File channels are safe for use by multiple concurrent threads. The {@link Channel#close close} method may be invoked at any time, as specified by the {@link Channel} interface. Only one operation that involves the channel's position or can change its file's size may be in progress at any given time; attempts to initiate a second such operation while the first is still in progress will block until the first operation completes. Other operations, in particular those that take an explicit position, may proceed concurrently; whether they in fact do so is dependent upon the underlying implementation and is therefore unspecified.

The view of a file provided by an instance of this class is guaranteed to be consistent with other views of the same file provided by other instances in the same program. The view provided by an instance of this class may or may not, however, be consistent with the views seen by other concurrently-running programs due to caching performed by the underlying operating system and delays induced by network-filesystem protocols. This is true regardless of the language in which these other programs are written, and whether they are running on the same machine or on some other machine. The exact nature of any such inconsistencies are system-dependent and are therefore unspecified.

This class does not define methods for opening existing files or for creating new ones; such methods may be added in a future release. In this release a file channel can be obtained from an existing {@link java.io.FileInputStream#getChannel FileInputStream}, {@link java.io.FileOutputStream#getChannel FileOutputStream}, or {@link java.io.RandomAccessFile#getChannel RandomAccessFile} object by invoking that object's getChannel method, which returns a file channel that is connected to the same underlying file.

The state of a file channel is intimately connected to that of the object whose getChannel method returned the channel. Changing the channel's position, whether explicitly or by reading or writing bytes, will change the file position of the originating object, and vice versa. Changing the file's length via the file channel will change the length seen via the originating object, and vice versa. Changing the file's content by writing bytes will change the content seen by the originating object, and vice versa.

At various points this class specifies that an instance that is "open for reading," "open for writing," or "open for reading and writing" is required. A channel obtained via the {@link java.io.FileInputStream#getChannel getChannel} method of a {@link java.io.FileInputStream} instance will be open for reading. A channel obtained via the {@link java.io.FileOutputStream#getChannel getChannel} method of a {@link java.io.FileOutputStream} instance will be open for writing. Finally, a channel obtained via the {@link java.io.RandomAccessFile#getChannel getChannel} method of a {@link java.io.RandomAccessFile} instance will be open for reading if the instance was created with mode "r" and will be open for reading and writing if the instance was created with mode "rw".

A file channel that is open for writing may be in append mode, for example if it was obtained from a file-output stream that was created by invoking the {@link java.io.FileOutputStream#FileOutputStream(java.io.File,boolean) FileOutputStream(File,boolean)} constructor and passing true for the second parameter. In this mode each invocation of a relative write operation first advances the position to the end of the file and then writes the requested data. Whether the advancement of the position and the writing of the data are done in a single atomic operation is system-dependent and therefore unspecified.

FileChannel

protected FileChannel()

Initializes a new instance of this class.

force

public abstract void force(boolean metaData)
                    throws IOException

lock

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.{@link #lock(long,long,boolean) lock}(0L, Long.MAX_VALUE, false) 

Returns:

A lock object representing the newly-acquired lock

Throws:

IOException - If some other I/O error occurs

lock

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 {@link AsynchronousCloseException} will be thrown.

If the invoking thread is interrupted while waiting to acquire the lock then its interrupt status will be set and a {@link FileLockInterruptionException} will be thrown. If the invoker's interrupt status is set when this method is invoked then that exception will be thrown immediately; the thread's interrupt status will not be changed.

The region specified by the position and size parameters need not be contained within, or even overlap, the actual underlying file. Lock regions are fixed in size; if a locked region initially contains the end of the file and the file grows beyond the region then the new portion of the file will not be covered by the lock. If a file is expected to grow in size and a lock on the entire file is required then a region starting at zero, and no smaller than the expected maximum size of the file, should be locked. The zero-argument {@link #lock()} method simply locks a region of size {@link Long#MAX_VALUE}.

Some operating systems do not support shared locks, in which case a request for a shared lock is automatically converted into a request for an exclusive lock. Whether the newly-acquired lock is shared or exclusive may be tested by invoking the resulting lock object's {@link FileLock#isShared() isShared} method.

File locks are held on behalf of the entire Java virtual machine. They are not suitable for controlling access to a file by multiple threads within the same virtual machine.

Parameters:

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

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

shared - 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:

IOException - If some other I/O error occurs

map

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

position

public abstract long position()
                       throws IOException

Returns this channel's file position.

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:

IOException - If some other I/O error occurs

position

public abstract FileChannel position(long newPosition)
                              throws IOException

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

Returns:

This file channel

Throws:

IOException - If some other I/O error occurs

read

public abstract int read(ByteBuffer dst)
                  throws IOException

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

Bytes are read starting at this channel's current file position, and then the file position is updated with the number of bytes actually read. Otherwise this method behaves exactly as specified in the {@link ReadableByteChannel} interface.

Parameters:

dst

Throws:

IOException

read

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 {@link #read(ByteBuffer)} method, except that bytes are read starting at the given file position rather than at the channel's current position. This method does not modify this channel's position. If the given position is greater than the file's current size then no bytes are read.

Parameters:

dst - The buffer into which bytes are to be transferred

position - 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:

IOException - If some other I/O error occurs

read

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

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

Bytes are read starting at this channel's current file position, and then the file position is updated with the number of bytes actually read. Otherwise this method behaves exactly as specified in the {@link ScatteringByteChannel} interface.

Parameters:

dsts

Throws:

IOException

read

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

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

Bytes are read starting at this channel's current file position, and then the file position is updated with the number of bytes actually read. Otherwise this method behaves exactly as specified in the {@link ScatteringByteChannel} interface.

Parameters:

dsts

offset

length

Throws:

IOException

size

public abstract long size()
                   throws IOException

Returns the current size of this channel's file.

Returns:

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

Throws:

IOException - 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 given position. An invocation of this method may or may not transfer all of the requested bytes; whether or not it does so depends upon the natures and states of the channels. Fewer than the requested number of bytes will be transferred if the source channel has fewer than count bytes remaining, or if the source channel is non-blocking and has fewer than count bytes immediately available in its input buffer.

This method does not modify this channel's position. If the given position is greater than the file's current size then no bytes are transferred. If the source channel has a position then bytes are read starting at that position and then the position is incremented by the number of bytes read.

This method is potentially much more efficient than a simple loop that reads from the source channel and writes to this channel. Many operating systems can transfer bytes directly from the source channel into the filesystem cache without actually copying them.

Parameters:

src - The source channel

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

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

Returns:

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

Throws:

IOException - 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 given position in this channel's file and write them to the target channel. An invocation of this method may or may not transfer all of the requested bytes; whether or not it does so depends upon the natures and states of the channels. Fewer than the requested number of bytes are transferred if this channel's file contains fewer than count bytes starting at the given position, or if the target channel is non-blocking and it has fewer than count bytes free in its output buffer.

This method does not modify this channel's position. If the given position is greater than the file's current size then no bytes are transferred. If the target channel has a position then bytes are written starting at that position and then the position is incremented by the number of bytes written.

This method is potentially much more efficient than a simple loop that reads from this channel and writes to the target channel. Many operating systems can transfer bytes directly from the filesystem cache to the target channel without actually copying them.

Parameters:

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

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

target - The target channel

Returns:

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

Throws:

IOException - If some other I/O error occurs

truncate

public abstract FileChannel truncate(long size)
                              throws IOException

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 - The new size, a non-negative byte count

Returns:

This file channel

Throws:

IOException - If some other I/O error occurs

tryLock

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.{@link #tryLock(long,long,boolean) 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:

IOException - If some other I/O error occurs

tryLock

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 {@link #tryLock()} method simply locks a region of size {@link Long#MAX_VALUE}.

Some operating systems do not support shared locks, in which case a request for a shared lock is automatically converted into a request for an exclusive lock. Whether the newly-acquired lock is shared or exclusive may be tested by invoking the resulting lock object's {@link FileLock#isShared() isShared} method.

File locks are held on behalf of the entire Java virtual machine. They are not suitable for controlling access to a file by multiple threads within the same virtual machine.

Parameters:

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

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

shared - 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:

IOException - If some other I/O error occurs

write

public abstract int write(ByteBuffer src)
                   throws IOException

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

Bytes are written starting at this channel's current file position unless the channel is in append mode, in which case the position is first advanced to the end of the file. The file is grown, if necessary, to accommodate the written bytes, and then the file position is updated with the number of bytes actually written. Otherwise this method behaves exactly as specified by the {@link WritableByteChannel} interface.

Parameters:

src

Throws:

IOException

write

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 {@link #write(ByteBuffer)} method, except that bytes are written starting at the given file position rather than at the channel's current position. This method does not modify this channel's position. If the given position is greater than the file's current size then the file will be grown to accommodate the new bytes; the values of any bytes between the previous end-of-file and the newly-written bytes are unspecified.

Parameters:

src - The buffer from which bytes are to be transferred

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

Returns:

The number of bytes written, possibly zero

Throws:

IOException - If some other I/O error occurs

write

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

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

Bytes are written starting at this channel's current file position unless the channel is in append mode, in which case the position is first advanced to the end of the file. The file is grown, if necessary, to accommodate the written bytes, and then the file position is updated with the number of bytes actually written. Otherwise this method behaves exactly as specified in the {@link GatheringByteChannel} interface.

Parameters:

srcs

Throws:

IOException

write

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

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

Bytes are written starting at this channel's current file position unless the channel is in append mode, in which case the position is first advanced to the end of the file. The file is grown, if necessary, to accommodate the written bytes, and then the file position is updated with the number of bytes actually written. Otherwise this method behaves exactly as specified in the {@link GatheringByteChannel} interface.

Parameters:

srcs

offset

length

Throws:

IOException