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java.lang.Objectjava.nio.channels.spi.AbstractInterruptibleChannel
java.nio.channels.FileChannel
org.classpath.icedtea.java.nio.channels.FileChannel
public abstract class FileChannel
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 modified
. 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
out
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
some other channel
, and vice
versa
, 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 FileInputStream
instance will be open for reading. A channel
obtained via the getChannel
method of a FileOutputStream
instance will be open for
writing. Finally, a channel obtained via the getChannel
method of a 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 FileOutputStream(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.
FileInputStream.getChannel()
,
FileOutputStream.getChannel()
,
RandomAccessFile.getChannel()
Nested Class Summary |
---|
Nested classes/interfaces inherited from class java.nio.channels.FileChannel |
---|
FileChannel.MapMode |
Constructor Summary | |
---|---|
protected |
FileChannel()
Initializes a new instance of this class. |
Method Summary | |
---|---|
static FileChannel |
open(Path file,
OpenOption... options)
Opens or creates a file, returning a file channel to access the file. |
static FileChannel |
open(Path file,
Set<? extends OpenOption> options,
FileAttribute<?>... attrs)
Opens or creates a file, returning a file channel to access the file. |
abstract FileChannel |
positionSBC(long newPosition)
Sets this channel's file position. |
abstract FileChannel |
truncateSBC(long size)
Truncates this channel's file to the given size. |
Methods inherited from class java.nio.channels.FileChannel |
---|
force, lock, lock, map, position, position, read, read, read, read, size, transferFrom, transferTo, truncate, tryLock, tryLock, write, write, write, write |
Methods inherited from class java.nio.channels.spi.AbstractInterruptibleChannel |
---|
begin, close, end, implCloseChannel, isOpen |
Methods inherited from class java.lang.Object |
---|
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait |
Methods inherited from interface org.classpath.icedtea.java.nio.channels.SeekableByteChannel |
---|
position, read, size, write |
Methods inherited from interface java.nio.channels.Channel |
---|
close, isOpen |
Constructor Detail |
---|
protected FileChannel()
Method Detail |
---|
public static FileChannel open(Path file, Set<? extends OpenOption> options, FileAttribute<?>... attrs) throws IOException
The options
parameter determines how the file is opened.
The READ
and WRITE
options determine if the file should be opened for reading and/or
writing. If neither option (or the APPEND
option) is contained in the array then the file is opened for reading.
By default reading or writing commences at the beginning of the file.
In the addition to READ
and WRITE
, the following
options may be present:
Option | Description |
---|---|
APPEND |
If this option is present then the file is opened for writing and
each invocation of the channel's write method 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. This option may not be used in conjunction
with the READ or TRUNCATE_EXISTING options. |
TRUNCATE_EXISTING |
If this option is present then the existing file is truncated to a size of 0 bytes. This option is ignored when the file is opened only for reading. |
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. |
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 CREATE_NEW option is also present or
the file is opened only for reading. |
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 close method. If the close method is not
invoked then a best effort attempt is made to delete the file
when the Java virtual machine terminates. |
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. |
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). |
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 attrs
parameter is an optional array of file file-attributes
to set atomically when creating the file.
The new channel is created by invoking the newFileChannel
method on the
provider that created the Path
.
file
- The path of the file to open or createoptions
- Options specifying how the file is openedattrs
- An optional list of file attributes to set atomically when
creating the file
IllegalArgumentException
- If the set contains an invalid combination of options
UnsupportedOperationException
- If the file
is associated with a provider that does not
support creating file channels, or an unsupported open option is
specified, or the array contains an attribute that cannot be set
atomically when creating the file
IOException
- If an I/O error occurs
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 SecurityManager.checkRead(String)
method is invoked to check
read access if the file is opened for reading. The SecurityManager.checkWrite(String)
method is invoked to check
write access if the file is opened for writingpublic static FileChannel open(Path file, OpenOption... options) throws IOException
An invocation of this method behaves in exactly the same way as the invocation
fc.open
(file, options, new FileAttribute<?>[0]);
file
- The path of the file to open or createoptions
- Options specifying how the file is opened
IllegalArgumentException
- If the set contains an invalid combination of options
UnsupportedOperationException
- If the file
is associated with a provider that does not
support creating file channels, or an unsupported open option is
specified
IOException
- If an I/O error occurs
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 SecurityManager.checkRead(String)
method is invoked to check
read access if the file is opened for reading. The SecurityManager.checkWrite(String)
method is invoked to check
write access if the file is opened for writingpublic abstract FileChannel positionSBC(long newPosition) throws IOException
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 accommodate the new bytes; the values of any bytes between the previous end-of-file and the newly-written bytes are unspecified.
positionSBC
in interface SeekableByteChannel
newPosition
- The new position, a non-negative integer counting
the number of bytes from the beginning of the file
ClosedChannelException
- If this channel is closed
IllegalArgumentException
- If the new position is negative
IOException
- If some other I/O error occurspublic abstract FileChannel truncateSBC(long size) throws IOException
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.
truncateSBC
in interface SeekableByteChannel
size
- The new size, a non-negative byte count
NonWritableChannelException
- If this channel was not opened for writing
ClosedChannelException
- If this channel is closed
IllegalArgumentException
- If the new size is negative
IOException
- If some other I/O error occurs
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