A channel for reading, writing, mapping, and manipulating a file.
A file channel is a SeekableByteChannel that is connected to
a file. It has a current position within its file which can
be both queried and position(long). The file itself 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. The
file may also have some associated metadata such as access
permissions, content type, and last-modification time; this class does not
define methods for metadata access.
In addition to the familiar read, write, and close operations of byte channels, this class defines the following file-specific operations:
Bytes may be read or
written at an absolute
position in a file in a way that does not affect the channel's current
position.
A region of a file may be mapped
directly into memory; for large files this is often much more efficient
than invoking the usual read or write methods.
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.
Bytes can be transferred from a file to, and vice, in a way that can be optimized by many operating systems
into a very fast transfer directly to or from the filesystem cache.
A region of a file may be locked
against access by other programs.
File channels are safe for use by multiple concurrent threads. The
close method may be invoked at any time, as specified
by the 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.
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.
A file channel is created by invoking one of the open
methods defined by this class. A file channel can also be obtained from an
existing FileInputStream, FileOutputStream, or RandomAccessFile object by invoking
that object's getChannel method, which returns a file channel that
is connected to the same underlying file. Where the file channel is obtained
from an existing stream or random access file then the state of the file
channel is intimately connected to that of the object whose getChannel
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.
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 getChannel method of a java.io.FileInputStream instance will be open for reading. A channel
obtained via the getChannel
method of a java.io.FileOutputStream instance will be open for
writing. Finally, a channel obtained via the getChannel method of a java.io.RandomAccessFile instance will be open for reading if the instance
was created with mode "r" and will be open for reading and writing
if the instance was created with mode "rw".
A file channel that is open for writing may be in
append mode, for example if it was obtained from a file-output stream
that was created by invoking the java.io.FileOutputStream.FileOutputStream(java.io.File,boolean) constructor and passing true 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.
extends
java.io.FileInputStream.getChannel(), java.io.FileOutputStream.getChannel(), java.io.RandomAccessFile.getChannel()