lock that overlaps the requested region is already held by
1111      *          this Java virtual machine, or if another thread is already
1112      *          blocked in this method and is attempting to lock an overlapping
1113      *          region of the same file
1114      *
1115      * @throws  IOException
1116      *          If some other I/O error occurs
1117      *
1118      * @see     #lock()
1119      * @see     #lock(long,long,boolean)
1120      * @see     #tryLock()
1121      */
1122     public abstract FileLock tryLock(long position, long size, boolean shared)
1123         throws IOException;
1124 
1125     /**
1126      * Attempts to acquire an exclusive lock on this channel's file.
1127      *
1128      * <p> An invocation of this method of the form {@code fc.tryLock()}
1129      * behaves in exactly the same way as the invocation
1130      *
1131      * <pre>
1132      *     fc.{@link #tryLock(long,long,boolean) tryLock}(0L, Long.MAX_VALUE, false) </pre>
1133      *
1134      * @return  A lock object representing the newly-acquired lock,
1135      *          or {@code null} if the lock could not be acquired
1136      *          because another program holds an overlapping lock
1137      *
1138      * @throws  ClosedChannelException
1139      *          If this channel is closed
1140      *
1141      * @throws  OverlappingFileLockException
1142      *          If a lock that overlaps the requested region is already held by
1143      *          this Java virtual machine, or if another thread is already
1144      *          blocked in this method and is attempting to lock an overlapping
1145      *          region
1146      *
1147      * @throws  IOException
1148      *          If some other I/O error occurs
1149      *
1150      * @see     #lock()
1151      * @see     #lock(long,long,boolean)
1152      * @see     #tryLock(long,long,boolean)
1153      */
1154     public final FileLock tryLock() throws IOException {
1155         return tryLock(0L, Long.MAX_VALUE, false);
1156     }
1157 
1158 }