FileChannelpublic abstract class FileChannel extends AbstractInterruptibleChannel implements ByteChannel, GatheringByteChannel, ScatteringByteChannel| A channel for reading, writing, mapping, and manipulating a file.
A file channel has a current position within its file which can
be both {@link #position() queried} and {@link #position(long)
modified}. The file itself contains a variable-length sequence
of bytes that can be read and written and whose current {@link #size
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 {@link #truncate 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:
read} or
{@link #write(ByteBuffer, long) 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 {@link #map 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 {@link #force 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 {@link #transferTo to
some other channel}, and {@link #transferFrom 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 {@link FileLock locked}
against access by other programs.
File channels 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. 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.
This class does not define methods for opening existing files or for
creating new ones; such methods may be added in a future release. In this
release a file channel can be obtained from an existing {@link
java.io.FileInputStream#getChannel FileInputStream}, {@link
java.io.FileOutputStream#getChannel FileOutputStream}, or {@link
java.io.RandomAccessFile#getChannel RandomAccessFile} object by invoking
that object's getChannel method, which returns a file channel that
is connected to the same underlying file.
The state of a 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 {@link
java.io.FileInputStream#getChannel getChannel} method of a {@link
java.io.FileInputStream} instance will be open for reading. A channel
obtained via the {@link java.io.FileOutputStream#getChannel getChannel}
method of a {@link java.io.FileOutputStream} instance will be open for
writing. Finally, a channel obtained via the {@link
java.io.RandomAccessFile#getChannel getChannel} method of a {@link
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 {@link
java.io.FileOutputStream#FileOutputStream(java.io.File,boolean)
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. |
| Constructors Summary |
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protected FileChannel()Initializes a new instance of this class.
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| Methods Summary |
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| public abstract void | force(boolean metaData)Forces any updates to this channel's file to be written to the storage
device that contains it.
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 metaData parameter can be used to limit the number of
I/O operations that this method is required to perform. Passing
false for this parameter indicates that only updates to the
file's content need be written to storage; passing 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. It may or
may not force changes that were made by modifying the content of a
{@link MappedByteBuffer mapped byte buffer} obtained by
invoking the {@link #map map} method. Invoking the {@link
MappedByteBuffer#force force} method of the mapped byte buffer will
force changes made to the buffer's content to be written.
| | public abstract java.nio.channels.FileLock | lock(long position, long size, boolean shared)Acquires a lock on the given region of this channel's file.
An invocation of this method will block until the region can be
locked, this channel is closed, or the invoking thread is interrupted,
whichever comes first.
If this channel is closed by another thread during an invocation of
this method then an {@link AsynchronousCloseException} will be thrown.
If the invoking thread is interrupted while waiting to acquire the
lock then its interrupt status will be set and a {@link
FileLockInterruptionException} will be thrown. If the invoker's
interrupt status is set when this method is invoked then that exception
will be thrown immediately; the thread's interrupt status will not be
changed.
The region specified by the position and 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 zero-argument
{@link #lock()} method simply locks a region of size {@link
Long#MAX_VALUE}.
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.
| | public final java.nio.channels.FileLock | lock()Acquires an exclusive lock on this channel's file.
An invocation of this method of the form fc.lock() behaves
in exactly the same way as the invocation
fc.{@link #lock(long,long,boolean) lock}(0L, Long.MAX_VALUE, false)
return lock(0L, Long.MAX_VALUE, false);
| | public abstract java.nio.MappedByteBuffer | map(java.nio.channels.FileChannel$MapMode mode, long position, long size)Maps a region of this channel's file directly into memory.
A region of a file may be mapped into memory in one of three modes:
Read-only: Any attempt to modify the resulting buffer
will cause a {@link java.nio.ReadOnlyBufferException} to be thrown.
({@link MapMode#READ_ONLY MapMode.READ_ONLY}) Read/write: Changes made to the resulting buffer will
eventually be propagated to the file; they may or may not be made
visible to other programs that have mapped the same file. ({@link
MapMode#READ_WRITE MapMode.READ_WRITE}) Private: Changes made to the resulting buffer will not
be propagated to the file and will not be visible to other programs
that have mapped the same file; instead, they will cause private
copies of the modified portions of the buffer to be created. ({@link
MapMode#PRIVATE MapMode.PRIVATE})
For a read-only mapping, this channel must have been opened for
reading; for a read/write or private mapping, this channel must have
been opened for both reading and writing.
The {@link MappedByteBuffer mapped byte buffer}
returned by this method will have a position of zero and a limit and
capacity of size; its mark will be undefined. The buffer and
the mapping that it represents will remain valid until the buffer itself
is garbage-collected.
A mapping, once established, is not dependent upon the file channel
that was used to create it. Closing the channel, in particular, has no
effect upon the validity of the mapping.
Many of the details of memory-mapped files are inherently dependent
upon the underlying operating system and are therefore unspecified. The
behavior of this method when the requested region is not completely
contained within this channel's file is unspecified. Whether changes
made to the content or size of the underlying file, by this program or
another, are propagated to the buffer is unspecified. The rate at which
changes to the buffer are propagated to the file is unspecified.
For most operating systems, mapping a file into memory is more
expensive than reading or writing a few tens of kilobytes of data via
the usual {@link #read read} and {@link #write write} methods. From the
standpoint of performance it is generally only worth mapping relatively
large files into memory.
| | public abstract long | position()Returns this channel's file position.
| | public abstract java.nio.channels.FileChannel | position(long newPosition)Sets this channel's file position.
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.
| | public abstract int | read(java.nio.ByteBuffer dst, long position)Reads a sequence of bytes from this channel into the given buffer,
starting at the given file position.
This method works in the same manner as the {@link
#read(ByteBuffer)} method, except that bytes are read starting at the
given file position rather than at the channel's current position. This
method does not modify this channel's position. If the given position
is greater than the file's current size then no bytes are read.
| | public abstract int | read(java.nio.ByteBuffer dst)Reads a sequence of bytes from this channel into the given buffer.
Bytes are read starting at this channel's current file position, and
then the file position is updated with the number of bytes actually
read. Otherwise this method behaves exactly as specified in the {@link
ReadableByteChannel} interface.
| | public abstract long | read(java.nio.ByteBuffer[] dsts, int offset, int length)Reads a sequence of bytes from this channel into a subsequence of the
given buffers.
Bytes are read starting at this channel's current file position, and
then the file position is updated with the number of bytes actually
read. Otherwise this method behaves exactly as specified in the {@link
ScatteringByteChannel} interface.
| | public final long | read(java.nio.ByteBuffer[] dsts)Reads a sequence of bytes from this channel into the given buffers.
Bytes are read starting at this channel's current file position, and
then the file position is updated with the number of bytes actually
read. Otherwise this method behaves exactly as specified in the {@link
ScatteringByteChannel} interface.
return read(dsts, 0, dsts.length);
| | public abstract long | size()Returns the current size of this channel's file.
| | public abstract long | transferFrom(java.nio.channels.ReadableByteChannel src, long position, long count)Transfers bytes into this channel's file from the given readable byte
channel.
An attempt is made to read up to count bytes from the
source channel and write them to this channel's file starting at the
given position. An invocation of this method may or may not
transfer all of the requested bytes; whether or not it does so depends
upon the natures and states of the channels. Fewer than the requested
number of bytes will be transferred if the source channel has fewer than
count bytes remaining, or if the source channel is non-blocking
and has fewer than count bytes immediately available in its
input buffer.
This method does not modify this channel's position. If the given
position is greater than the file's current size then no bytes are
transferred. If the source channel has a position then bytes are read
starting at that position and then the position is incremented by the
number of bytes read.
This method is potentially much more efficient than a simple loop
that reads from the source channel and writes to this channel. Many
operating systems can transfer bytes directly from the source channel
into the filesystem cache without actually copying them.
| | public abstract long | transferTo(long position, long count, java.nio.channels.WritableByteChannel target)Transfers bytes from this channel's file to the given writable byte
channel.
An attempt is made to read up to count bytes starting at
the given position in this channel's file and write them to the
target channel. An invocation of this method may or may not transfer
all of the requested bytes; whether or not it does so depends upon the
natures and states of the channels. Fewer than the requested number of
bytes are transferred if this channel's file contains fewer than
count bytes starting at the given position, or if the
target channel is non-blocking and it has fewer than count
bytes free in its output buffer.
This method does not modify this channel's position. If the given
position is greater than the file's current size then no bytes are
transferred. If the target channel has a position then bytes are
written starting at that position and then the position is incremented
by the number of bytes written.
This method is potentially much more efficient than a simple loop
that reads from this channel and writes to the target channel. Many
operating systems can transfer bytes directly from the filesystem cache
to the target channel without actually copying them.
| | public abstract java.nio.channels.FileChannel | truncate(long size)Truncates this channel's file to the given size.
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.
| | public abstract java.nio.channels.FileLock | tryLock(long position, long size, boolean shared)Attempts to acquire a lock on the given region of this channel's file.
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
null. If it fails to acquire a lock for any other reason then
an appropriate exception is thrown.
The region specified by the position and 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 zero-argument
{@link #tryLock()} method simply locks a region of size {@link
Long#MAX_VALUE}.
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.
| | public final java.nio.channels.FileLock | tryLock()Attempts to acquire an exclusive lock on this channel's file.
An invocation of this method of the form fc.tryLock()
behaves in exactly the same way as the invocation
fc.{@link #tryLock(long,long,boolean) tryLock}(0L, Long.MAX_VALUE, false)
return tryLock(0L, Long.MAX_VALUE, false);
| | public abstract int | write(java.nio.ByteBuffer src, long position)Writes a sequence of bytes to this channel from the given buffer,
starting at the given file position.
This method works in the same manner as the {@link
#write(ByteBuffer)} method, except that bytes are written starting at
the given file position rather than at the channel's current position.
This method does not modify this channel's position. If the given
position is greater than the file's current size 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.
| | public abstract int | write(java.nio.ByteBuffer src)Writes a sequence of bytes to this channel from the given buffer.
Bytes are written starting at this channel's current file position
unless the channel is in append mode, in which case the position is
first advanced to the end of the file. The file is grown, if necessary,
to accommodate the written bytes, and then the file position is updated
with the number of bytes actually written. Otherwise this method
behaves exactly as specified by the {@link WritableByteChannel}
interface.
| | public abstract long | write(java.nio.ByteBuffer[] srcs, int offset, int length)Writes a sequence of bytes to this channel from a subsequence of the
given buffers.
Bytes are written starting at this channel's current file position
unless the channel is in append mode, in which case the position is
first advanced to the end of the file. The file is grown, if necessary,
to accommodate the written bytes, and then the file position is updated
with the number of bytes actually written. Otherwise this method
behaves exactly as specified in the {@link GatheringByteChannel}
interface.
| | public final long | write(java.nio.ByteBuffer[] srcs)Writes a sequence of bytes to this channel from the given buffers.
Bytes are written starting at this channel's current file position
unless the channel is in append mode, in which case the position is
first advanced to the end of the file. The file is grown, if necessary,
to accommodate the written bytes, and then the file position is updated
with the number of bytes actually written. Otherwise this method
behaves exactly as specified in the {@link GatheringByteChannel}
interface.
return write(srcs, 0, srcs.length);
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