A file channel has a current position within its file which can
* be both {@link #position() queried In addition to the familiar read, write, and close operations of byte
* channels, this class defines the following file-specific operations: Bytes may be {@link #read(ByteBuffer, long) } and {@link #position(long)
* modified}. The file itself contains a variable-length sequence
* of bytes that can be read and written and whose current {@link #size
* 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 {@link #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.
*
* read
*
*
} or
* {@link #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 {@link #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 {@link #force 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 {@link #transferTo to
* some other channel}, and {@link #transferFrom 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 {@link FileLock locked}
* against access by other programs.
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.
*
*
* @see java.io.FileInputStream#getChannel()
* @see java.io.FileOutputStream#getChannel()
* @see java.io.RandomAccessFile#getChannel()
*
* @author Mark Reinhold
* @author Mike McCloskey
* @author JSR-51 Expert Group
* @since 1.4
*/
public abstract class FileChannel
extends AbstractInterruptibleChannel
implements ByteChannel, GatheringByteChannel, ScatteringByteChannel
{
/**
* Initializes a new instance of this class.
*/
protected FileChannel() { }
// -- Channel operations --
/**
* 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. 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. 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. 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. 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. 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. 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.
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. *
* * @param size * The new size, a non-negative byte count * * @return This file channel * * @throws NonWritableChannelException * If this channel was not opened for writing * * @throws ClosedChannelException * If this channel is closed * * @throws IllegalArgumentException * If the new size is negative * * @throws IOException * If some other I/O error occurs */ public abstract FileChannel truncate(long size) 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
* {@link MappedByteBuffer mapped byte buffer} obtained by
* invoking the {@link #map map} method. Invoking the {@link
* MappedByteBuffer#force force} method of the mapped byte buffer will
* force changes made to the buffer's content to be written.
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.
* * @param position * The position within the file at which the transfer is to begin; * must be non-negative * * @param count * The maximum number of bytes to be transferred; must be * non-negative * * @param target * The target channel * * @return The number of bytes, possibly zero, * that were actually transferred * * @throws IllegalArgumentException * If the preconditions on the parameters do not hold * * @throws NonReadableChannelException * If this channel was not opened for reading * * @throws NonWritableChannelException * If the target channel was not opened for writing * * @throws ClosedChannelException * If either this channel or the target channel is closed * * @throws AsynchronousCloseException * If another thread closes either channel * while the transfer is in progress * * @throws ClosedByInterruptException * If another thread interrupts the current thread while the * transfer is in progress, thereby closing both channels and * setting the current thread's interrupt status * * @throws IOException * If some other I/O error occurs */ public abstract long transferTo(long position, long count, WritableByteChannel target) 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.
* * @param src * The source channel * * @param position * The position within the file at which the transfer is to begin; * must be non-negative * * @param count * The maximum number of bytes to be transferred; must be * non-negative * * @return The number of bytes, possibly zero, * that were actually transferred * * @throws IllegalArgumentException * If the preconditions on the parameters do not hold * * @throws NonReadableChannelException * If the source channel was not opened for reading * * @throws NonWritableChannelException * If this channel was not opened for writing * * @throws ClosedChannelException * If either this channel or the source channel is closed * * @throws AsynchronousCloseException * If another thread closes either channel * while the transfer is in progress * * @throws ClosedByInterruptException * If another thread interrupts the current thread while the * transfer is in progress, thereby closing both channels and * setting the current thread's interrupt status * * @throws IOException * If some other I/O error occurs */ public abstract long transferFrom(ReadableByteChannel src, long position, long count) 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.
* * @param dst * The buffer into which bytes are to be transferred * * @param position * The file position at which the transfer is to begin; * must be non-negative * * @return 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 IllegalArgumentException * If the position is negative * * @throws NonReadableChannelException * If this channel was not opened for reading * * @throws ClosedChannelException * If this channel is closed * * @throws AsynchronousCloseException * If another thread closes this channel * while the read operation is in progress * * @throws 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 * interrupt status * * @throws IOException * If some other I/O error occurs */ public abstract int read(ByteBuffer dst, 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.
* * @param src * The buffer from which bytes are to be transferred * * @param position * The file position at which the transfer is to begin; * must be non-negative * * @return The number of bytes written, possibly zero * * @throws IllegalArgumentException * If the position is negative * * @throws NonWritableChannelException * If this channel was not opened for writing * * @throws ClosedChannelException * If this channel is closed * * @throws AsynchronousCloseException * If another thread closes this channel * while the write operation is in progress * * @throws 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 * interrupt status * * @throws IOException * If some other I/O error occurs */ public abstract int write(ByteBuffer src, long position) throws IOException; // -- Memory-mapped buffers -- /** * A typesafe enumeration for file-mapping modes. * * @since 1.4 * * @see java.nio.channels.FileChannel#map */ public static class MapMode { /** * Mode for a read-only mapping. */ public static final MapMode READ_ONLY = new MapMode("READ_ONLY"); /** * Mode for a read/write mapping. */ public static final MapMode READ_WRITE = new MapMode("READ_WRITE"); /** * Mode for a private (copy-on-write) mapping. */ public static final MapMode PRIVATE = new MapMode("PRIVATE"); private final String name; private MapMode(String name) { this.name = name; } /** * Returns a string describing this file-mapping mode. * * @return A descriptive string */ public String toString() { return name; } } /** * Maps a region of this channel's file directly into memory. * *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 {@link java.nio.ReadOnlyBufferException} to be thrown. * ({@link 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. ({@link * 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. ({@link * 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 {@link MappedByteBuffer mapped byte buffer 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 {@link #read read} and {@link #write write} methods. From the
* standpoint of performance it is generally only worth mapping relatively
* large files into memory. 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. An invocation of this method of the form fc.lock() behaves
* in exactly the same way as the invocation
*
* 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. An invocation of this method of the form fc.tryLock()
* behaves in exactly the same way as the invocation
*
* }
* 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.
*
*
* fc.{@link #lock(long,long,boolean) lock}(0L, Long.MAX_VALUE, false)
*
* @return A lock object representing the newly-acquired lock
*
* @throws ClosedChannelException
* If this channel is closed
*
* @throws AsynchronousCloseException
* If another thread closes this channel while the invoking
* thread is blocked in this method
*
* @throws FileLockInterruptionException
* If the invoking thread is interrupted while blocked in this
* method
*
* @throws 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
*
* @throws NonWritableChannelException
* If this channel was not opened for writing
*
* @throws IOException
* If some other I/O error occurs
*
* @see #lock(long,long,boolean)
* @see #tryLock()
* @see #tryLock(long,long,boolean)
*/
public final FileLock lock() throws IOException {
return lock(0L, Long.MAX_VALUE, false);
}
/**
* Attempts to acquire a lock on the given region of this channel's file.
*
*
* fc.{@link #tryLock(long,long,boolean) tryLock}(0L, Long.MAX_VALUE, false)
*
* @return 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
*
* @throws 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
*
* @throws IOException
* If some other I/O error occurs
*
* @see #lock()
* @see #lock(long,long,boolean)
* @see #tryLock(long,long,boolean)
*/
public final FileLock tryLock() throws IOException {
return tryLock(0L, Long.MAX_VALUE, false);
}
}