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java.nio.channels
abstract public class: FileChannel [javadoc | source]
java.lang.Object
   java.nio.channels.spi.AbstractInterruptibleChannel
      java.nio.channels.FileChannel

All Implemented Interfaces:
    GatheringByteChannel, SeekableByteChannel, ScatteringByteChannel, Channel, InterruptibleChannel

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

A file channel is a SeekableByteChannel that is connected to a file. It 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:

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.

A file channel is created by invoking one of the open methods defined by this class. A file channel can also 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. Where the file channel is obtained from an existing stream or random access file then the state of the 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 java.io.FileInputStream instance will be open for reading. A channel obtained via the getChannel method of a java.io.FileOutputStream instance will be open for writing. Finally, a channel obtained via the getChannel method of a 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 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.

Nested Class Summary:
public static class  FileChannel.MapMode  A typesafe enumeration for file-mapping modes. 
Constructor:
 protected FileChannel() 
Method from java.nio.channels.FileChannel Summary:
force,   lock,   lock,   map,   open,   open,   position,   position,   read,   read,   read,   read,   size,   transferFrom,   transferTo,   truncate,   tryLock,   tryLock,   write,   write,   write,   write
Methods from java.nio.channels.spi.AbstractInterruptibleChannel:
begin,   blockedOn,   close,   end,   implCloseChannel,   isOpen
Methods from java.lang.Object:
clone,   equals,   finalize,   getClass,   hashCode,   notify,   notifyAll,   toString,   wait,   wait,   wait
Method from java.nio.channels.FileChannel Detail:
 abstract public  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.

 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. lock (0L, Long.MAX_VALUE, false) 
 abstract public 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 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 #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 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.

 abstract public MappedByteBuffer map(MapMode mode,
    long position,
    long size) throws IOException
    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 java.nio.ReadOnlyBufferException to be thrown. ( 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 )

    • 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 )

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

 public static FileChannel open(Path path,
    OpenOption options) throws IOException 
    Opens or creates a file, returning a file channel to access the file.

    An invocation of this method behaves in exactly the same way as the invocation

        fc. open (file, opts, new FileAttribute<?>[0]);
    
    where {@code opts} is a set of the options specified in the {@code options} array.
 public static FileChannel open(Path path,
    Set<OpenOption> options,
    FileAttribute<?> attrs) throws IOException 
    Opens or creates a file, returning a file channel to access the file.

    The {@code options} parameter determines how the file is opened. The READ and WRITE options determine if the file should be opened for reading and/or writing. If neither option (or the APPEND option) is contained in the array then the file is opened for reading. By default reading or writing commences at the beginning of the file.

    In the addition to {@code READ} and {@code WRITE}, the following options may be present:
    Option Description
    APPEND If this option is present then the file is opened for writing and each invocation of the channel's {@code write} method 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. This option may not be used in conjunction with the {@code READ} or {@code TRUNCATE_EXISTING} options.
    TRUNCATE_EXISTING If this option is present then the existing file is truncated to a size of 0 bytes. This option is ignored when the file is opened only for reading.
    CREATE_NEW If this option is present then a new file is created, failing if the file already exists. When creating a file the check for the existence of the file and the creation of the file if it does not exist is atomic with respect to other file system operations. This option is ignored when the file is opened only for reading.
    CREATE If this option is present then an existing file is opened if it exists, otherwise a new file is created. When creating a file the check for the existence of the file and the creation of the file if it does not exist is atomic with respect to other file system operations. This option is ignored if the {@code CREATE_NEW} option is also present or the file is opened only for reading.
    DELETE_ON_CLOSE When this option is present then the implementation makes a best effort attempt to delete the file when closed by the the close method. If the {@code close} method is not invoked then a best effort attempt is made to delete the file when the Java virtual machine terminates.
    SPARSE When creating a new file this option is a hint that the new file will be sparse. This option is ignored when not creating a new file.
    SYNC Requires that every update to the file's content or metadata be written synchronously to the underlying storage device. (see Synchronized I/O file integrity).
    DSYNC Requires that every update to the file's content be written synchronously to the underlying storage device. (see Synchronized I/O file integrity).

    An implementation may also support additional options.

    The {@code attrs} parameter is an optional array of file file-attributes to set atomically when creating the file.

    The new channel is created by invoking the newFileChannel method on the provider that created the {@code Path}.

 abstract public long position() throws IOException
    Returns this channel's file position.

 abstract public 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.

 abstract public 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 ReadableByteChannel interface.

 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 ScatteringByteChannel interface.

 abstract public 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 #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.

 abstract public 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 ScatteringByteChannel interface.

 abstract public long size() throws IOException
    Returns the current size of this channel's file.

 abstract public 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.

 abstract public 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.

 abstract public 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.

 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. tryLock (0L, Long.MAX_VALUE, false) 
 abstract public 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 #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 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.

 abstract public 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 WritableByteChannel interface.

 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 GatheringByteChannel interface.

 abstract public 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 #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.

 abstract public 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 GatheringByteChannel interface.