java.nio.channels
Class FileChannel

java.lang.Object
  java.nio.channels.spi.AbstractInterruptibleChannel
      java.nio.channels.FileChannel

All Implemented Interfaces:

Closeable, ByteChannel, Channel, GatheringByteChannel, InterruptibleChannel, ReadableByteChannel, ScatteringByteChannel, WritableByteChannel

public abstract class FileChannel
extends AbstractInterruptibleChannel
implements GatheringByteChannel, ScatteringByteChannel, ByteChannel

An abstract channel type for interaction with a platform file.

A FileChannel defines the methods for reading, writing, memory mapping, and manipulating the logical state of a platform file. This type does not have a method for opening files, since this behaviour has been delegated to the FileInputStream, FileOutputStream, and RandomAccessFile types.

FileChannels created from a FileInputStream, or a RandomAccessFile created in mode "r", are read-only. FileChannels created from a FileOutputStream are write-only. FileChannels created from a RandomAccessFile created in mode "rw" are read/write. FileChannels created from a RandomAccessFile that was opened in append-mode will also be in append-mode -- meaning that each write will be proceeded by a seek to the end of file. Some platforms will seek and write atomically, others will not.

FileChannels has a virtual pointer into the file which is referred to as a file position. The position can be manipulated by repositioning it within the file, and its current position can be queried.

FileChannels also have an associated size. The size of the file is the number of bytes that it currently contains. The size can be manipulated by adding more bytes to the end of the file (which increases the size) or truncating the file (which decreases the size). The current size can also be queried.

FileChannels have operations beyond the simple read, write, and close. They can also:

• request that cached data be forced onto the disk
• lock ranges of bytes associated with the file
• transfer data directly to another channel in a manner that has the potential to be optimized by the platform
• memory-mapping files into NIO buffers to provide efficient manipulation of file data
• read and write to the file at absolute byte offsets in a fashion that does not modify the current position

FileChannels are thread-safe. Only one operation involving manipulation of the file position may be in-flight at once. Subsequent calls to such operations will block, and one of those blocked will be freed to continue when the first operation has completed. There is no ordered queue or fairness applied to the blocked threads.

It is undefined whether operations that do not manipulate the file position will also block when there are any other operations in-flight.

The logical view of the underlying file is consistent across all FileChannels and IO streams opened on the same file by the same JVM process. Therefore modifications performed via a channel will be visible to the stream, and vice versa; including modifications to the file position, content, size, etc.

Nested Class Summary

static class

FileChannel.MapMode A type of file mapping modes.

Constructor Summary

protected

FileChannel() Protected default constructor.

Method Summary

abstract void	force(boolean metadata) Request that all updates to the channel are committed to the storage device.
FileLock	lock() Obtain an exclusive lock on this file.
abstract FileLock	lock(long position, long size, boolean shared) Obtain a lock on a specified region of the file.
abstract MappedByteBuffer	map(FileChannel.MapMode mode, long position, long size) Maps the file into memory.There can be three modes:Read-only,Read/write and Private.
abstract long	position() Returns the current value of the file position pointer.
abstract FileChannel	position(long offset) Sets the file position pointer to a new value.
abstract int	read(ByteBuffer buffer) Reads bytes from the channel into the given byte buffer.
long	read(ByteBuffer[] buffers) Reads bytes from the channel into all the given byte buffers.
abstract long	read(ByteBuffer[] buffers, int start, int number) Reads bytes from the file channel into a subset of the given byte buffers.
abstract int	read(ByteBuffer buffer, long position) Reads bytes from the file channel into the given buffer starting from the given file position.
abstract long	size() Returns the size of the file underlying this channel, in bytes.
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 data from the file to the given channel.
abstract FileChannel	truncate(long size) Truncates the file underlying this channel to a given size.
FileLock	tryLock() Attempts to acquire an exclusive lock on this file without blocking.
abstract FileLock	tryLock(long position, long size, boolean shared) Attempts to acquire an exclusive lock on this file without blocking.
abstract int	write(ByteBuffer src) Writes bytes from the given byte buffer into the file channel.
long	write(ByteBuffer[] buffers) Writes bytes from all the given byte buffers into the file channel.
abstract long	write(ByteBuffer[] buffers, int offset, int length) Writes a subset of the given bytes from the buffers to the channel.
abstract int	write(ByteBuffer buffer, long position) Writes bytes from the given buffer to the file channel starting at the given file position.

Methods inherited from class java.nio.channels.spi.AbstractInterruptibleChannel

begin, close, end, implCloseChannel, isOpen

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Methods inherited from interface java.nio.channels.Channel

close, isOpen

Constructor Detail

FileChannel

protected FileChannel()

Protected default constructor.

Method Detail

force

public abstract void force(boolean metadata)
                    throws IOException

Request that all updates to the channel are committed to the storage device.

When this method returns all modifications made to the platform file underlying this channel will be committed to a local storage device. If the file is not hosted locally, such as a networked file system, then applications cannot be certain that the modifications have been committed.

There are no assurances given that changes made to the file using methods defined elsewhere will be committed. For example, changes made via a mapped byte buffer may not be committed.

The metadata parameter indicated whether the update should include the file's metadata such as last modification time, last access time, etc. Note that passing true may invoke an underlying write to the operating system (if the platform is maintaining metadata such as last access time), even if the channel is opened read-only.

Parameters:

metadata - true if the file metadata should be flushed in addition to the file content, and false otherwise.

Throws:

ClosedChannelException - if the channel is already closed.

IOException - some other problem occurred.

lock

public final FileLock lock()
                    throws IOException

Obtain an exclusive lock on this file.

This is a convenience method for acquiring a maximum length lock on a file. It is equivalent to:

 fileChannel.lock(0L, Long.MAX_VALUE, false)
 

Returns:

the lock object representing the locked file area.

Throws:

ClosedChannelException - the file channel is closed.

NonWritableChannelException - this channel was not opened for writing.

OverlappingFileLockException - Either a lock is already held that overlaps this lock request, or another thread is waiting to acquire a lock that will overlap with this request.

FileLockInterruptionException - The calling thread was interrupted while waiting to acquire the lock.

AsynchronousCloseException - The channel was closed while the calling thread was waiting to acquire the lock.

IOException - some other problem occurred obtaining the requested lock.

lock

public abstract FileLock lock(long position,
                              long size,
                              boolean shared)
                       throws IOException

Obtain a lock on a specified region of the file.

This is the blocking version of lock acquisition, see also the tryLock() methods.

Attempts to acquire an overlapping lock region will fail. The attempt will fail if the overlapping lock has already been obtained, or if another thread is currently waiting to acquire the overlapping lock.

If the request is not for an overlapping lock, the thread calling this method will block until the lock is obtained (likely by no contention or another process releasing a lock), or this thread being interrupted or the channel closed.

If the lock is obtained successfully then the FileLock object returned represents the lock for subsequent operations on the locked region.

If the thread is interrupted while waiting for the lock, the thread is set to the interrupted state, and throws a FileLockInterruptionException. If the channel is closed while the thread is waiting to obtain the lock then the thread throws a AsynchronousCloseException.

There is no requirement for the position and size to be within the current start and length of the file.

Some platforms do not support shared locks, and if a request is made for a shared lock on such a platform this method will attempt to acquire an exclusive lock instead. It is undefined whether the lock obtained is advisory or mandatory.

Parameters:

position - the starting position for the lock region

size - the length of the lock, in bytes

shared - a flag indicating whether an attempt should be made to acquire a shared lock.

Returns:

the file lock object

Throws:

IllegalArgumentException - if the parameters are invalid.

ClosedChannelException - if the channel is already closed.

OverlappingFileLockException - if the requested region overlaps an existing lock or pending lock request.

NonReadableChannelException - if the channel is not open in read-mode and shared is true.

NonWritableChannelException - if the channel is not open in write mode and shared is false.

AsynchronousCloseException - if the channel is closed by another thread while this method is in operation.

FileLockInterruptionException - if the thread is interrupted while in the state of waiting on the desired file lock.

IOException - if some other IO problem occurs.

map

public abstract MappedByteBuffer map(FileChannel.MapMode mode,
                                     long position,
                                     long size)
                              throws IOException

Maps the file into memory.There can be three modes:Read-only,Read/write and Private. After mapping, the memory and the file channel do not affect each other. Note : mapping a file into memory is usually expensive.

Parameters:

mode - one of three modes to map

position - the starting position of the file

size - the size to map

Returns:

the mapped byte buffer

Throws:

NonReadableChannelException - If the file is not opened for reading but the given mode is "READ_ONLY"

NonWritableChannelException - If the file is not opened for writing but the mode is not "READ_ONLY"

IllegalArgumentException - If the given parameters of position and size are not correct

IOException - If any I/O error occurs

position

public abstract long position()
                       throws IOException

Returns the current value of the file position pointer.

Returns:

the current position as a positive integer number of bytes from the start of the file.

Throws:

ClosedChannelException - if the channel is already closed.

IOException - if some other IO problem occurs.

position

public abstract FileChannel position(long offset)
                              throws IOException

Sets the file position pointer to a new value.

The argument is the number of bytes counted from the start of the file. The position cannot be set to a value that is negative. The new position can be set beyond the current file size. If set beyond the current file size, attempts to read will return end of file, and writes will succeed, but fill-in the bytes between the current end of file and the position with the required number of (unspecified) byte values.

Parameters:

offset - the new file position, in bytes.

Returns:

the receiver.

Throws:

IllegalArgumentException - if the new position is negative.

ClosedChannelException - if the channel is already closed.

IOException - if some other IO problem occurs.

read

public abstract int read(ByteBuffer buffer)
                  throws IOException

Reads bytes from the channel into the given byte buffer.

The bytes are read starting at the current file position, and after some number of bytes are read (up to the remaining number of bytes in the buffer) the file position is increased by the number of bytes actually read.

Specified by:

read in interface ReadableByteChannel

Parameters:

buffer - the byte buffer to receive the bytes.

Returns:

the number of bytes actually read.

Throws:

ClosedChannelException - if the channel was already closed.

AsynchronousCloseException - if another thread closes the channel during the read.

ClosedByInterruptException - if another thread interrupt the calling thread during the read.

IOException - another IO exception occurs, details are in the message.

See Also:

ReadableByteChannel.read(java.nio.ByteBuffer)

read

public abstract int read(ByteBuffer buffer,
                         long position)
                  throws IOException

Reads bytes from the file channel into the given buffer starting from the given file position.

The bytes are read starting at the given file position (up to the remaining number of bytes in the buffer). The number of bytes actually read is returned.

If the position is beyond the current end of file, then no bytes are read.

Note that file position is unmodified by this method.

Parameters:

buffer - the buffer to receive the bytes

position - the (non-negative) position at which to read the bytes.

Returns:

the number of bytes actually read.

Throws:

IllegalArgumentException - if position is less than -1.

ClosedChannelException - if the channel is already closed.

NonReadableChannelException - if the channel was not opened in read-mode.

AsynchronousCloseException - if the channel is closed by another thread while this method is in operation.

ClosedByInterruptException - if another thread interrupts the calling thread while the operation is in progress. The calling thread will have the interrupt state set, and the channel will be closed.

IOException - some other IO error occurred.

read

public final long read(ByteBuffer[] buffers)
                throws IOException

Reads bytes from the channel into all the given byte buffers.

The bytes are read starting at the current file position, and after some number of bytes are read (up to the remaining number of bytes in all the buffers) the file position is increased by the number of bytes actually read.

This method behaves exactly like:

 read(buffers, 0, buffers.length);
 

Specified by:

read in interface ScatteringByteChannel

Parameters:

buffers - the array of byte buffers to receive the bytes being read.

Returns:

the number of bytes actually read.

Throws:

ClosedChannelException - if the channel is closed.

ClosedByInterruptException - if the thread is interrupted in its IO operation by another thread closing the channel.

AsynchronousCloseException - if the read is interrupted by another thread sending an explicit interrupt.

IOException - if some other type of exception occurs. Details are in the message.

See Also:

ScatteringByteChannel.read(java.nio.ByteBuffer[])

read

public abstract long read(ByteBuffer[] buffers,
                          int start,
                          int number)
                   throws IOException

Reads bytes from the file channel into a subset of the given byte buffers.

Specified by:

read in interface ScatteringByteChannel

Parameters:

buffers - the array of byte buffers into which the bytes will be read.

start - the index of the first buffer to read.

number - the maximum number of buffers to read.

Returns:

the number of bytes actually read.

Throws:

ClosedChannelException - the channel is currently closed.

AsynchronousCloseException - the channel was closed by another thread while the write was underway.

ClosedByInterruptException - the thread was interrupted by another thread while the write was underway.

IOException - if some other type of exception occurs. Details are in the message.

See Also:

ScatteringByteChannel.read(java.nio.ByteBuffer[], int, int)

size

public abstract long size()
                   throws IOException

Returns the size of the file underlying this channel, in bytes.

Returns:

the size of the file in bytes.

Throws:

ClosedChannelException - if the channel is closed.

IOException - if a problem occurs getting the size of the file.

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. It may be very efficient. By invoking this method, it will read form the source channel and write into the file channel. Note: no guarantee whether all bytes may be transferred. And it does not modify the position of the channel.

Parameters:

src - the source channel to read

position - the non-negative position to begin

count - the non-negative bytes to be transferred

Returns:

the number of bytes that are transferred.

Throws:

IllegalArgumentException - If the parameters are not correct

NonReadableChannelException - If the source channel is not readable

NonWritableChannelException - If this channel is not writable

ClosedChannelException - If either channel has already been closed

AsynchronousCloseException - If either channel is closed by other threads during this operation

ClosedByInterruptException - If the thread is interrupted during this operation

IOException - If any I/O error occurs

transferTo

public abstract long transferTo(long position,
                                long count,
                                WritableByteChannel target)
                         throws IOException

Transfers data from the file to the given channel. It may be very efficient. By invoking this method, it will read form the file and write into the writable channel. Note: no guarantee whether all bytes may be transfered.And it does not modify the position of the channel.

Parameters:

position - the non-negative position to begin

count - the non-negative bytes to be transferred

target - the target channel to write into

Returns:

the number of bytes that were transferred.

Throws:

IllegalArgumentException - If the parameters are not correct

NonReadableChannelException - If this channel is not readable

NonWritableChannelException - If the target channel is not writable

ClosedChannelException - If either channel has already been closed

AsynchronousCloseException - If either channel is closed by other threads during this operation

ClosedByInterruptException - If the thread is interrupted during this operation

IOException - If any I/O error occurs

truncate

public abstract FileChannel truncate(long size)
                              throws IOException

Truncates the file underlying this channel to a given size.

Any bytes beyond the given size are removed from the file. If there are no bytes beyond the given size then the file contents are unmodified.

If the file position is currently greater than the given size, then it is set to be the given size.

Parameters:

size - the maximum size of the underlying file

Returns:

this channel

Throws:

IllegalArgumentException - the requested size is negative.

ClosedChannelException - the channel is closed.

NonWritableChannelException - the channel cannot be written.

IOException - some other IO problem occurred.

tryLock

public final FileLock tryLock()
                       throws IOException

Attempts to acquire an exclusive lock on this file without blocking.

This is a convenience method for attempting to acquire a maximum length lock on the file. It is equivalent to:

 fileChannel.tryLock(0L, Long.MAX_VALUE, false)
 

The method returns null if the acquisition would result in an overlapped lock with another OS process.

Returns:

the file lock object, or null if the lock would overlap an existing exclusive lock in another OS process.

Throws:

ClosedChannelException - the file channel is closed.

OverlappingFileLockException - Either a lock is already held that overlaps this lock request, or another thread is waiting to acquire a lock that will overlap with this request.

IOException - if any I/O error occurs

tryLock

public abstract FileLock tryLock(long position,
                                 long size,
                                 boolean shared)
                          throws IOException

Attempts to acquire an exclusive lock on this file without blocking.

The method returns null if the acquisition would result in an overlapped lock with another OS process.

Parameters:

position - the starting position

size - the size of file to lock

shared - true if share

Returns:

the file lock object, or null if the lock would overlap an existing exclusive lock in another OS process.

Throws:

IllegalArgumentException - If any parameters are bad

ClosedChannelException - the file channel is closed.

OverlappingFileLockException - Either a lock is already held that overlaps this lock request, or another thread is waiting to acquire a lock that will overlap with this request.

IOException - if any I/O error occurs

write

public abstract int write(ByteBuffer src)
                   throws IOException

Writes bytes from the given byte buffer into the file channel.

The bytes are written starting at the current file position, and after some number of bytes are written (up to the remaining number of bytes in the buffer) the file position is increased by the number of bytes actually written.

Specified by:

write in interface WritableByteChannel

Parameters:

src - the source buffer to write

Returns:

the number of bytes actually written.

Throws:

ClosedChannelException - if the channel was already closed.

AsynchronousCloseException - if another thread closes the channel during the write.

ClosedByInterruptException - if another thread interrupt the calling thread during the write.

IOException - another IO exception occurs, details are in the message.

See Also:

WritableByteChannel.write(java.nio.ByteBuffer)

write

public abstract int write(ByteBuffer buffer,
                          long position)
                   throws IOException

Writes bytes from the given buffer to the file channel starting at the given file position.

The bytes are written starting at the given file position (up to the remaining number of bytes in the buffer). The number of bytes actually written is returned.

If the position is beyond the current end of file, then the file is first extended up to the given position by the required number of unspecified byte values.

Note that file position is unmodified by this method.

Parameters:

buffer - the buffer containing the bytes to be written.

position - the (non-negative) position at which to write the bytes.

Returns:

the number of bytes actually written.

Throws:

IllegalArgumentException - if position is less than -1.

ClosedChannelException - if the channel is already closed.

NonWritableChannelException - if the channel was not opened in write-mode.

AsynchronousCloseException - if the channel is closed by another thread while this method is in operation.

ClosedByInterruptException - if another thread interrupts the calling thread while the operation is in progress. The calling thread will have the interrupt state set, and the channel will be closed.

IOException - some other IO error occurred.

write

public final long write(ByteBuffer[] buffers)
                 throws IOException

Writes bytes from all the given byte buffers into the file channel.

The bytes are written starting at the current file position, and after some number of bytes are written (up to the remaining number of bytes in all the buffers) the file position is increased by the number of bytes actually written.

This method behaves exactly like:

 write(buffers, 0, buffers.length);
 

Specified by:

write in interface GatheringByteChannel

Parameters:

buffers - the buffers containing bytes to be written.

Returns:

the number of bytes actually written.

Throws:

ClosedChannelException - if the channel is closed.

ClosedByInterruptException - if the thread is interrupted in its IO operation by another thread closing the channel.

AsynchronousCloseException - if the write is interrupted by another thread sending an explicit interrupt.

IOException - if some other type of exception occurs. Details are in the message.

See Also:

GatheringByteChannel.write(java.nio.ByteBuffer[])

write

public abstract long write(ByteBuffer[] buffers,
                           int offset,
                           int length)
                    throws IOException

Description copied from interface: GatheringByteChannel

Writes a subset of the given bytes from the buffers to the channel.

This method attempts to write all of the remaining() bytes from length byte buffers, in order, starting at buffers[offset]. The number of bytes actually written is returned.

If a write operation is in progress, subsequent threads will block until the write is completed, and will then contend for the ability to write.

Specified by:

write in interface GatheringByteChannel

Parameters:

buffers - the array of byte buffers containing the source of remaining bytes that will be attempted to be written.

offset - the index of the first buffer to write.

length - the number of buffers to write.

Returns:

the number of bytes actually written.

Throws:

ClosedChannelException - the channel is currently closed.

AsynchronousCloseException - the channel was closed by another thread while the write was underway.

ClosedByInterruptException - the thread was interrupted by another thread while the write was underway.

IOException - if some other type of exception occurs. Details are in the message.

See Also:

GatheringByteChannel.write(java.nio.ByteBuffer[], int, int)