filechannel.java

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/*
 * @(#)FileChannel.java	1.38 03/01/23
 *
 * Copyright 2003 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */

package java.nio.channels;

import java.io.*;
import java.nio.ByteBuffer;
import java.nio.MappedByteBuffer;
import java.nio.channels.spi.AbstractInterruptibleChannel;


/**
 * A channel for reading, writing, mapping, and manipulating a file.
 *
 * <p> A file channel has a current <i>position</i> within its file which can
 * be both {@link #position() </code>queried<code>} and {@link #position(long)
 * </code>modified<code>}.  The file itself contains a variable-length sequence
 * of bytes that can be read and written and whose current {@link #size
 * </code><i>size</i><code>} can be queried.  The size of the file increases
 * when bytes are written beyond its current size; the size of the file
 * decreases when it is {@link #truncate </code><i>truncated</i><code>}.  The
 * file may also have some associated <i>metadata</i> such as access
 * permissions, content type, and last-modification time; this class does not
 * define methods for metadata access.
 *
 * <p> In addition to the familiar read, write, and close operations of byte
 * channels, this class defines the following file-specific operations: </p>
 *
 * <ul>
 *
 *   <li><p> Bytes may be {@link #read(ByteBuffer, long) </code>read<code>} or
 *   {@link #write(ByteBuffer, long) </code>written<code>} at an absolute
 *   position in a file in a way that does not affect the channel's current
 *   position.  </p></li>
 *
 *   <li><p> A region of a file may be {@link #map </code>mapped<code>}
 *   directly into memory; for large files this is often much more efficient
 *   than invoking the usual <tt>read</tt> or <tt>write</tt> methods.
 *   </p></li>
 *
 *   <li><p> Updates made to a file may be {@link #force </code>forced
 *   out<code>} to the underlying storage device, ensuring that data are not
 *   lost in the event of a system crash.  </p></li>
 *
 *   <li><p> Bytes can be transferred from a file {@link #transferTo </code>to
 *   some other channel<code>}, and {@link #transferFrom </code>vice
 *   versa<code>}, in a way that can be optimized by many operating systems
 *   into a very fast transfer directly to or from the filesystem cache.
 *   </p></li>
 *
 *   <li><p> A region of a file may be {@link FileLock </code>locked<code>}
 *   against access by other programs.  </p></li>
 *
 * </ul>
 *
 * <p> File channels are safe for use by multiple concurrent threads.  The
 * {@link Channel#close close} method may be invoked at any time, as specified
 * by the {@link Channel} interface.  Only one operation that involves the
 * channel's position or can change its file's size may be in progress at any
 * given time; attempts to initiate a second such operation while the first is
 * still in progress will block until the first operation completes.  Other
 * operations, in particular those that take an explicit position, may proceed
 * concurrently; whether they in fact do so is dependent upon the underlying
 * implementation and is therefore unspecified.
 *
 * <p> The view of a file provided by an instance of this class is guaranteed
 * to be consistent with other views of the same file provided by other
 * instances in the same program.  The view provided by an instance of this
 * class may or may not, however, be consistent with the views seen by other
 * concurrently-running programs due to caching performed by the underlying
 * operating system and delays induced by network-filesystem protocols.  This
 * is true regardless of the language in which these other programs are
 * written, and whether they are running on the same machine or on some other
 * machine.  The exact nature of any such inconsistencies are system-dependent
 * and are therefore unspecified.
 *
 * <p> This class does not define methods for opening existing files or for
 * creating new ones; such methods may be added in a future release.  In this
 * release a file channel can be obtained from an existing {@link
 * java.io.FileInputStream#getChannel FileInputStream}, {@link
 * java.io.FileOutputStream#getChannel FileOutputStream}, or {@link
 * java.io.RandomAccessFile#getChannel RandomAccessFile} object by invoking
 * that object's <tt>getChannel</tt> method, which returns a file channel that
 * is connected to the same underlying file.
 *
 * <p> The state of a file channel is intimately connected to that of the
 * object whose <tt>getChannel</tt> method returned the channel.  Changing the
 * channel's position, whether explicitly or by reading or writing bytes, will
 * change the file position of the originating object, and vice versa.
 * Changing the file's length via the file channel will change the length seen
 * via the originating object, and vice versa.  Changing the file's content by
 * writing bytes will change the content seen by the originating object, and
 * vice versa.
 *
 * <a name="open-mode"><p> At various points this class specifies that an
 * instance that is "open for reading," "open for writing," or "open for
 * reading and writing" is required.  A channel obtained via the {@link
 * java.io.FileInputStream#getChannel getChannel} method of a {@link
 * java.io.FileInputStream} instance will be open for reading.  A channel
 * obtained via the {@link java.io.FileOutputStream#getChannel getChannel}
 * method of a {@link java.io.FileOutputStream} instance will be open for
 * writing.  Finally, a channel obtained via the {@link
 * java.io.RandomAccessFile#getChannel getChannel} method of a {@link
 * java.io.RandomAccessFile} instance will be open for reading if the instance
 * was created with mode <tt>"r"</tt> and will be open for reading and writing
 * if the instance was created with mode <tt>"rw"</tt>.
 *
 * <a name="append-mode"><p> A file channel that is open for writing may be in
 * <i>append mode</i>, for example if it was obtained from a file-output stream
 * that was created by invoking the {@link
 * java.io.FileOutputStream#FileOutputStream(java.io.File,boolean)
 * FileOutputStream(File,boolean)} constructor and passing <tt>true</tt> for
 * the second parameter.  In this mode each invocation of a relative write
 * operation first advances the position to the end of the file and then writes
 * the requested data.  Whether the advancement of the position and the writing
 * of the data are done in a single atomic operation is system-dependent and
 * therefore unspecified.
 *
 *
 * @see java.io.FileInputStream#getChannel()
 * @see java.io.FileOutputStream#getChannel()
 * @see java.io.RandomAccessFile#getChannel()
 *
 * @author Mark Reinhold
 * @author Mike McCloskey
 * @author JSR-51 Expert Group
 * @version 1.38, 03/01/23
 * @since 1.4
 */

public abstract class FileChannel
    extends AbstractInterruptibleChannel
    implements ByteChannel, GatheringByteChannel, ScatteringByteChannel
{

    /**
     * Initializes a new instance of this class.
     */
    protected FileChannel() { }


    // -- Channel operations --

    /**
     * Reads a sequence of bytes from this channel into the given buffer.
     *
     * <p> Bytes are read starting at this channel's current file position, and
     * then the file position is updated with the number of bytes actually
     * read.  Otherwise this method behaves exactly as specified in the {@link
     * ReadableByteChannel} interface. </p>
     */
    public abstract int read(ByteBuffer dst) throws IOException;

    /**
     * Reads a sequence of bytes from this channel into a subsequence of the
     * given buffers.
     *
     * <p> Bytes are read starting at this channel's current file position, and
     * then the file position is updated with the number of bytes actually
     * read.  Otherwise this method behaves exactly as specified in the {@link
     * ScatteringByteChannel} interface.  </p>
     */
    public abstract long read(ByteBuffer[] dsts, int offset, int length)
	throws IOException;

    /**
     * Reads a sequence of bytes from this channel into the given buffers.
     *
     * <p> Bytes are read starting at this channel's current file position, and
     * then the file position is updated with the number of bytes actually
     * read.  Otherwise this method behaves exactly as specified in the {@link
     * ScatteringByteChannel} interface.  </p>
     */
    public final long read(ByteBuffer[] dsts) throws IOException {
	return read(dsts, 0, dsts.length);
    }

    /**
     * Writes a sequence of bytes to this channel from the given buffer.
     *
     * <p> Bytes are written starting at this channel's current file position
     * unless the channel is in append mode, in which case the position is
     * first advanced to the end of the file.  The file is grown, if necessary,
     * to accomodate the written bytes, and then the file position is updated
     * with the number of bytes actually written.  Otherwise this method
     * behaves exactly as specified by the {@link WritableByteChannel}
     * interface. </p>
     */ 
    public abstract int write(ByteBuffer src) throws IOException;

    /**
     * Writes a sequence of bytes to this channel from a subsequence of the
     * given buffers.
     *
     * <p> Bytes are written starting at this channel's current file position
     * unless the channel is in append mode, in which case the position is
     * first advanced to the end of the file.  The file is grown, if necessary,
     * to accomodate the written bytes, and then the file position is updated
     * with the number of bytes actually written.  Otherwise this method
     * behaves exactly as specified in the {@link GatheringByteChannel}
     * interface.  </p>
     */ 
    public abstract long write(ByteBuffer[] srcs, int offset, int length)
	throws IOException;

    /**
     * Writes a sequence of bytes to this channel from the given buffers.
     *
     * <p> Bytes are written starting at this channel's current file position
     * unless the channel is in append mode, in which case the position is
     * first advanced to the end of the file.  The file is grown, if necessary,
     * to accomodate the written bytes, and then the file position is updated
     * with the number of bytes actually written.  Otherwise this method
     * behaves exactly as specified in the {@link GatheringByteChannel}
     * interface.  </p>
     */ 
    public final long write(ByteBuffer[] srcs) throws IOException {
	return write(srcs, 0, srcs.length);
    }


    // -- Other operations --

    /**
     * Returns this channel's file position.  </p>
     *
     * @return  This channel's file position,
     *          a non-negative integer counting the number of bytes
     *          from the beginning of the file to the current position
     *
     * @throws  ClosedChannelException
     *          If this channel is closed
     *
     * @throws  IOException
     *          If some other I/O error occurs
     */
    public abstract long position() throws IOException;

    /**
     * Sets this channel's file position.
     *
     * <p> Setting the position to a value that is greater than the file's
     * current size is legal but does not change the size of the file.  A later
     * attempt to read bytes at such a position will immediately return an
     * end-of-file indication.  A later attempt to write bytes at such a
     * position will cause the file to be grown to accomodate the new bytes;
     * the values of any bytes between the previous end-of-file and the
     * newly-written bytes are unspecified.  </p>
     *
     * @param  newPosition
     *         The new position, a non-negative integer counting
     *         the number of bytes from the beginning of the file
     *
     * @return  This file channel
     *
     * @throws  ClosedChannelException
     *          If this channel is closed
     *
     * @throws  IllegalArgumentException
     *          If the new position is negative
     *
     * @throws  IOException
     *          If some other I/O error occurs
     */
    public abstract FileChannel position(long newPosition) throws IOException;

    /**
     * Returns the current size of this channel's file. </p>
     *
     * @return  The current size of this channel's file,
     *          measured in bytes
     *
     * @throws  ClosedChannelException
     *          If this channel is closed
     *
     * @throws  IOException
     *          If some other I/O error occurs
     */
    public abstract long size() throws IOException;

    /**
     * Truncates this channel's file to the given size.
     *
     * <p> If the given size is less than the file's current size then the file
     * is truncated, discarding any bytes beyond the new end of the file.  If
     * the given size is greater than or equal to the file's current size then
     * the file is not modified.  In either case, if this channel's file
     * position is greater than the given size then it is set to that size.
     * </p>
     *
     * @param  size
     *         The new size, a non-negative byte count
     *
     * @return  This file channel
     *
     * @throws  NonWritableChannelException
     *          If this channel was not opened for writing
     *
     * @throws  ClosedChannelException
     *          If this channel is closed
     *
     * @throws  IllegalArgumentException
     *          If the new size is negative
     *
     * @throws  IOException
     *          If some other I/O error occurs
     */
    public abstract FileChannel truncate(long size) throws IOException;

    /**
     * Forces any updates to this channel's file to be written to the storage
     * device that contains it.
     *
     * <p> If this channel's file resides on a local storage device then when
     * this method returns it is guaranteed that all changes made to the file
     * since this channel was created, or since this method was last invoked,
     * will have been written to that device.  This is useful for ensuring that
     * critical information is not lost in the event of a system crash.
     *
     * <p> If the file does not reside on a local device then no such guarantee