/*

 * Copyright (c) 2007, 2018, Oracle and/or its affiliates. All rights reserved.

 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

 *

 * This code is free software; you can redistribute it and/or modify it

 * under the terms of the GNU General Public License version 2 only, as

 * published by the Free Software Foundation.  Oracle designates this

 * particular file as subject to the "Classpath" exception as provided

 * by Oracle in the LICENSE file that accompanied this code.

 *

 * This code is distributed in the hope that it will be useful, but WITHOUT

 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

 * version 2 for more details (a copy is included in the LICENSE file that

 * accompanied this code).

 *

 * You should have received a copy of the GNU General Public License version

 * 2 along with this work; if not, write to the Free Software Foundation,

 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

 *

 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

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package java.nio.channels;

import java.nio.file.*;

import java.nio.file.attribute.FileAttribute;

import java.nio.file.spi.*;

import java.nio.ByteBuffer;

import java.io.IOException;

import java.util.concurrent.Future;

import java.util.concurrent.ExecutorService;

import java.util.Set;

import java.util.HashSet;

import java.util.Collections;

/**

 * An asynchronous channel for reading, writing, and manipulating a file.

 *

 * <p> An asynchronous file channel is created when a file is opened by invoking

 * one of the {@link #open open} methods defined by this class. The file contains

 * a variable-length sequence of bytes that can be read and written and whose

 * current size can be {@link #size() 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 truncated}.

 *

 * <p> An asynchronous file channel does not have a <i>current position</i>

 * within the file. Instead, the file position is specified to each read and

 * write method that initiates asynchronous operations. A {@link CompletionHandler}

 * is specified as a parameter and is invoked to consume the result of the I/O

 * operation. This class also defines read and write methods that initiate

 * asynchronous operations, returning a {@link Future} to represent the pending

 * result of the operation. The {@code Future} may be used to check if the

 * operation has completed, wait for its completion, and retrieve the result.

 *

 * <p> In addition to read and write operations, this class defines the

 * following operations: </p>

 *

 * <ul>

 *

 *   <li><p> Updates made to a file may be {@link #force <i>forced

 *   out</i>} to the underlying storage device, ensuring that data are not

 *   lost in the event of a system crash.  </p></li>

 *

 *   <li><p> A region of a file may be {@link #lock <i>locked</i>} against

 *   access by other programs.  </p></li>

 *

 * </ul>

 *

 * <p> An {@code AsynchronousFileChannel} is associated with a thread pool to

 * which tasks are submitted to handle I/O events and dispatch to completion

 * handlers that consume the results of I/O operations on the channel. The

 * completion handler for an I/O operation initiated on a channel is guaranteed

 * to be invoked by one of the threads in the thread pool (This ensures that the

 * completion handler is run by a thread with the expected <em>identity</em>).

 * Where an I/O operation completes immediately, and the initiating thread is

 * itself a thread in the thread pool, then the completion handler may be invoked

 * directly by the initiating thread. When an {@code AsynchronousFileChannel} is

 * created without specifying a thread pool then the channel is associated with

 * a system-dependent default thread pool that may be shared with other

 * channels. The default thread pool is configured by the system properties

 * defined by the {@link AsynchronousChannelGroup} class.

 *

 * <p> Channels of this type 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. This causes all outstanding asynchronous

 * operations on the channel to complete with the exception {@link

 * AsynchronousCloseException}. Multiple read and write operations may be

 * outstanding at the same time. When multiple read and write operations are

 * outstanding then the ordering of the I/O operations, and the order that the

 * completion handlers are invoked, is not specified; they are not, in particular,

 * guaranteed to execute in the order that the operations were initiated. The

 * {@link java.nio.ByteBuffer ByteBuffers} used when reading or writing are not

 * safe for use by multiple concurrent I/O operations. Furthermore, after an I/O

 * operation is initiated then care should be taken to ensure that the buffer is

 * not accessed until after the operation has completed.

 *

 * <p> As with {@link FileChannel}, 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.

 *

 * @since 1.7

 */

public abstract class AsynchronousFileChannel

    implements AsynchronousChannel

{

    /**

     * Initializes a new instance of this class.

     */

    protected AsynchronousFileChannel() {

    }

    /**

     * Opens or creates a file for reading and/or writing, returning an

     * asynchronous file channel to access the file.

     *

     * <p> The {@code options} parameter determines how the file is opened.

     * The {@link StandardOpenOption#READ READ} and {@link StandardOpenOption#WRITE

     * WRITE} options determines if the file should be opened for reading and/or

     * writing. If neither option is contained in the array then an existing file

     * is opened for  reading.

     *

     * <p> In addition to {@code READ} and {@code WRITE}, the following options

     * may be present:

     *

     * <table class="striped">

     * <caption style="display:none">additional options</caption>

     * <thead>

     * <tr> <th scope="col">Option</th> <th scope="col">Description</th> </tr>

     * </thead>

     * <tbody>

     * <tr>

     *   <th scope="row"> {@link StandardOpenOption#TRUNCATE_EXISTING TRUNCATE_EXISTING} </th>

     *   <td> When opening an existing file, the file is first truncated to a

     *   size of 0 bytes. This option is ignored when the file is opened only

     *   for reading.</td>

     * </tr>

     * <tr>

     *   <th scope="row"> {@link StandardOpenOption#CREATE_NEW CREATE_NEW} </th>

     *   <td> If this option is present then a new file is created, failing if

     *   the file already exists. When creating a file the check for the

     *   existence of the file and the creation of the file if it does not exist

     *   is atomic with respect to other file system operations. This option is

     *   ignored when the file is opened only for reading. </td>

     * </tr>

     * <tr>

     *   <th scope="row" > {@link StandardOpenOption#CREATE CREATE} </th>

     *   <td> If this option is present then an existing file is opened if it

     *   exists, otherwise a new file is created. When creating a file the check

     *   for the existence of the file and the creation of the file if it does

     *   not exist is atomic with respect to other file system operations. This

     *   option is ignored if the {@code CREATE_NEW} option is also present or

     *   the file is opened only for reading. </td>

     * </tr>

     * <tr>

     *   <th scope="row" > {@link StandardOpenOption#DELETE_ON_CLOSE DELETE_ON_CLOSE} </th>

     *   <td> When this option is present then the implementation makes a

     *   <em>best effort</em> attempt to delete the file when closed by

     *   the {@link #close close} method. If the {@code close} method is not

     *   invoked then a <em>best effort</em> attempt is made to delete the file

     *   when the Java virtual machine terminates. </td>

     * </tr>

     * <tr>

     *   <th scope="row">{@link StandardOpenOption#SPARSE SPARSE} </th>

     *   <td> When creating a new file this option is a <em>hint</em> that the

     *   new file will be sparse. This option is ignored when not creating

     *   a new file. </td>

     * </tr>

     * <tr>

     *   <th scope="row"> {@link StandardOpenOption#SYNC SYNC} </th>

     *   <td> Requires that every update to the file's content or metadata be

     *   written synchronously to the underlying storage device. (see <a

     *   href="../file/package-summary.html#integrity"> Synchronized I/O file

     *   integrity</a>). </td>

     * </tr>

     * <tr>

     *   <th scope="row"> {@link StandardOpenOption#DSYNC DSYNC} </th>

     *   <td> Requires that every update to the file's content be written

     *   synchronously to the underlying storage device. (see <a

     *   href="../file/package-summary.html#integrity"> Synchronized I/O file

     *   integrity</a>). </td>

     * </tr>

     * </tbody>

     * </table>

     *

     * <p> An implementation may also support additional options.

     *

     * <p> The {@code executor} parameter is the {@link ExecutorService} to

     * which tasks are submitted to handle I/O events and dispatch completion

     * results for operations initiated on resulting channel.

     * The nature of these tasks is highly implementation specific and so care

     * should be taken when configuring the {@code Executor}. Minimally it

     * should support an unbounded work queue and should not run tasks on the

     * caller thread of the {@link ExecutorService#execute execute} method.

     * Shutting down the executor service while the channel is open results in

     * unspecified behavior.

     *

     * <p> The {@code attrs} parameter is an optional array of file {@link

     * FileAttribute file-attributes} to set atomically when creating the file.

     *

     * <p> The new channel is created by invoking the {@link

     * FileSystemProvider#newFileChannel newFileChannel} method on the

     * provider that created the {@code Path}.

     *

     * @param   file

     *          The path of the file to open or create

     * @param   options

     *          Options specifying how the file is opened

     * @param   executor

     *          The thread pool or {@code null} to associate the channel with

     *          the default thread pool

     * @param   attrs

     *          An optional list of file attributes to set atomically when

     *          creating the file

     *

     * @return  A new asynchronous file channel

     *

     * @throws  IllegalArgumentException

     *          If the set contains an invalid combination of options

     * @throws  UnsupportedOperationException

     *          If the {@code file} is associated with a provider that does not

     *          support creating asynchronous file channels, or an unsupported

     *          open option is specified, or the array contains an attribute that

     *          cannot be set atomically when creating the file

     * @throws  IOException

     *          If an I/O error occurs

     * @throws  SecurityException

     *          If a security manager is installed and it denies an

     *          unspecified permission required by the implementation.

     *          In the case of the default provider, the {@link

     *          SecurityManager#checkRead(String)} method is invoked to check

     *          read access if the file is opened for reading. The {@link

     *          SecurityManager#checkWrite(String)} method is invoked to check

     *          write access if the file is opened for writing

     */

    public static AsynchronousFileChannel open(Path file,

                                               Set<? extends OpenOption> options,

                                               ExecutorService executor,

                                               FileAttribute<?>... attrs)

        throws IOException

    {

        FileSystemProvider provider = file.getFileSystem().provider();

        return provider.newAsynchronousFileChannel(file, options, executor, attrs);

    }

    @SuppressWarnings({"unchecked", "rawtypes"}) // generic array construction

    private static final FileAttribute<?>[] NO_ATTRIBUTES = new FileAttribute[0];

    /**

     * Opens or creates a file for reading and/or writing, returning an

     * asynchronous file channel to access the file.

     *

     * <p> An invocation of this method behaves in exactly the same way as the

     * invocation

     * <pre>

     *     ch.{@link #open(Path,Set,ExecutorService,FileAttribute[])

     *       open}(file, opts, null, new FileAttribute<?>[0]);

     * </pre>

     * where {@code opts} is a {@code Set} containing the options specified to

     * this method.

     *

     * <p> The resulting channel is associated with default thread pool to which

     * tasks are submitted to handle I/O events and dispatch to completion

     * handlers that consume the result of asynchronous operations performed on

     * the resulting channel.

     *

     * @param   file

     *          The path of the file to open or create

     * @param   options

     *          Options specifying how the file is opened

     *

     * @return  A new asynchronous file channel

     *

     * @throws  IllegalArgumentException

     *          If the set contains an invalid combination of options

     * @throws  UnsupportedOperationException

     *          If the {@code file} is associated with a provider that does not

     *          support creating file channels, or an unsupported open option is

     *          specified

     * @throws  IOException

     *          If an I/O error occurs

     * @throws  SecurityException

     *          If a security manager is installed and it denies an

     *          unspecified permission required by the implementation.

     *          In the case of the default provider, the {@link

     *          SecurityManager#checkRead(String)} method is invoked to check

     *          read access if the file is opened for reading. The {@link

     *          SecurityManager#checkWrite(String)} method is invoked to check

     *          write access if the file is opened for writing

     */

    public static AsynchronousFileChannel open(Path file, OpenOption... options)

        throws IOException

    {

        Set<OpenOption> set;

        if (options.length == 0) {

            set = Collections.emptySet();

        } else {

            set = new HashSet<>();

            Collections.addAll(set, options);

        }

        return open(file, set, null, NO_ATTRIBUTES);

    }

    /**

     * Returns the current size of this channel's file.

     *

     * @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. </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 AsynchronousFileChannel 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

     * is made.

     *

     * <p> The {@code metaData} parameter can be used to limit the number of

     * I/O operations that this method is required to perform.  Passing

     * {@code false} for this parameter indicates that only updates to the

     * file's content need be written to storage; passing {@code true}

     * indicates that updates to both the file's content and metadata must be

     * written, which generally requires at least one more I/O operation.

     * Whether this parameter actually has any effect is dependent upon the

     * underlying operating system and is therefore unspecified.

     *

     * <p> Invoking this method may cause an I/O operation to occur even if the

     * channel was only opened for reading.  Some operating systems, for

     * example, maintain a last-access time as part of a file's metadata, and

     * this time is updated whenever the file is read.  Whether or not this is

     * actually done is system-dependent and is therefore unspecified.

     *

     * <p> This method is only guaranteed to force changes that were made to

     * this channel's file via the methods defined in this class.

     *

     * @param   metaData

     *          If {@code true} then this method is required to force changes

     *          to both the file's content and metadata to be written to

     *          storage; otherwise, it need only force content changes to be

     *          written

     *

     * @throws  ClosedChannelException

     *          If this channel is closed

     *

     * @throws  IOException

     *          If some other I/O error occurs

     */

    public abstract void force(boolean metaData) throws IOException;

    /**

     * Acquires a lock on the given region of this channel's file.

     *

     * <p> This method initiates an operation to acquire a lock on the given

     * region of this channel's file. The {@code handler} parameter is a

     * completion handler that is invoked when the lock is acquired (or the

     * operation fails). The result passed to the completion handler is the

     * resulting {@code FileLock}.

     *

     * <p> The region specified by the {@code position} and {@code size}

     * parameters need not be contained within, or even overlap, the actual

     * underlying file.  Lock regions are fixed in size; if a locked region

     * initially contains the end of the file and the file grows beyond the

     * region then the new portion of the file will not be covered by the lock.

     * If a file is expected to grow in size and a lock on the entire file is

     * required then a region starting at zero, and no smaller than the

     * expected maximum size of the file, should be locked.  The two-argument

     * {@link #lock(Object,CompletionHandler)} method simply locks a region

     * of size {@link Long#MAX_VALUE}. If a lock that overlaps the requested

     * region is already held by this Java virtual machine, or this method has

     * been invoked to lock an overlapping region and that operation has not

     * completed, then this method throws {@link OverlappingFileLockException}.

     *

     * <p> Some operating systems do not support a mechanism to acquire a file

     * lock in an asynchronous manner. Consequently an implementation may

     * acquire the file lock in a background thread or from a task executed by

     * a thread in the associated thread pool. If there are many lock operations

     * outstanding then it may consume threads in the Java virtual machine for

     * indefinite periods.

     *

     * <p> Some operating systems do not support shared locks, in which case a

     * request for a shared lock is automatically converted into a request for

     * an exclusive lock.  Whether the newly-acquired lock is shared or

     * exclusive may be tested by invoking the resulting lock object's {@link

     * FileLock#isShared() isShared} method.

     *

     * <p> File locks are held on behalf of the entire Java virtual machine.

     * They are not suitable for controlling access to a file by multiple

     * threads within the same virtual machine.

     *

     * @param   <A>

     *          The type of the attachment

     * @param   position

     *          The position at which the locked region is to start; must be

     *          non-negative

     * @param   size

     *          The size of the locked region; must be non-negative, and the sum

     *          {@code position} + {@code size} must be non-negative

     * @param   shared

     *          {@code true} to request a shared lock, in which case this

     *          channel must be open for reading (and possibly writing);

     *          {@code false} to request an exclusive lock, in which case this

     *          channel must be open for writing (and possibly reading)

     * @param   attachment

     *          The object to attach to the I/O operation; can be {@code null}

     * @param   handler

     *          The handler for consuming the result

     *

     * @throws  OverlappingFileLockException

     *          If a lock that overlaps the requested region is already held by

     *          this Java virtual machine, or there is already a pending attempt

     *          to lock an overlapping region

     * @throws  IllegalArgumentException

     *          If the preconditions on the parameters do not hold

     * @throws  NonReadableChannelException

     *          If {@code shared} is true but this channel was not opened for reading

     * @throws  NonWritableChannelException

     *          If {@code shared} is false but this channel was not opened for writing

     */

    public abstract <A> void lock(long position,

                                  long size,

                                  boolean shared,

                                  A attachment,

                                  CompletionHandler<FileLock,? super A> handler);

    /**

     * Acquires an exclusive lock on this channel's file.

     *

     * <p> This method initiates an operation to acquire a lock on the given

     * region of this channel's file. The {@code handler} parameter is a

     * completion handler that is invoked when the lock is acquired (or the

     * operation fails). The result passed to the completion handler is the

     * resulting {@code FileLock}.

     *

     * <p> An invocation of this method of the form {@code ch.lock(att,handler)}

     * behaves in exactly the same way as the invocation

     * <pre>

     *     ch.{@link #lock(long,long,boolean,Object,CompletionHandler) lock}(0L, Long.MAX_VALUE, false, att, handler)

     * </pre>

     *

     * @param   <A>

     *          The type of the attachment

     * @param   attachment

     *          The object to attach to the I/O operation; can be {@code null}

     * @param   handler

     *          The handler for consuming the result

     *

     * @throws  OverlappingFileLockException

     *          If a lock is already held by this Java virtual machine, or there

     *          is already a pending attempt to lock a region

     * @throws  NonWritableChannelException

     *          If this channel was not opened for writing

     */

    public final <A> void lock(A attachment,

                               CompletionHandler<FileLock,? super A> handler)

    {

        lock(0L, Long.MAX_VALUE, false, attachment, handler);

    }

    /**

     * Acquires a lock on the given region of this channel's file.

     *

     * <p> This method initiates an operation to acquire a lock on the given

     * region of this channel's file.  The method behaves in exactly the same

     * manner as the {@link #lock(long, long, boolean, Object, CompletionHandler)}

     * method except that instead of specifying a completion handler, this

     * method returns a {@code Future} representing the pending result. The

     * {@code Future}'s {@link Future#get() get} method returns the {@link

     * FileLock} on successful completion.

     *

     * @param   position