FileChannel JDK 1.6)

Description

publicabstract abstract class FileChannel

A channel for reading, writing, mapping, and manipulating a file.

A file channel has a current position within its file which can be both queried and modified. The file itself contains a variable-length sequence of bytes that can be read and written and whose current size can be queried. The size of the file increases when bytes are written beyond its current size; the size of the file decreases when it is truncated. The file may also have some associated metadata such as access permissions, content type, and last-modification time; this class does not define methods for metadata access.

In addition to the familiar read, write, and close operations of byte channels, this class defines the following file-specific operations:

Bytes may be read or written at an absolute position in a file in a way that does not affect the channel's current position.
A region of a file may be mapped directly into memory; for large files this is often much more efficient than invoking the usual read or write methods.
Updates made to a file may be forced out to the underlying storage device, ensuring that data are not lost in the event of a system crash.
Bytes can be transferred from a file to some other channel, and vice versa, in a way that can be optimized by many operating systems into a very fast transfer directly to or from the filesystem cache.
A region of a file may be locked against access by other programs.

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

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

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 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.

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 getChannel method of a FileInputStream instance will be open for reading. A channel obtained via the getChannel method of a FileOutputStream instance will be open for writing. Finally, a channel obtained via the getChannel method of a RandomAccessFile instance will be open for reading if the instance was created with mode "r" and will be open for reading and writing if the instance was created with mode "rw".

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

See also: FileInputStream.getChannel() FileOutputStream.getChannel() RandomAccessFile.getChannel()

Methods

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protectedfinal void begin () [Inherited From AbstractInterruptibleChannel]

Marks the beginning of an I/O operation that might block indefinitely.

publicfinal void close () throws IOException [Inherited From AbstractInterruptibleChannel]

Closes this channel.

protectedfinal void end (boolean completed) throws AsynchronousCloseException [Inherited From AbstractInterruptibleChannel]

Marks the end of an I/O operation that might block indefinitely.

publicabstract 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. 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 - 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
Throws:
- ClosedChannelException - If this channel is closed
- IOException - If some other I/O error occurs

protectedabstract void implCloseChannel () throws IOException [Inherited From AbstractInterruptibleChannel]

Closes this channel.

publicfinal boolean isOpen () [Inherited From AbstractInterruptibleChannel]

publicfinal 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)

Returns: A lock object representing the newly-acquired lock
Throws:
- 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
- IOException - If some other I/O error occurs
See Also: FileChannel.lock(long,long,boolean), FileChannel.tryLock(), FileChannel.tryLock(long,long,boolean),

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

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

publicabstract MappedByteBuffer map (MapMode mode, long position, long size) throws IOException

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

publicabstract long position () throws IOException

Returns this channel's file position.

publicabstract FileChannel position (long newPosition) throws IOException

Sets this channel's file position.

publicabstract int read (ByteBuffer dst) throws IOException

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

publicfinal long read (ByteBuffer dsts) throws IOException [Specified in ScatteringByteChannel]

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

publicabstract long read (ByteBuffer dsts, int offset, int length) throws IOException [Specified in ScatteringByteChannel]

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

publicabstract 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.

publicabstract long size () throws IOException

Returns the current size of this channel's file.

publicabstract long transferFrom (ReadableByteChannel src, long position, long count) throws IOException

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

publicabstract long transferTo (long position, long count, WritableByteChannel target) throws IOException

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

publicabstract FileChannel truncate (long size) throws IOException

Truncates this channel's file to the given size.

publicfinal 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)

Returns: A lock object representing the newly-acquired lock, or null if the lock could not be acquired because another program holds an overlapping lock
Throws:
- ClosedChannelException - If this channel is 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
- IOException - If some other I/O error occurs
See Also: FileChannel.lock(), FileChannel.lock(long,long,boolean), FileChannel.tryLock(long,long,boolean),

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

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

publicabstract int write (ByteBuffer src) throws IOException

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

publicfinal long write (ByteBuffer srcs) throws IOException [Specified in GatheringByteChannel]

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

publicabstract long write (ByteBuffer srcs, int offset, int length) throws IOException [Specified in GatheringByteChannel]

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

publicabstract 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.