An asynchronous channel for reading, writing, and manipulating a file.
An asynchronous file channel is created when a file is opened by invoking
one of the 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 queried. The size of the file increases
when bytes are written beyond its current size; the size of the file decreases
when it is 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 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 Future to represent the pending
result of the operation. The 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 forced 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 locked against
access by other programs.
An 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 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 AsynchronousChannelGroup class.
Channels of this type are safe for use by multiple concurrent threads. The
close method may be invoked at any time, as specified
by the Channel interface. This causes all outstanding asynchronous
operations on the channel to complete with the exception 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
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 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.
implements