Java Doc for FileChannel.java in  » 6.0-JDK-Core » io-nio » java » nio » channels » Java Source Code / Java DocumentationJava Source Code and Java Documentation

A file channel has a current position within its file which can be both FileChannel.position() queried and FileChannel.position(long)modified . The file itself contains a variable-length sequence of bytes that can be read and written and whose current FileChannel.sizesize 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 FileChannel.truncate 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 FileChannel.read(ByteBuffer,long) read or FileChannel.write(ByteBuffer,long) 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 FileChannel.map 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 FileChannel.force forcedout 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 FileChannel.transferTo tosome other channel , and FileChannel.transferFrom viceversa , 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 FileLock locked against access by other programs.

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.

An invocation of this method will block until the region can be locked, this channel is closed, or the invoking thread is interrupted, whichever comes first.

If this channel is closed by another thread during an invocation of this method then an AsynchronousCloseException will be thrown.

If the invoking thread is interrupted while waiting to acquire the lock then its interrupt status will be set and a FileLockInterruptionException will be thrown.

A region of a file may be mapped into memory in one of three modes:

• Read-only: Any attempt to modify the resulting buffer will cause a java.nio.ReadOnlyBufferException to be thrown. ( MapMode.READ_ONLY 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.

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.

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.

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.

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.

This method works in the same manner as the FileChannel.read(ByteBuffer) method, except that bytes are read starting at the given file position rather than at the channel's current position.

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 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.

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.

This method does not block.

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.

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.

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.

This method works in the same manner as the FileChannel.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 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 MappedByteBuffer mapped byte buffer obtained by invoking the FileChannel.map map method. Invoking the MappedByteBuffer.force force method of the mapped byte buffer will force changes made to the buffer's content to be written.

An invocation of this method will block until the region can be locked, this channel is closed, or the invoking thread is interrupted, whichever comes first.

If this channel is closed by another thread during an invocation of this method then an AsynchronousCloseException will be thrown.

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

The region specified by the position 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 FileChannel.lock() method simply locks a region of size 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 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.

An invocation of this method of the form fc.lock() behaves in exactly the same way as the invocation

 fc.
FileChannel.lock(long,long,boolean) lock (0L, Long.MAX_VALUE, false) 

A region of a file may be mapped into memory in one of three modes:

• Read-only: Any attempt to modify the resulting buffer will cause a java.nio.ReadOnlyBufferException to be thrown. ( MapMode.READ_ONLY 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 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 MapMode.PRIVATE )

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 MappedByteBuffer 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 FileChannel.read read and FileChannel.write write methods. From the standpoint of performance it is generally only worth mapping relatively large files into memory.

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.

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.

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.

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.

This method works in the same manner as the FileChannel.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.

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.

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.

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.

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 FileChannel.tryLock() method simply locks a region of size 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 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.

An invocation of this method of the form fc.tryLock() behaves in exactly the same way as the invocation

 fc.
FileChannel.tryLock(long,long,boolean) tryLock (0L, Long.MAX_VALUE, false) 

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.

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.

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.

This method works in the same manner as the FileChannel.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.