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java.nio.channels

public abstract Class FileChannel

extends AbstractInterruptibleChannel
implements SeekableByteChannel, GatheringByteChannel, ScatteringByteChannel
Class Inheritance
All Implemented Interfaces
java.nio.channels.ScatteringByteChannel, java.nio.channels.ReadableByteChannel, java.nio.channels.Channel, java.io.Closeable, java.lang.AutoCloseable, java.nio.channels.GatheringByteChannel, java.nio.channels.WritableByteChannel, java.nio.channels.SeekableByteChannel, java.nio.channels.ByteChannel
Known Direct Subclasses
sun.nio.ch.FileChannelImpl
Imports
java.io.IOException, java.lang.foreign.Arena, .MemorySegment, java.nio.ByteBuffer, .MappedByteBuffer, java.nio.channels.spi.AbstractInterruptibleChannel, java.nio.file.FileAlreadyExistsException, .OpenOption, .Path, .StandardOpenOption, java.nio.file.attribute.FileAttribute, java.nio.file.spi.FileSystemProvider, java.util.Set, .HashSet, .Collections, jdk.internal.javac.PreviewFeature
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:

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 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 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.

Authors
Mark Reinhold, Mike McCloskey, JSR-51 Expert Group
Since
1.4
See Also
java.io.FileInputStream#getChannel(), java.io.FileOutputStream#getChannel(), java.io.RandomAccessFile#getChannel()

Nested and Inner Type Summary

Modifier and TypeClass and Description
public static class
FileChannel.MapMode

A file-mapping mode.

Field Summary

Modifier and TypeField and Description
private static final FileAttribute<?>[]
NO_ATTRIBUTES

Constructor Summary

AccessConstructor and Description
protected
FileChannel()

Initializes a new instance of this class.

Method Summary

Modifier and TypeMethod and Description
public abstract void
force(boolean
If true 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
metaData)

Forces any updates to this channel's file to be written to the storage device that contains it.

public abstract FileLock

Returns:

A lock object representing the newly-acquired lock
lock(long
The position at which the locked region is to start; must be non-negative
position, long
The size of the locked region; must be non-negative, and the sum position + 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 truncated
size, boolean
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)
shared)

Acquires a lock on the given region of this channel's file.

public final FileLock

Returns:

A lock object representing the newly-acquired lock
lock()

Acquires an exclusive lock on this channel's file.

public abstract MappedByteBuffer

Returns:

The mapped byte buffer
map(FileChannel.MapMode
One of the constants READ_ONLY, READ_WRITE, or PRIVATE defined in the 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 mode
mode, long
The position within the file at which the mapped region is to start; must be non-negative
position, long
The size of the region to be mapped; must be non-negative and no greater than java.lang.Integer#MAX_VALUE
size)

Maps a region of this channel's file directly into memory.

public MemorySegment

Returns:

A new mapped memory segment
map(FileChannel.MapMode
The file mapping mode, see FileChannel#map(FileChannel.MapMode, long, long); the mapping mode might affect the behavior of the returned memory mapped segment (see MemorySegment#force())
mode, long
The offset (expressed in bytes) within the file at which the mapped segment is to start
offset, long
The size (in bytes) of the mapped memory backing the memory segment
size, Arena
The segment arena
arena)

Maps a region of this channel's file into a new mapped memory segment, with the given offset, size and arena.

public static FileChannel

Returns:

A new file channel
open(Path
The path of the file to open or create
path, Set<? extends OpenOption>
Options specifying how the file is opened
options, FileAttribute<?>...
An optional list of file attributes to set atomically when creating the file
attrs)

Opens or creates a file, returning a file channel to access the file.

public static FileChannel

Returns:

A new file channel
open(Path
The path of the file to open or create
path, OpenOption...
Options specifying how the file is opened
options)

Opens or creates a file, returning a file channel to access the file.

public abstract long

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
position()

Redeclares java.nio.channels.SeekableByteChannel.position.

Returns this channel's file position.

public abstract FileChannel

Returns:

This file channel
position(long
The new position, a non-negative integer counting the number of bytes from the beginning of the file
newPosition)

Redeclares java.nio.channels.SeekableByteChannel.position.

Sets this channel's file position.

public abstract int
read(ByteBuffer dst)

Redeclares java.nio.channels.SeekableByteChannel.read, java.nio.channels.ReadableByteChannel.read.

Reads a sequence of bytes from this channel into the given buffer.

public abstract long
read(ByteBuffer[] dsts, int offset, int length)

Redeclares java.nio.channels.ScatteringByteChannel.read.

Reads a sequence of bytes from this channel into a subsequence of the given buffers.

public final long
read(ByteBuffer[] dsts)

Implements java.nio.channels.ScatteringByteChannel.read.

Reads a sequence of bytes from this channel into the given buffers.

public abstract int

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
read(ByteBuffer
The buffer into which bytes are to be transferred
dst, long
The file position at which the transfer is to begin; must be non-negative
position)

Reads a sequence of bytes from this channel into the given buffer, starting at the given file position.

public abstract long

Returns:

The current size of this channel's file, measured in bytes
size()

Redeclares java.nio.channels.SeekableByteChannel.size.

Returns the current size of this channel's file.

public abstract long

Returns:

The number of bytes, possibly zero, that were actually transferred
transferFrom(ReadableByteChannel
The source channel
src, long
The file position at which the transfer is to begin; must be non-negative
position, long
The maximum number of bytes to be transferred; must be non-negative
count)

Transfers bytes into this channel's file from the given readable byte channel.

public abstract long

Returns:

The number of bytes, possibly zero, that were actually transferred
transferTo(long
The position within the file at which the transfer is to begin; must be non-negative
position, long
The maximum number of bytes to be transferred; must be non-negative
count, WritableByteChannel
The target channel
target)

Transfers bytes from this channel's file to the given writable byte channel.

public abstract FileChannel

Returns:

This file channel
truncate(long
The new size, a non-negative byte count
size)

Redeclares java.nio.channels.SeekableByteChannel.truncate.

Truncates this channel's file to the given size.

public abstract FileLock

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
tryLock(long
The position at which the locked region is to start; must be non-negative
position, long
The size of the locked region; must be non-negative, and the sum position + 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 truncated
size, boolean
true to request a shared lock, false to request an exclusive lock
shared)

Attempts to acquire a lock on the given region of this channel's file.

public final FileLock

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
tryLock()

Attempts to acquire an exclusive lock on this channel's file.

public abstract int
write(ByteBuffer src)

Redeclares java.nio.channels.SeekableByteChannel.write, java.nio.channels.WritableByteChannel.write.

Writes a sequence of bytes to this channel from the given buffer.

public abstract long
write(ByteBuffer[] srcs, int offset, int length)

Redeclares java.nio.channels.GatheringByteChannel.write.

Writes a sequence of bytes to this channel from a subsequence of the given buffers.

public final long
write(ByteBuffer[] srcs)

Implements java.nio.channels.GatheringByteChannel.write.

Writes a sequence of bytes to this channel from the given buffers.

public abstract int

Returns:

The number of bytes written, possibly zero
write(ByteBuffer
The buffer from which bytes are to be transferred
src, long
The file position at which the transfer is to begin; must be non-negative
position)

Writes a sequence of bytes to this channel from the given buffer, starting at the given file position.

Inherited from java.nio.channels.spi.AbstractInterruptibleChannel:
begincloseendimplCloseChannelisOpen

Field Detail

NO_ATTRIBUTESback to summary
private static final FileAttribute<?>[] NO_ATTRIBUTES
Annotations
@SuppressWarnings:rawtypes

Constructor Detail

FileChannelback to summary
protected FileChannel()

Initializes a new instance of this class.

Method Detail

forceback to summary
public abstract void force(boolean metaData) throws IOException

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, or the methods defined by java.io.FileOutputStream or java.io.RandomAccessFile when the channel was obtained with the getChannel method. It may or may not force changes that were made by modifying the content of a mapped byte buffer obtained by invoking the map method. Invoking the force method of the mapped byte buffer will force changes made to the buffer's content to be written.

Parameters
metaData:boolean

If true 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

Exceptions
IOException:
If some other I/O error occurs
ClosedChannelException:
If this channel is closed
lockback to summary
public abstract FileLock lock(long position, long size, boolean shared) throws IOException

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 interrupted status will be set and a FileLockInterruptionException will be thrown. If the invoker's interrupted status is set when this method is invoked then that exception will be thrown immediately; the thread's interrupted 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 lock() method simply locks a region of size Long#MAX_VALUE. If the position is non-negative and the size is zero, then a lock of size Long.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:long

The position at which the locked region is to start; must be non-negative

size:long

The size of the locked region; must be non-negative, and the sum position + 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 truncated

shared:boolean

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:FileLock

A lock object representing the newly-acquired lock

Exceptions
IOException:
If some other I/O error occurs
IllegalArgumentException:
If the preconditions on the parameters do not hold
ClosedChannelException:
If this channel is closed
AsynchronousCloseException:
If another thread closes this channel while the invoking thread is blocked in this method
FileLockInterruptionException:
If the invoking thread is interrupted while blocked in this method
OverlappingFileLockException:
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
NonReadableChannelException:
If shared is true but this channel was not opened for reading
NonWritableChannelException:
If shared is false but this channel was not opened for writing
See Also
lock(), tryLock(), tryLock(long, long, boolean)
lockback to summary
public final FileLock lock() throws IOException

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.lock(0L, Long.MAX_VALUE, false) 
fc.lock(0L, Long.MAX_VALUE, false)
Returns:FileLock

A lock object representing the newly-acquired lock

Exceptions
IOException:
If some other I/O error occurs
ClosedChannelException:
If this channel is closed
AsynchronousCloseException:
If another thread closes this channel while the invoking thread is blocked in this method
FileLockInterruptionException:
If the invoking thread is interrupted while blocked in this method
OverlappingFileLockException:
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 file
NonWritableChannelException:
If this channel was not opened for writing
See Also
lock(long, long, boolean), tryLock(), tryLock(long, long, boolean)
mapback to summary
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 java.nio.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 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 read and write methods. From the standpoint of performance it is generally only worth mapping relatively large files into memory.

Parameters
mode:FileChannel.MapMode

One of the constants READ_ONLY, READ_WRITE, or PRIVATE defined in the 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 mode

position:long

The position within the file at which the mapped region is to start; must be non-negative

size:long

The size of the region to be mapped; must be non-negative and no greater than java.lang.Integer#MAX_VALUE

Returns:MappedByteBuffer

The mapped byte buffer

Exceptions
IOException:
If some other I/O error occurs
NonReadableChannelException:
If the mode is READ_ONLY or an implementation specific map mode requiring read access, but this channel was not opened for reading
NonWritableChannelException:
If the mode is READ_WRITE, PRIVATE or an implementation specific map mode requiring write access, but this channel was not opened for both reading and writing
IllegalArgumentException:
If the preconditions on the parameters do not hold
UnsupportedOperationException:
If an unsupported map mode is specified
See Also
java.nio.channels.FileChannel.MapMode, java.nio.MappedByteBuffer
mapback to summary
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 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.

Implementation Specification

The default implementation of this method throws UnsupportedOperationException.

Parameters
mode:FileChannel.MapMode

The file mapping mode, see FileChannel#map(FileChannel.MapMode, long, long); the mapping mode might affect the behavior of the returned memory mapped segment (see MemorySegment#force())

offset:long

The offset (expressed in bytes) within the file at which the mapped segment is to start

size:long

The size (in bytes) of the mapped memory backing the memory segment

arena:Arena

The segment arena

Returns:MemorySegment

A new mapped memory segment

Exceptions
IOException:
If some other I/O error occurs
IllegalArgumentException:
If offset < 0, size < 0 or offset + size overflows the range of long
IllegalStateException:
If arena.isAlive() == false
WrongThreadException:
If arena is a confined scoped arena, and this method is called from a thread T, other than the scoped arena's owner thread
NonReadableChannelException:
If the mode is READ_ONLY or an implementation specific map mode requiring read access, but this channel was not opened for reading
NonWritableChannelException:
If the mode is READ_WRITE, PRIVATE or an implementation specific map mode requiring write access, but this channel was not opened for both reading and writing
UnsupportedOperationException:
If an unsupported map mode is specified
Since
22
openback to summary
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. 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:

additional options
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 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 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.

Parameters
path:Path

The path of the file to open or create

options:Set<? extends OpenOption>

Options specifying how the file is opened

attrs:FileAttribute<?>[]

An optional list of file attributes to set atomically when creating the file

Returns:FileChannel

A new file channel

Exceptions
IOException:
If an I/O error occurs
IllegalArgumentException:
If the set contains an invalid combination of options
UnsupportedOperationException:
If the path 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
FileAlreadyExistsException:
If a file of that name already exists and the CREATE_NEW option is specified and the file is being opened for writing (optional specific exception)
Since
1.7
openback to summary
public static FileChannel open(Path path, OpenOption... options) throws IOException

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]); 
fc.open(file, opts, new FileAttribute<?>[0]);
where opts is a set of the options specified in the options array.
Parameters
path:Path

The path of the file to open or create

options:OpenOption[]

Options specifying how the file is opened

Returns:FileChannel

A new file channel

Exceptions
IOException:
If an I/O error occurs
IllegalArgumentException:
If the set contains an invalid combination of options
UnsupportedOperationException:
If the path is associated with a provider that does not support creating file channels, or an unsupported open option is specified
FileAlreadyExistsException:
If a file of that name already exists and the CREATE_NEW option is specified and the file is being opened for writing (optional specific exception)
Since
1.7
positionback to summary
public abstract long position() throws IOException

Redeclares java.nio.channels.SeekableByteChannel.position.

Returns this channel's file position.

Returns:long

This channel's file position, a non-negative integer counting the number of bytes from the beginning of the file to the current position

Exceptions
IOException:
If some other I/O error occurs
ClosedChannelException:
If this channel is closed
positionback to summary
public abstract FileChannel position(long newPosition) throws IOException

Redeclares java.nio.channels.SeekableByteChannel.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.

Parameters
newPosition:long

The new position, a non-negative integer counting the number of bytes from the beginning of the file

Returns:FileChannel

This file channel

Exceptions
IOException:
If some other I/O error occurs
ClosedChannelException:
If this channel is closed
IllegalArgumentException:
If the new position is negative
readback to summary
public abstract int read(ByteBuffer dst) throws IOException

Redeclares java.nio.channels.SeekableByteChannel.read, java.nio.channels.ReadableByteChannel.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.

Exceptions
IOException:

Doc from java.nio.channels.ReadableByteChannel.read.

If some other I/O error occurs

ClosedChannelException:
If this channel is closed
AsynchronousCloseException:
If another thread closes this channel while the read operation is in progress
ClosedByInterruptException:
If another thread interrupts the current thread while the read operation is in progress, thereby closing the channel and setting the current thread's interrupted status
NonReadableChannelException:
If this channel was not opened for reading
readback to summary
public abstract long read(ByteBuffer[] dsts, int offset, int length) throws IOException

Redeclares java.nio.channels.ScatteringByteChannel.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.

Exceptions
IOException:

Doc from java.nio.channels.ScatteringByteChannel.read.

If some other I/O error occurs

ClosedChannelException:
If this channel is closed
AsynchronousCloseException:
If another thread closes this channel while the read operation is in progress
ClosedByInterruptException:
If another thread interrupts the current thread while the read operation is in progress, thereby closing the channel and setting the current thread's interrupted status
NonReadableChannelException:
If this channel was not opened for reading
readback to summary
public final long read(ByteBuffer[] dsts) throws IOException

Implements java.nio.channels.ScatteringByteChannel.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.

Exceptions
IOException:

Doc from java.nio.channels.ScatteringByteChannel.read.

If some other I/O error occurs

ClosedChannelException:
If this channel is closed
AsynchronousCloseException:
If another thread closes this channel while the read operation is in progress
ClosedByInterruptException:
If another thread interrupts the current thread while the read operation is in progress, thereby closing the channel and setting the current thread's interrupted status
NonReadableChannelException:
If this channel was not opened for reading
readback to summary
public abstract int read(ByteBuffer dst, long position) throws IOException

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:ByteBuffer

The buffer into which bytes are to be transferred

position:long

The file position at which the transfer is to begin; must be non-negative

Returns:int

The number of bytes read, possibly zero, or -1 if the given position is greater than or equal to the file's current size

Exceptions
IOException:
If some other I/O error occurs
IllegalArgumentException:
If the position is negative or the buffer is read-only
NonReadableChannelException:
If this channel was not opened for reading
ClosedChannelException:
If this channel is closed
AsynchronousCloseException:
If another thread closes this channel while the read operation is in progress
ClosedByInterruptException:
If another thread interrupts the current thread while the read operation is in progress, thereby closing the channel and setting the current thread's interrupted status
sizeback to summary
public abstract long size() throws IOException

Redeclares java.nio.channels.SeekableByteChannel.size.

Returns the current size of this channel's file.

Returns:long

The current size of this channel's file, measured in bytes

Exceptions
IOException:
If some other I/O error occurs
ClosedChannelException:
If this channel is closed
transferFromback to summary
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 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. 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:ReadableByteChannel

The source channel

position:long

The file position at which the transfer is to begin; must be non-negative

count:long

The maximum number of bytes to be transferred; must be non-negative

Returns:long

The number of bytes, possibly zero, that were actually transferred

Exceptions
IOException:
If some other I/O error occurs
IllegalArgumentException:
If the preconditions on the parameters do not hold
NonReadableChannelException:
If the source channel was not opened for reading
NonWritableChannelException:
If this channel was not opened for writing
ClosedChannelException:
If either this channel or the source channel is closed
AsynchronousCloseException:
If another thread closes either channel while the transfer is in progress
ClosedByInterruptException:
If another thread interrupts the current thread while the transfer is in progress, thereby closing both channels and setting the current thread's interrupted status
transferToback to summary
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 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 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:long

The position within the file at which the transfer is to begin; must be non-negative

count:long

The maximum number of bytes to be transferred; must be non-negative

target:WritableByteChannel

The target channel

Returns:long

The number of bytes, possibly zero, that were actually transferred

Exceptions
IOException:
If some other I/O error occurs
IllegalArgumentException:
If the preconditions on the parameters do not hold
NonReadableChannelException:
If this channel was not opened for reading
NonWritableChannelException:
If the target channel was not opened for writing
ClosedChannelException:
If either this channel or the target channel is closed
AsynchronousCloseException:
If another thread closes either channel while the transfer is in progress
ClosedByInterruptException:
If another thread interrupts the current thread while the transfer is in progress, thereby closing both channels and setting the current thread's interrupted status
truncateback to summary
public abstract FileChannel truncate(long size) throws IOException

Redeclares java.nio.channels.SeekableByteChannel.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.

Parameters
size:long

The new size, a non-negative byte count

Returns:FileChannel

This file channel

Exceptions
IOException:
If some other I/O error occurs
NonWritableChannelException:
If this channel was not opened for writing
ClosedChannelException:
If this channel is closed
IllegalArgumentException:
If the new size is negative
tryLockback to summary
public abstract FileLock tryLock(long position, long size, boolean shared) throws IOException

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 tryLock() method simply locks a region of size Long#MAX_VALUE. If the position is non-negative and the size is zero, then a lock of size Long.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:long

The position at which the locked region is to start; must be non-negative

size:long

The size of the locked region; must be non-negative, and the sum position + 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 truncated

shared:boolean

true to request a shared lock, false to request an exclusive lock

Returns:FileLock

A lock object representing the newly-acquired lock, or null if the lock could not be acquired because another program holds an overlapping lock

Exceptions
IOException:
If some other I/O error occurs
IllegalArgumentException:
If the preconditions on the parameters do not hold
ClosedChannelException:
If this channel is closed
OverlappingFileLockException:
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 file
NonReadableChannelException:
If shared is true but this channel was not opened for reading
NonWritableChannelException:
If shared is false but this channel was not opened for writing
See Also
lock(), lock(long, long, boolean), tryLock()
tryLockback to summary
public final FileLock tryLock() throws IOException

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.tryLock(0L, Long.MAX_VALUE, false) 
fc.tryLock(0L, Long.MAX_VALUE, false)
Returns:FileLock

A lock object representing the newly-acquired lock, or null if the lock could not be acquired because another program holds an overlapping lock

Exceptions
IOException:
If some other I/O error occurs
ClosedChannelException:
If this channel is closed
OverlappingFileLockException:
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
NonWritableChannelException:
If this channel was not opened for writing
See Also
lock(), lock(long, long, boolean), tryLock(long, long, boolean)
writeback to summary
public abstract int write(ByteBuffer src) throws IOException

Redeclares java.nio.channels.SeekableByteChannel.write, java.nio.channels.WritableByteChannel.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.

Exceptions
IOException:

Doc from java.nio.channels.WritableByteChannel.write.

If some other I/O error occurs

ClosedChannelException:
If this channel is closed
AsynchronousCloseException:
If another thread closes this channel while the write operation is in progress
ClosedByInterruptException:
If another thread interrupts the current thread while the write operation is in progress, thereby closing the channel and setting the current thread's interrupted status
NonWritableChannelException:
If this channel was not opened for writing
writeback to summary
public abstract long write(ByteBuffer[] srcs, int offset, int length) throws IOException

Redeclares java.nio.channels.GatheringByteChannel.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.

Exceptions
IOException:

Doc from java.nio.channels.GatheringByteChannel.write.

If some other I/O error occurs

ClosedChannelException:
If this channel is closed
AsynchronousCloseException:
If another thread closes this channel while the write operation is in progress
ClosedByInterruptException:
If another thread interrupts the current thread while the write operation is in progress, thereby closing the channel and setting the current thread's interrupted status
NonWritableChannelException:
If this channel was not opened for writing
writeback to summary
public final long write(ByteBuffer[] srcs) throws IOException

Implements java.nio.channels.GatheringByteChannel.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.

Exceptions
IOException:

Doc from java.nio.channels.GatheringByteChannel.write.

If some other I/O error occurs

ClosedChannelException:
If this channel is closed
AsynchronousCloseException:
If another thread closes this channel while the write operation is in progress
ClosedByInterruptException:
If another thread interrupts the current thread while the write operation is in progress, thereby closing the channel and setting the current thread's interrupted status
NonWritableChannelException:
If this channel was not opened for writing
writeback to summary
public abstract int write(ByteBuffer src, long position) throws IOException

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:ByteBuffer

The buffer from which bytes are to be transferred

position:long

The file position at which the transfer is to begin; must be non-negative

Returns:int

The number of bytes written, possibly zero

Exceptions
IOException:
If some other I/O error occurs
IllegalArgumentException:
If the position is negative
NonWritableChannelException:
If this channel was not opened for writing
ClosedChannelException:
If this channel is closed
AsynchronousCloseException:
If another thread closes this channel while the write operation is in progress
ClosedByInterruptException:
If another thread interrupts the current thread while the write operation is in progress, thereby closing the channel and setting the current thread's interrupted status
java.nio.channels back to summary

public Class FileChannel.MapMode

extends Object
Class Inheritance
A file-mapping mode.
Since
1.4
See Also
java.nio.channels.FileChannel#map

Field Summary

Modifier and TypeField and Description
private final String
name

public static final FileChannel.MapMode
PRIVATE

Mode for a private (copy-on-write) mapping.

public static final FileChannel.MapMode
READ_ONLY

Mode for a read-only mapping.

public static final FileChannel.MapMode
READ_WRITE

Mode for a read/write mapping.

Constructor Summary

AccessConstructor and Description
private
MapMode(String
the name of the map mode
name)

Constructs an instance of this class.

Method Summary

Modifier and TypeMethod and Description
public String

Returns:

A descriptive string
toString()

Overrides java.lang.Object.toString.

Returns a string describing this file-mapping mode.

Inherited from java.lang.Object:
cloneequalsfinalizegetClasshashCodenotifynotifyAllwaitwaitwait

Field Detail

nameback to summary
private final String name
PRIVATEback to summary
public static final FileChannel.MapMode PRIVATE

Mode for a private (copy-on-write) mapping.

READ_ONLYback to summary
public static final FileChannel.MapMode READ_ONLY

Mode for a read-only mapping.

READ_WRITEback to summary
public static final FileChannel.MapMode READ_WRITE

Mode for a read/write mapping.

Constructor Detail

MapModeback to summary
private MapMode(String name)

Constructs an instance of this class. This constructor may be used by code in java.base to create file mapping modes beyond the file mapping modes defined here.

Parameters
name:String

the name of the map mode

Method Detail

toStringback to summary
public String toString()

Overrides java.lang.Object.toString.

Returns a string describing this file-mapping mode.

Returns:String

A descriptive string