- All Implemented Interfaces:
Closeable
,AutoCloseable
,ByteChannel
,Channel
,GatheringByteChannel
,InterruptibleChannel
,ReadableByteChannel
,ScatteringByteChannel
,SeekableByteChannel
,WritableByteChannel
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
orwritten
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 usualread
orwrite
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
, andvice 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. Closing the channel will
close the originating object.
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. In this mode the behavior of the method to
write at a given position is also
system-dependent.
- Since:
- 1.4
- See Also:
-
Nested Class Summary
-
Constructor Summary
-
Method Summary
Modifier and TypeMethodDescriptionabstract void
force
(boolean metaData) Forces any updates to this channel's file to be written to the storage device that contains it.final FileLock
lock()
Acquires an exclusive lock on this channel's file.abstract FileLock
lock
(long position, long size, boolean shared) Acquires a lock on the given region of this channel's file.abstract MappedByteBuffer
map
(FileChannel.MapMode mode, long position, long size) Maps a region of this channel's file directly into memory.map
(FileChannel.MapMode mode, long offset, long size, Arena arena) Maps a region of this channel's file into a new mapped memory segment, with the given offset, size and arena.static FileChannel
open
(Path path, OpenOption... options) Opens or creates a file, returning a file channel to access the file.static FileChannel
open
(Path path, Set<? extends OpenOption> options, FileAttribute<?>... attrs) Opens or creates a file, returning a file channel to access the file.abstract long
position()
Returns this channel's file position.abstract FileChannel
position
(long newPosition) Sets this channel's file position.abstract int
read
(ByteBuffer dst) Reads a sequence of bytes from this channel into the given buffer.final long
read
(ByteBuffer[] dsts) Reads a sequence of bytes from this channel into the given buffers.abstract long
read
(ByteBuffer[] dsts, int offset, int length) Reads a sequence of bytes from this channel into a subsequence of the given buffers.abstract int
read
(ByteBuffer dst, long position) Reads a sequence of bytes from this channel into the given buffer, starting at the given file position.abstract long
size()
Returns the current size of this channel's file.abstract long
transferFrom
(ReadableByteChannel src, long position, long count) Transfers bytes into this channel's file from the given readable byte channel.abstract long
transferTo
(long position, long count, WritableByteChannel target) Transfers bytes from this channel's file to the given writable byte channel.abstract FileChannel
truncate
(long size) Truncates this channel's file to the given size.final FileLock
tryLock()
Attempts to acquire an exclusive lock on this channel's file.abstract FileLock
tryLock
(long position, long size, boolean shared) Attempts to acquire a lock on the given region of this channel's file.abstract int
write
(ByteBuffer src) Writes a sequence of bytes to this channel from the given buffer.final long
write
(ByteBuffer[] srcs) Writes a sequence of bytes to this channel from the given buffers.abstract long
write
(ByteBuffer[] srcs, int offset, int length) Writes a sequence of bytes to this channel from a subsequence of the given buffers.abstract int
write
(ByteBuffer src, long position) Writes a sequence of bytes to this channel from the given buffer, starting at the given file position.Methods declared in class java.nio.channels.spi.AbstractInterruptibleChannel
begin, close, end, implCloseChannel, isOpen
-
Constructor Details
-
FileChannel
protected FileChannel()Initializes a new instance of this class.
-
-
Method Details
-
open
public static FileChannel open(Path path, Set<? extends OpenOption> options, FileAttribute<?>... attrs) throws IOException Opens or creates a file, returning a file channel to access the file.The
options
parameter determines how the file is opened. TheREAD
andWRITE
options determine if the file should be opened for reading and/or writing. If neither option (or theAPPEND
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
andWRITE
, 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. The effect of writing at a given position with this option present is unspecified. This option may not be used in conjunction with theREAD
orTRUNCATE_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 close
method. If theclose
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 filefile-attributes
to set atomically when creating the file.The new channel is created by invoking the
newFileChannel
method on the provider that created thePath
.- Parameters:
path
- 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- Returns:
- A new file channel
- Throws:
IllegalArgumentException
- If the set contains an invalid combination of optionsUnsupportedOperationException
- If thepath
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 fileFileAlreadyExistsException
- If a file of that name already exists and theCREATE_NEW
option is specified and the file is being opened for writing (optional specific exception)IOException
- If an I/O error occursSecurityException
- If a security manager is installed and it denies an unspecified permission required by the implementation. In the case of the default provider, theSecurityManager.checkRead(String)
method is invoked to check read access if the file is opened for reading. TheSecurityManager.checkWrite(String)
method is invoked to check write access if the file is opened for writing- Since:
- 1.7
-
open
Opens or creates a file, returning a file channel to access the file.An invocation of this method behaves in exactly the same way as the invocation
fc.
open
(file, opts, new FileAttribute<?>[0]);opts
is a set of the options specified in theoptions
array.- Parameters:
path
- The path of the file to open or createoptions
- Options specifying how the file is opened- Returns:
- A new file channel
- Throws:
IllegalArgumentException
- If the set contains an invalid combination of optionsUnsupportedOperationException
- If thepath
is associated with a provider that does not support creating file channels, or an unsupported open option is specifiedFileAlreadyExistsException
- If a file of that name already exists and theCREATE_NEW
option is specified and the file is being opened for writing (optional specific exception)IOException
- If an I/O error occursSecurityException
- If a security manager is installed and it denies an unspecified permission required by the implementation. In the case of the default provider, theSecurityManager.checkRead(String)
method is invoked to check read access if the file is opened for reading. TheSecurityManager.checkWrite(String)
method is invoked to check write access if the file is opened for writing- Since:
- 1.7
-
read
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
ReadableByteChannel
interface.- Specified by:
read
in interfaceReadableByteChannel
- Specified by:
read
in interfaceSeekableByteChannel
- Parameters:
dst
- The buffer into which bytes are to be transferred- Returns:
- The number of bytes read, possibly zero, or
-1
if the channel has reached end-of-stream - Throws:
ClosedChannelException
- If this channel is closedAsynchronousCloseException
- If another thread closes this channel while the read operation is in progressClosedByInterruptException
- If another thread interrupts the current thread while the read operation is in progress, thereby closing the channel and setting the current thread's interrupt statusNonReadableChannelException
- If this channel was not opened for readingIOException
- If some other I/O error occurs
-
read
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
ScatteringByteChannel
interface.- Specified by:
read
in interfaceScatteringByteChannel
- Parameters:
dsts
- The buffers into which bytes are to be transferredoffset
- The offset within the buffer array of the first buffer into which bytes are to be transferred; must be non-negative and no larger thandsts.length
length
- The maximum number of buffers to be accessed; must be non-negative and no larger thandsts.length
-offset
- Returns:
- The number of bytes read, possibly zero,
or
-1
if the channel has reached end-of-stream - Throws:
ClosedChannelException
- If this channel is closedAsynchronousCloseException
- If another thread closes this channel while the read operation is in progressClosedByInterruptException
- If another thread interrupts the current thread while the read operation is in progress, thereby closing the channel and setting the current thread's interrupt statusNonReadableChannelException
- If this channel was not opened for readingIOException
- If some other I/O error occurs
-
read
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
ScatteringByteChannel
interface.- Specified by:
read
in interfaceScatteringByteChannel
- Parameters:
dsts
- The buffers into which bytes are to be transferred- Returns:
- The number of bytes read, possibly zero,
or
-1
if the channel has reached end-of-stream - Throws:
ClosedChannelException
- If this channel is closedAsynchronousCloseException
- If another thread closes this channel while the read operation is in progressClosedByInterruptException
- If another thread interrupts the current thread while the read operation is in progress, thereby closing the channel and setting the current thread's interrupt statusNonReadableChannelException
- If this channel was not opened for readingIOException
- If some other I/O error occurs
-
write
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
WritableByteChannel
interface.- Specified by:
write
in interfaceSeekableByteChannel
- Specified by:
write
in interfaceWritableByteChannel
- Parameters:
src
- The buffer from which bytes are to be retrieved- Returns:
- The number of bytes written, possibly zero
- Throws:
ClosedChannelException
- If this channel is closedAsynchronousCloseException
- If another thread closes this channel while the write operation is in progressClosedByInterruptException
- If another thread interrupts the current thread while the write operation is in progress, thereby closing the channel and setting the current thread's interrupt statusNonWritableChannelException
- If this channel was not opened for writingIOException
- If some other I/O error occurs
-
write
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
GatheringByteChannel
interface.- Specified by:
write
in interfaceGatheringByteChannel
- Parameters:
srcs
- The buffers from which bytes are to be retrievedoffset
- The offset within the buffer array of the first buffer from which bytes are to be retrieved; must be non-negative and no larger thansrcs.length
length
- The maximum number of buffers to be accessed; must be non-negative and no larger thansrcs.length
-offset
- Returns:
- The number of bytes written, possibly zero
- Throws:
ClosedChannelException
- If this channel is closedAsynchronousCloseException
- If another thread closes this channel while the write operation is in progressClosedByInterruptException
- If another thread interrupts the current thread while the write operation is in progress, thereby closing the channel and setting the current thread's interrupt statusNonWritableChannelException
- If this channel was not opened for writingIOException
- If some other I/O error occurs
-
write
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
GatheringByteChannel
interface.- Specified by:
write
in interfaceGatheringByteChannel
- Parameters:
srcs
- The buffers from which bytes are to be retrieved- Returns:
- The number of bytes written, possibly zero
- Throws:
ClosedChannelException
- If this channel is closedAsynchronousCloseException
- If another thread closes this channel while the write operation is in progressClosedByInterruptException
- If another thread interrupts the current thread while the write operation is in progress, thereby closing the channel and setting the current thread's interrupt statusNonWritableChannelException
- If this channel was not opened for writingIOException
- If some other I/O error occurs
-
position
Returns this channel's file position.- Specified by:
position
in interfaceSeekableByteChannel
- Returns:
- This channel's file position, a non-negative integer counting the number of bytes from the beginning of the file to the current position
- Throws:
ClosedChannelException
- If this channel is closedIOException
- If some other I/O error occurs
-
position
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.
- Specified by:
position
in interfaceSeekableByteChannel
- Parameters:
newPosition
- The new position, a non-negative integer counting the number of bytes from the beginning of the file- Returns:
- This file channel
- Throws:
ClosedChannelException
- If this channel is closedIllegalArgumentException
- If the new position is negativeIOException
- If some other I/O error occurs
-
size
Returns the current size of this channel's file.- Specified by:
size
in interfaceSeekableByteChannel
- Returns:
- The current size of this channel's file, measured in bytes
- Throws:
ClosedChannelException
- If this channel is closedIOException
- If some other I/O error occurs
-
truncate
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.
- Specified by:
truncate
in interfaceSeekableByteChannel
- Parameters:
size
- The new size, a non-negative byte count- Returns:
- This file channel
- Throws:
NonWritableChannelException
- If this channel was not opened for writingClosedChannelException
- If this channel is closedIllegalArgumentException
- If the new size is negativeIOException
- If some other I/O error occurs
-
force
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. Passingfalse
for this parameter indicates that only updates to the file's content need be written to storage; passingtrue
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, or the methods defined by
FileOutputStream
orRandomAccessFile
when the channel was obtained with thegetChannel
method. It may or may not force changes that were made by modifying the content of amapped byte buffer
obtained by invoking themap
method. Invoking theforce
method of the mapped byte buffer will force changes made to the buffer's content to be written.- Parameters:
metaData
- Iftrue
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 closedIOException
- If some other I/O error occurs
-
transferTo
public abstract long transferTo(long position, long count, WritableByteChannel target) throws IOException 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 givenposition
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 thancount
bytes starting at the givenposition
, or if the target channel is non-blocking and it has fewer thancount
bytes free in its output buffer.This method does not modify this channel's position. If the given position is greater than or equal to 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.
- Parameters:
position
- The position within the file at which the transfer is to begin; must be non-negativecount
- The maximum number of bytes to be transferred; must be non-negativetarget
- The target channel- Returns:
- The number of bytes, possibly zero, that were actually transferred
- Throws:
IllegalArgumentException
- If the preconditions on the parameters do not holdNonReadableChannelException
- If this channel was not opened for readingNonWritableChannelException
- If the target channel was not opened for writingClosedChannelException
- If either this channel or the target channel is closedAsynchronousCloseException
- If another thread closes either channel while the transfer is in progressClosedByInterruptException
- If another thread interrupts the current thread while the transfer is in progress, thereby closing both channels and setting the current thread's interrupt statusIOException
- If some other I/O error occurs
-
transferFrom
public abstract long transferFrom(ReadableByteChannel src, long position, long count) throws IOException 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 givenposition
. 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 thancount
bytes remaining, or if the source channel is non-blocking and has fewer thancount
bytes immediately available in its input buffer. No bytes are transferred, and zero is returned, if the source has reached end-of-stream.This method does not modify this channel's position. If the given position is greater than or equal to 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. 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.
- Parameters:
src
- The source channelposition
- The file position at which the transfer is to begin; must be non-negativecount
- The maximum number of bytes to be transferred; must be non-negative- Returns:
- The number of bytes, possibly zero, that were actually transferred
- Throws:
IllegalArgumentException
- If the preconditions on the parameters do not holdNonReadableChannelException
- If the source channel was not opened for readingNonWritableChannelException
- If this channel was not opened for writingClosedChannelException
- If either this channel or the source channel is closedAsynchronousCloseException
- If another thread closes either channel while the transfer is in progressClosedByInterruptException
- If another thread interrupts the current thread while the transfer is in progress, thereby closing both channels and setting the current thread's interrupt statusIOException
- If some other I/O error occurs
-
read
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
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 or equal to the file's current size then no bytes are read.- Parameters:
dst
- The buffer into which bytes are to be transferredposition
- The file position at which the transfer is to begin; must be non-negative- Returns:
- The number of bytes read, possibly zero, or
-1
if the given position is greater than or equal to the file's current size - Throws:
IllegalArgumentException
- If the position is negative or the buffer is read-onlyNonReadableChannelException
- If this channel was not opened for readingClosedChannelException
- If this channel is closedAsynchronousCloseException
- If another thread closes this channel while the read operation is in progressClosedByInterruptException
- If another thread interrupts the current thread while the read operation is in progress, thereby closing the channel and setting the current thread's interrupt statusIOException
- If some other I/O error occurs
-
write
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
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 or equal to 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.If the file is open in append mode, then the effect of invoking this method is unspecified.
- Parameters:
src
- The buffer from which bytes are to be transferredposition
- The file position at which the transfer is to begin; must be non-negative- Returns:
- The number of bytes written, possibly zero
- Throws:
IllegalArgumentException
- If the position is negativeNonWritableChannelException
- If this channel was not opened for writingClosedChannelException
- If this channel is closedAsynchronousCloseException
- If another thread closes this channel while the write operation is in progressClosedByInterruptException
- If another thread interrupts the current thread while the write operation is in progress, thereby closing the channel and setting the current thread's interrupt statusIOException
- If some other I/O error occurs
-
map
public abstract MappedByteBuffer map(FileChannel.MapMode mode, long position, long size) throws IOException Maps a region of this channel's file directly into memory.The
mode
parameter specifies how the region of the file is mapped and may be one of the following modes:Read-only: Any attempt to modify the resulting buffer will cause a
ReadOnlyBufferException
to be thrown. (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. (
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. (
MapMode.PRIVATE
)
An implementation may support additional map modes.
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
mapped byte buffer
returned by this method will have a position of zero and a limit and capacity ofsize
; 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
read
andwrite
methods. From the standpoint of performance it is generally only worth mapping relatively large files into memory.- Parameters:
mode
- One of the constantsREAD_ONLY
,READ_WRITE
, orPRIVATE
defined in theFileChannel.MapMode
class, according to whether the file is to be mapped read-only, read/write, or privately (copy-on-write), respectively, or an implementation specific map modeposition
- The position within the file at which the mapped region is to start; must be non-negativesize
- The size of the region to be mapped; must be non-negative and no greater thanInteger.MAX_VALUE
- Returns:
- The mapped byte buffer
- Throws:
NonReadableChannelException
- If themode
isREAD_ONLY
or an implementation specific map mode requiring read access, but this channel was not opened for readingNonWritableChannelException
- If themode
isREAD_WRITE
,PRIVATE
or an implementation specific map mode requiring write access, but this channel was not opened for both reading and writingIllegalArgumentException
- If the preconditions on the parameters do not holdUnsupportedOperationException
- If an unsupported map mode is specifiedIOException
- If some other I/O error occurs- See Also:
-
map
public MemorySegment map(FileChannel.MapMode mode, long offset, long size, Arena arena) throws IOException Maps a region of this channel's file into a new mapped memory segment, with the given offset, size and arena. The address of the returned memory segment is the starting address of the mapped off-heap region backing the segment.The lifetime of the returned segment is controlled by the provided arena. For instance, if the provided arena is a closeable arena, the returned segment will be unmapped when the provided closeable arena is closed.
If the specified mapping mode is READ_ONLY, the resulting segment will be read-only (see
MemorySegment.isReadOnly()
).The content of a mapped memory segment can change at any time, for example if the content of the corresponding region of the mapped file is changed by this (or another) program. Whether such changes occur, and when they occur, is operating-system dependent and therefore unspecified.
All or part of a mapped memory segment may become inaccessible at any time, for example if the backing mapped file is truncated. An attempt to access an inaccessible region of a mapped memory segment will not change the segment's content and will cause an unspecified exception to be thrown either at the time of the access or at some later time. It is therefore strongly recommended that appropriate precautions be taken to avoid the manipulation of a mapped file by this (or another) program, except to read or write the file's content.
- Implementation Requirements:
- The default implementation of this method throws
UnsupportedOperationException
. - Implementation Note:
- When obtaining a mapped segment from a newly created file channel, the initialization state of the contents of the block of mapped memory associated with the returned mapped memory segment is unspecified and should not be relied upon.
- Parameters:
mode
- The file mapping mode, seemap(FileChannel.MapMode, long, long)
; the mapping mode might affect the behavior of the returned memory mapped segment (seeMemorySegment.force()
)offset
- The offset (expressed in bytes) within the file at which the mapped segment is to startsize
- The size (in bytes) of the mapped memory backing the memory segmentarena
- The segment arena- Returns:
- A new mapped memory segment
- Throws:
IllegalArgumentException
- Ifoffset < 0
,size < 0
oroffset + size
overflows the range oflong
IllegalStateException
- Ifarena.isAlive() == false
WrongThreadException
- Ifarena
is a confined scoped arena, and this method is called from a threadT
, other than the scoped arena's owner threadNonReadableChannelException
- If themode
isREAD_ONLY
or an implementation specific map mode requiring read access, but this channel was not opened for readingNonWritableChannelException
- If themode
isREAD_WRITE
,PRIVATE
or an implementation specific map mode requiring write access, but this channel was not opened for both reading and writingIOException
- If some other I/O error occursUnsupportedOperationException
- If an unsupported map mode is specified- Since:
- 22
-
lock
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
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
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
andsize
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-argumentlock()
method simply locks a region of sizeLong.MAX_VALUE
. If theposition
is non-negative and thesize
is zero, then a lock of sizeLong.MAX_VALUE - position
is returned.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
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.
- Parameters:
position
- The position at which the locked region is to start; must be non-negativesize
- The size of the locked region; must be non-negative, and the sumposition
+size
must be non-negative. A value of zero means to lock all bytes from the specified starting position to the end of the file, regardless of whether the file is subsequently extended or truncatedshared
-true
to request a shared lock, in which case this channel must be open for reading (and possibly writing);false
to request an exclusive lock, in which case this channel must be open for writing (and possibly reading)- Returns:
- A lock object representing the newly-acquired lock
- Throws:
IllegalArgumentException
- If the preconditions on the parameters do not holdClosedChannelException
- If this channel is closedAsynchronousCloseException
- If another thread closes this channel while the invoking thread is blocked in this methodFileLockInterruptionException
- If the invoking thread is interrupted while blocked in this methodOverlappingFileLockException
- 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 regionNonReadableChannelException
- Ifshared
istrue
but this channel was not opened for readingNonWritableChannelException
- Ifshared
isfalse
but this channel was not opened for writingIOException
- If some other I/O error occurs- See Also:
-
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 invocationfc.
lock
(0L, Long.MAX_VALUE, false)- Returns:
- A lock object representing the newly-acquired lock
- Throws:
ClosedChannelException
- If this channel is closedAsynchronousCloseException
- If another thread closes this channel while the invoking thread is blocked in this methodFileLockInterruptionException
- If the invoking thread is interrupted while blocked in this methodOverlappingFileLockException
- 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 fileNonWritableChannelException
- If this channel was not opened for writingIOException
- If some other I/O error occurs- See Also:
-
tryLock
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
andsize
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-argumenttryLock()
method simply locks a region of sizeLong.MAX_VALUE
. If theposition
is non-negative and thesize
is zero, then a lock of sizeLong.MAX_VALUE - position
is returned.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
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.
- Parameters:
position
- The position at which the locked region is to start; must be non-negativesize
- The size of the locked region; must be non-negative, and the sumposition
+size
must be non-negative. A value of zero means to lock all bytes from the specified starting position to the end of the file, regardless of whether the file is subsequently extended or truncatedshared
-true
to request a shared lock,false
to request an exclusive lock- Returns:
- A lock object representing the newly-acquired lock,
or
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 holdClosedChannelException
- If this channel is closedOverlappingFileLockException
- 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 fileNonReadableChannelException
- Ifshared
istrue
but this channel was not opened for readingNonWritableChannelException
- Ifshared
isfalse
but this channel was not opened for writingIOException
- If some other I/O error occurs- See Also:
-
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 invocationfc.
tryLock
(0L, Long.MAX_VALUE, false)- Returns:
- A lock object representing the newly-acquired lock,
or
null
if the lock could not be acquired because another program holds an overlapping lock - Throws:
ClosedChannelException
- If this channel is closedOverlappingFileLockException
- 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 regionNonWritableChannelException
- If this channel was not opened for writingIOException
- If some other I/O error occurs- See Also:
-