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}. * *
An asynchronous file channel does not have a current position * 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. * *
In addition to read and write operations, this class defines the * following operations:
* *Updates made to a file may be {@link #force forced * out} to the underlying storage device, ensuring that data are not * lost in the event of a system crash.
A region of a file may be {@link #lock locked} against * access by other programs.
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 identity). * 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. * *
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. * *
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. * *
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. * *
In addition to {@code READ} and {@code WRITE}, the following options * may be present: * *
| Option | Description |
|---|---|
| {@link StandardOpenOption#TRUNCATE_EXISTING TRUNCATE_EXISTING} | *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. | *
| {@link StandardOpenOption#CREATE_NEW CREATE_NEW} | *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. | *
| {@link StandardOpenOption#CREATE CREATE} | *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. | *
| {@link StandardOpenOption#DELETE_ON_CLOSE DELETE_ON_CLOSE} | *When this option is present then the implementation makes a * best effort attempt to delete the file when closed by the * the {@link #close close} method. If the {@code close} method is not * invoked then a best effort attempt is made to delete the file * when the Java virtual machine terminates. | *
| {@link StandardOpenOption#SPARSE SPARSE} | *When creating a new file this option is a hint that the * new file will be sparse. This option is ignored when not creating * a new file. | *
| {@link StandardOpenOption#SYNC SYNC} | *Requires that every update to the file's content or metadata be * written synchronously to the underlying storage device. (see Synchronized I/O file * integrity). | *
| {@link StandardOpenOption#DSYNC DSYNC} | *Requires that every update to the file's content be written * synchronously to the underlying storage device. (see Synchronized I/O file * integrity). | *
An implementation may also support additional options. * *
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. * *
The {@code attrs} parameter is an optional array of file {@link * FileAttribute file-attributes} to set atomically when creating the file. * *
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 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. * *
An invocation of this method behaves in exactly the same way as the * invocation *
* ch.{@link #open(Path,Set,ExecutorService,FileAttribute[])
* open}(file, opts, null, new FileAttribute<?>[0]);
*
* where {@code opts} is a {@code Set} containing the options specified to
* this method.
*
* 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 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. 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.
*
* If the file does not reside on a local device then no such guarantee
* is made.
*
* 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.
*
* 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.
*
* 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.
*
* 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}.
*
* 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}.
*
* 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.
*
* 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.
*
* 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 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 void lock(long position,
long size,
boolean shared,
A attachment,
CompletionHandler 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}.
*
* An invocation of this method of the form {@code ch.lock(att,handler)}
* behaves in exactly the same way as the invocation
* 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
* 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)
*
* @return a {@code Future} object representing the pending 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 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 Future This method initiates an operation to acquire an exclusive lock on this
* channel's file. The method returns a {@code Future} representing the
* pending result of the operation. The {@code Future}'s {@link Future#get()
* get} method returns the {@link FileLock} on successful completion.
*
* An invocation of this method behaves in exactly the same way as the
* invocation
* This method does not block. An invocation always returns immediately,
* either having acquired a lock on the requested region or having failed to
* do so. If it fails to acquire a lock because an overlapping lock is held
* by another program then it returns {@code null}. If it fails to acquire
* a lock for any other reason then an appropriate exception is thrown.
*
* @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,
* {@code false} to request an exclusive lock
*
* @return A lock object representing the newly-acquired lock,
* or {@code null} if the lock could not be acquired
* because another program holds an overlapping lock
*
* @throws IllegalArgumentException
* If the preconditions on the parameters do not hold
* @throws ClosedChannelException
* If this channel is closed
* @throws OverlappingFileLockException
* If a lock that overlaps the requested region is already held by
* this Java virtual machine, or if another thread is already
* blocked in this method and is attempting to lock an overlapping
* region of the same file
* @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
*
* @throws IOException
* If some other I/O error occurs
*
* @see #lock(Object,CompletionHandler)
* @see #lock(long,long,boolean,Object,CompletionHandler)
* @see #tryLock()
*/
public abstract FileLock tryLock(long position, long size, boolean shared)
throws IOException;
/**
* Attempts to acquire an exclusive lock on this channel's file.
*
* An invocation of this method of the form {@code ch.tryLock()}
* behaves in exactly the same way as the invocation
*
* This method initiates the reading of a sequence of bytes from this
* channel into the given buffer, starting at the given file position. The
* result of the read is the number of bytes read or {@code -1} if the given
* position is greater than or equal to the file's size at the time that the
* read is attempted.
*
* This method works in the same manner as the {@link
* AsynchronousByteChannel#read(ByteBuffer,Object,CompletionHandler)}
* method, except that bytes are read starting at the given file position.
* If the given file position is greater than the file's size at the time
* that the read is attempted then no bytes are read.
*
* @param dst
* The buffer into which bytes are to be transferred
* @param position
* The file position at which the transfer is to begin;
* must be non-negative
* @param attachment
* The object to attach to the I/O operation; can be {@code null}
* @param handler
* The handler for consuming the result
*
* @throws IllegalArgumentException
* If the position is negative or the buffer is read-only
* @throws NonReadableChannelException
* If this channel was not opened for reading
*/
public abstract void read(ByteBuffer dst,
long position,
A attachment,
CompletionHandler This method initiates the reading of a sequence of bytes from this
* channel into the given buffer, starting at the given file position. This
* method returns a {@code Future} representing the pending result of the
* operation. The {@code Future}'s {@link Future#get() get} method returns
* the number of bytes read or {@code -1} if the given position is greater
* than or equal to the file's size at the time that the read is attempted.
*
* This method works in the same manner as the {@link
* AsynchronousByteChannel#read(ByteBuffer)} method, except that bytes are
* read starting at the given file position. If the given file position is
* greater than the file's size at the time that the read is attempted then
* no bytes are read.
*
* @param dst
* The buffer into which bytes are to be transferred
* @param position
* The file position at which the transfer is to begin;
* must be non-negative
*
* @return A {@code Future} object representing the pending result
*
* @throws IllegalArgumentException
* If the position is negative or the buffer is read-only
* @throws NonReadableChannelException
* If this channel was not opened for reading
*/
public abstract Future This method works in the same manner as the {@link
* AsynchronousByteChannel#write(ByteBuffer,Object,CompletionHandler)}
* method, except that bytes are written starting at the given file position.
* If the given position is greater than the file's size, at the time that
* the write is attempted, then the file will be grown to accommodate the new
* bytes; the values of any bytes between the previous end-of-file and the
* newly-written bytes are unspecified.
*
* @param src
* The buffer from which bytes are to be transferred
* @param position
* The file position at which the transfer is to begin;
* must be non-negative
* @param attachment
* The object to attach to the I/O operation; can be {@code null}
* @param handler
* The handler for consuming the result
*
* @throws IllegalArgumentException
* If the position is negative
* @throws NonWritableChannelException
* If this channel was not opened for writing
*/
public abstract void write(ByteBuffer src,
long position,
A attachment,
CompletionHandler This method initiates the writing of a sequence of bytes to this
* channel from the given buffer, starting at the given file position. The
* method returns a {@code Future} representing the pending result of the
* write operation. The {@code Future}'s {@link Future#get() get} method
* returns the number of bytes written.
*
* This method works in the same manner as the {@link
* AsynchronousByteChannel#write(ByteBuffer)} method, except that bytes are
* written starting at the given file position. If the given position is
* greater than the file's size, at the time that the write is attempted,
* then the file will be grown to accommodate the new bytes; the values of
* any bytes between the previous end-of-file and the newly-written bytes
* are unspecified.
*
* @param src
* The buffer from which bytes are to be transferred
* @param position
* The file position at which the transfer is to begin;
* must be non-negative
*
* @return A {@code Future} object representing the pending result
*
* @throws IllegalArgumentException
* If the position is negative
* @throws NonWritableChannelException
* If this channel was not opened for writing
*/
public abstract Future
* ch.{@link #lock(long,long,boolean,Object,CompletionHandler) lock}(0L, Long.MAX_VALUE, false, att, handler)
*
*
* @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 void lock(A attachment,
CompletionHandler
* ch.{@link #lock(long,long,boolean) lock}(0L, Long.MAX_VALUE, false)
*
*
* @return a {@code Future} object representing the pending 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 Future
* ch.{@link #tryLock(long,long,boolean) tryLock}(0L, Long.MAX_VALUE, false)
*
* @return A lock object representing the newly-acquired lock,
* or {@code null} if the lock could not be acquired
* because another program holds an overlapping lock
*
* @throws ClosedChannelException
* If this channel is closed
* @throws OverlappingFileLockException
* If a lock that overlaps the requested region is already held by
* this Java virtual machine, or if another thread is already
* blocked in this method and is attempting to lock an overlapping
* region
* @throws NonWritableChannelException
* If {@code shared} is false but this channel was not opened for writing
*
* @throws IOException
* If some other I/O error occurs
*
* @see #lock(Object,CompletionHandler)
* @see #lock(long,long,boolean,Object,CompletionHandler)
* @see #tryLock(long,long,boolean)
*/
public final FileLock tryLock() throws IOException {
return tryLock(0L, Long.MAX_VALUE, false);
}
/**
* Reads a sequence of bytes from this channel into the given buffer,
* starting at the given file position.
*
*