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Android APIs
public abstract class

FileChannel

extends AbstractInterruptibleChannel
implements ByteChannel GatheringByteChannel ScatteringByteChannel
java.lang.Object
   ↳ java.nio.channels.spi.AbstractInterruptibleChannel
     ↳ java.nio.channels.FileChannel

Class Overview

An abstract channel type for interaction with a platform file.

A FileChannel defines the methods for reading, writing, memory mapping, and manipulating the logical state of a platform file. This type does not have a method for opening files, since this behavior has been delegated to the FileInputStream, FileOutputStream and RandomAccessFile types.

FileChannels created from a FileInputStream or a RandomAccessFile created in mode "r", are read-only. FileChannels created from a FileOutputStream are write-only. FileChannels created from a RandomAccessFile created in mode "rw" are read/write. FileChannels created from a RandomAccessFile that was opened in append-mode will also be in append-mode -- meaning that each write will be proceeded by a seek to the end of file.

FileChannels have a virtual pointer into the file which is referred to as a file position. The position can be manipulated by moving it within the file, and the current position can be queried.

FileChannels also have an associated size. The size of the file is the number of bytes that it currently contains. The size can be manipulated by adding more bytes to the end of the file (which increases the size) or truncating the file (which decreases the size). The current size can also be queried.

FileChannels have operations beyond the simple read, write, and close. They can also:

  • request that cached data be forced onto the disk,
  • lock ranges of bytes associated with the file,
  • transfer data directly to another channel in a manner that has the potential to be optimized by the platform,
  • memory-mapping files into NIO buffers to provide efficient manipulation of file data,
  • read and write to the file at absolute byte offsets in a fashion that does not modify the current position.

FileChannels are thread-safe. Only one operation involving manipulation of the file position may be executed at the same time. Subsequent calls to such operations will block, and one of those blocked will be freed to continue when the first operation has completed. There is no ordered queue or fairness applied to the blocked threads.

It is undefined whether operations that do not manipulate the file position will also block when there are any other operations in-flight.

The logical view of the underlying file is consistent across all FileChannels and I/O streams opened on the same file by the same VM. Therefore, modifications performed via a channel will be visible to the stream and vice versa; this includes modifications to the file position, content, size, etc.

Summary

Nested Classes
class FileChannel.MapMode MapMode defines file mapping mode constants. 
Protected Constructors
FileChannel()
Protected default constructor.
Public Methods
abstract void force(boolean metadata)
Requests that all updates to this channel are committed to the storage device.
final FileLock lock()
Obtains an exclusive lock on this file.
abstract FileLock lock(long position, long size, boolean shared)
Obtains a lock on a specified region of the file.
abstract MappedByteBuffer map(FileChannel.MapMode mode, long position, long size)
Maps the file into memory.
abstract long position()
Returns the current value of the file position pointer.
abstract FileChannel position(long offset)
Sets the file position pointer to a new value.
abstract int read(ByteBuffer buffer, long position)
Reads bytes from this file channel into the given buffer starting from the specified file position.
abstract int read(ByteBuffer buffer)
Reads bytes from this file channel into the given buffer.
abstract long read(ByteBuffer[] buffers, int start, int number)
Reads bytes from this file channel into a subset of the given buffers.
final long read(ByteBuffer[] buffers)
Reads bytes from this file channel and stores them in the specified array of buffers.
abstract long size()
Returns the size of the file underlying this channel in bytes.
abstract long transferFrom(ReadableByteChannel src, long position, long count)
Reads up to count bytes from src and stores them in this channel's file starting at position.
abstract long transferTo(long position, long count, WritableByteChannel target)
Reads up to count bytes from this channel's file starting at position and writes them to target.
abstract FileChannel truncate(long size)
Truncates the file underlying this channel to a given size.
abstract FileLock tryLock(long position, long size, boolean shared)
Attempts to acquire an exclusive lock on this file without blocking.
final FileLock tryLock()
Attempts to acquire an exclusive lock on this file without blocking.
final long write(ByteBuffer[] buffers)
Writes bytes from all the given byte buffers to this file channel.
abstract long write(ByteBuffer[] buffers, int offset, int length)
Attempts to write a subset of the given bytes from the buffers to this file channel.
abstract int write(ByteBuffer src)
Writes bytes from the given byte buffer to this file channel.
abstract int write(ByteBuffer buffer, long position)
Writes bytes from the given buffer to this file channel starting at the given file position.
[Expand]
Inherited Methods
From class java.nio.channels.spi.AbstractInterruptibleChannel
From class java.lang.Object
From interface java.io.Closeable
From interface java.nio.channels.Channel
From interface java.nio.channels.GatheringByteChannel
From interface java.nio.channels.InterruptibleChannel
From interface java.nio.channels.ReadableByteChannel
From interface java.nio.channels.ScatteringByteChannel
From interface java.nio.channels.WritableByteChannel

Protected Constructors

protected FileChannel ()

Since: API Level 1

Protected default constructor.

Public Methods

public abstract void force (boolean metadata)

Since: API Level 1

Requests that all updates to this channel are committed to the storage device.

When this method returns, all modifications made to the platform file underlying this channel have been committed if the file resides on a local storage device. If the file is not hosted locally, for example on a networked file system, then applications cannot be certain that the modifications have been committed.

There are no assurances given that changes made to the file using methods defined elsewhere will be committed. For example, changes made via a mapped byte buffer may not be committed.

The metadata parameter indicates whether the update should include the file's metadata such as last modification time, last access time, etc. Note that passing true may invoke an underlying write to the operating system (if the platform is maintaining metadata such as last access time), even if the channel is opened read-only.

Parameters
metadata true if the file metadata should be flushed in addition to the file content, false otherwise.
Throws
ClosedChannelException if this channel is already closed.
IOException if another I/O error occurs.

public final FileLock lock ()

Since: API Level 1

Obtains an exclusive lock on this file.

This is a convenience method for acquiring a maximum length lock on a file. It is equivalent to: fileChannel.lock(0L, Long.MAX_VALUE, false);

Returns
  • the lock object representing the locked file area.
Throws
ClosedChannelException the file channel is closed.
NonWritableChannelException this channel was not opened for writing.
OverlappingFileLockException either a lock is already held that overlaps this lock request, or another thread is waiting to acquire a lock that will overlap with this request.
FileLockInterruptionException the calling thread was interrupted while waiting to acquire the lock.
AsynchronousCloseException the channel was closed while the calling thread was waiting to acquire the lock.
IOException if another I/O error occurs while obtaining the requested lock.

public abstract FileLock lock (long position, long size, boolean shared)

Since: API Level 1

Obtains a lock on a specified region of the file.

This is the blocking version of lock acquisition, see also the tryLock() methods.

Attempts to acquire an overlapping lock region will fail. The attempt will fail if the overlapping lock has already been obtained, or if another thread is currently waiting to acquire the overlapping lock.

If the request is not for an overlapping lock, the thread calling this method will block until the lock is obtained (likely by no contention or another process releasing a lock), or until this thread is interrupted or the channel is closed.

If the lock is obtained successfully then the FileLock object returned represents the lock for subsequent operations on the locked region.

If the thread is interrupted while waiting for the lock, the thread is set to the interrupted state and throws a FileLockInterruptionException. If this channel is closed while the thread is waiting to obtain the lock then the thread throws a AsynchronousCloseException.

There is no requirement for the position and size to be within the current start and length of the file.

Some platforms do not support shared locks, and if a request is made for a shared lock on such a platform, this method will attempt to acquire an exclusive lock instead. It is undefined whether the lock obtained is advisory or mandatory.

Parameters
position the starting position for the locked region.
size the length of the locked region in bytes.
shared a flag indicating whether an attempt should be made to acquire a shared lock.
Returns
  • the file lock object.
Throws
IllegalArgumentException if position or size is negative.
ClosedChannelException if this channel is closed.
OverlappingFileLockException if the requested region overlaps an existing lock or pending lock request.
NonReadableChannelException if the channel is not opened in read-mode but shared is true.
NonWritableChannelException if the channel is not opened in write mode but shared is false.
AsynchronousCloseException if this channel is closed by another thread while this method is executing.
FileLockInterruptionException if the thread is interrupted while in the state of waiting on the desired file lock.
IOException if another I/O error occurs.

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

Since: API Level 1

Maps the file into memory. There can be three modes: read-only, read/write and private. After mapping, changes made to memory or the file channel do not affect the other storage place.

Note: mapping a file into memory is usually expensive.

Parameters
mode one of the three mapping modes.
position the starting position of the file.
size the size of the region to map into memory.
Returns
  • the mapped byte buffer.
Throws
NonReadableChannelException if the FileChannel is not opened for reading but the given mode is "READ_ONLY".
NonWritableChannelException if the FileChannel is not opened for writing but the given mode is not "READ_ONLY".
IllegalArgumentException if the given parameters of position and size are not correct. Both must be non negative. size also must not be bigger than max integer.
IOException if any I/O error occurs.

public abstract long position ()

Since: API Level 1

Returns the current value of the file position pointer.

Returns
  • the current position as a positive integer number of bytes from the start of the file.
Throws
ClosedChannelException if this channel is closed.
IOException if another I/O error occurs.

public abstract FileChannel position (long offset)

Since: API Level 1

Sets the file position pointer to a new value.

The argument is the number of bytes counted from the start of the file. The position cannot be set to a value that is negative. The new position can be set beyond the current file size. If set beyond the current file size, attempts to read will return end of file. Write operations will succeed but they will fill the bytes between the current end of file and the new position with the required number of (unspecified) byte values.

Parameters
offset the new file position, in bytes.
Returns
  • the receiver.
Throws
IllegalArgumentException if the new position is negative.
ClosedChannelException if this channel is closed.
IOException if another I/O error occurs.

public abstract int read (ByteBuffer buffer, long position)

Since: API Level 1

Reads bytes from this file channel into the given buffer starting from the specified file position.

The bytes are read starting at the given file position (up to the remaining number of bytes in the buffer). The number of bytes actually read is returned.

If position is beyond the current end of file, then no bytes are read.

Note that the file position is unmodified by this method.

Parameters
buffer the buffer to receive the bytes.
position the (non-negative) position at which to read the bytes.
Returns
  • the number of bytes actually read.
Throws
AsynchronousCloseException if this channel is closed by another thread while this method is executing.
ClosedByInterruptException if another thread interrupts the calling thread while this operation is in progress. The calling thread will have the interrupt state set, and the channel will be closed.
ClosedChannelException if this channel is closed.
IllegalArgumentException if position is less than 0.
IOException if another I/O error occurs.
NonReadableChannelException if the channel has not been opened in a mode that permits reading.

public abstract int read (ByteBuffer buffer)

Since: API Level 1

Reads bytes from this file channel into the given buffer.

The maximum number of bytes that will be read is the remaining number of bytes in the buffer when the method is invoked. The bytes will be copied into the buffer starting at the buffer's current position.

The call may block if other threads are also attempting to read from this channel.

Upon completion, the buffer's position is set to the end of the bytes that have been read. The buffer's limit is not changed.

Parameters
buffer the byte buffer to receive the bytes.
Returns
  • the number of bytes actually read.
Throws
AsynchronousCloseException if another thread closes the channel during the read.
ClosedByInterruptException if another thread interrupts the calling thread during the read.
ClosedChannelException if this channel is closed.
IOException if another I/O error occurs, details are in the message.
NonReadableChannelException if the channel has not been opened in a mode that permits reading.

public abstract long read (ByteBuffer[] buffers, int start, int number)

Since: API Level 1

Reads bytes from this file channel into a subset of the given buffers. This method attempts to read all remaining() bytes from length byte buffers, in order, starting at targets[offset]. It increases the file position by the number of bytes actually read. The number of bytes actually read is returned.

If a read operation is in progress, subsequent threads will block until the read is completed and will then contend for the ability to read.

Parameters
buffers the array of byte buffers into which the bytes will be copied.
start the index of the first buffer to store bytes in.
number the maximum number of buffers to store bytes in.
Returns
  • the number of bytes actually read.
Throws
AsynchronousCloseException if this channel is closed by another thread during this read operation.
ClosedByInterruptException if the thread is interrupted by another thread during this read operation.
ClosedChannelException if this channel is closed.
IndexOutOfBoundsException if start < 0 or number < 0, or if start + number is greater than the size of buffers.
IOException if another I/O error occurs; details are in the message.
NonReadableChannelException if the channel has not been opened in a mode that permits reading.

public final long read (ByteBuffer[] buffers)

Since: API Level 1

Reads bytes from this file channel and stores them in the specified array of buffers. This method attempts to read as many bytes as can be stored in the buffer array from this channel and returns the number of bytes actually read. It also increases the file position by the number of bytes read.

If a read operation is in progress, subsequent threads will block until the read is completed and will then contend for the ability to read.

Calling this method is equivalent to calling read(buffers, 0, buffers.length);

Parameters
buffers the array of byte buffers into which the bytes will be copied.
Returns
  • the number of bytes actually read.
Throws
AsynchronousCloseException if this channel is closed by another thread during this read operation.
ClosedByInterruptException if the thread is interrupted by another thread during this read operation.
ClosedChannelException if this channel is closed.
IOException if another I/O error occurs; details are in the message.
NonReadableChannelException if the channel has not been opened in a mode that permits reading.

public abstract long size ()

Since: API Level 1

Returns the size of the file underlying this channel in bytes.

Returns
  • the size of the file in bytes.
Throws
ClosedChannelException if this channel is closed.
IOException if an I/O error occurs while getting the size of the file.

public abstract long transferFrom (ReadableByteChannel src, long position, long count)

Since: API Level 1

Reads up to count bytes from src and stores them in this channel's file starting at position. No bytes are transferred if position is larger than the size of this channel's file. Less than count bytes are transferred if there are less bytes remaining in the source channel or if the source channel is non-blocking and has less than count bytes immediately available in its output buffer.

Note that this channel's position is not modified.

Parameters
src the source channel to read bytes from.
position the non-negative start position.
count the non-negative number of bytes to transfer.
Returns
  • the number of bytes that are transferred.
Throws
IllegalArgumentException if the parameters are invalid.
NonReadableChannelException if the source channel is not readable.
NonWritableChannelException if this channel is not writable.
ClosedChannelException if either channel has already been closed.
AsynchronousCloseException if either channel is closed by other threads during this operation.
ClosedByInterruptException if the thread is interrupted during this operation.
IOException if any I/O error occurs.

public abstract long transferTo (long position, long count, WritableByteChannel target)

Since: API Level 1

Reads up to count bytes from this channel's file starting at position and writes them to target. No bytes are transferred if position is larger than the size of this channel's file. Less than count bytes are transferred if there less bytes available from this channel's file or if the target channel is non-blocking and has less than count bytes free in its input buffer.

Note that this channel's position is not modified.

Parameters
position the non-negative position to begin.
count the non-negative number of bytes to transfer.
target the target channel to write to.
Returns
  • the number of bytes that were transferred.
Throws
IllegalArgumentException if the parameters are invalid.
NonReadableChannelException if this channel is not readable.
NonWritableChannelException if the target channel is not writable.
ClosedChannelException if either channel has already been closed.
AsynchronousCloseException if either channel is closed by other threads during this operation.
ClosedByInterruptException if the thread is interrupted during this operation.
IOException if any I/O error occurs.

public abstract FileChannel truncate (long size)

Since: API Level 1

Truncates the file underlying this channel to a given size. Any bytes beyond the given size are removed from the file. If there are no bytes beyond the given size then the file contents are unmodified.

If the file position is currently greater than the given size, then it is set to the new size.

Parameters
size the maximum size of the underlying file.
Returns
  • this channel.
Throws
IllegalArgumentException if the requested size is negative.
ClosedChannelException if this channel is closed.
NonWritableChannelException if the channel cannot be written to.
IOException if another I/O error occurs.

public abstract FileLock tryLock (long position, long size, boolean shared)

Since: API Level 1

Attempts to acquire an exclusive lock on this file without blocking. The method returns null if the acquisition would result in an overlapped lock with another OS process.

It is possible to acquire a lock for any region even if it's completely outside of the file's size. The size of the lock is fixed. If the file grows outside of the lock that region of the file won't be locked by this lock.

Parameters
position the starting position.
size the size of file to lock.
shared true if the lock is shared.
Returns
  • the file lock object, or null if the lock would overlap with an existing exclusive lock in another OS process.
Throws
IllegalArgumentException if any parameters are invalid.
ClosedChannelException if the file channel is closed.
OverlappingFileLockException if a lock is already held that overlaps this lock request or another thread is waiting to acquire a lock that will overlap with this request.
IOException if any I/O error occurs.

public final FileLock tryLock ()

Since: API Level 1

Attempts to acquire an exclusive lock on this file without blocking.

This is a convenience method for attempting to acquire a maximum length lock on the file. It is equivalent to: fileChannel.tryLock(0L, Long.MAX_VALUE, false);

The method returns null if the acquisition would result in an overlapped lock with another OS process.

Returns
  • the file lock object, or null if the lock would overlap with an existing exclusive lock in another OS process.
Throws
ClosedChannelException if the file channel is closed.
OverlappingFileLockException if a lock already exists that overlaps this lock request or another thread is waiting to acquire a lock that will overlap with this request.
IOException if any I/O error occurs.

public final long write (ByteBuffer[] buffers)

Since: API Level 1

Writes bytes from all the given byte buffers to this file channel.

The bytes are written starting at the current file position, and after the bytes are written (up to the remaining number of bytes in all the buffers), the file position is increased by the number of bytes actually written.

Calling this method is equivalent to calling write(buffers, 0, buffers.length);

Parameters
buffers the buffers containing bytes to write.
Returns
  • the number of bytes actually written.
Throws
AsynchronousCloseException if this channel is closed by another thread during this write operation.
ClosedByInterruptException if another thread interrupts the calling thread while this operation is in progress. The interrupt state of the calling thread is set and the channel is closed.
ClosedChannelException if this channel is closed.
IOException if another I/O error occurs; details are in the message.
NonWritableChannelException if this channel was not opened for writing.

public abstract long write (ByteBuffer[] buffers, int offset, int length)

Since: API Level 1

Attempts to write a subset of the given bytes from the buffers to this file channel. This method attempts to write all remaining() bytes from length byte buffers, in order, starting at sources[offset]. The number of bytes actually written is returned.

If a write operation is in progress, subsequent threads will block until the write is completed and then contend for the ability to write.

Parameters
buffers the array of byte buffers that is the source for bytes written to this channel.
offset the index of the first buffer in buffers to get bytes from.
length the number of buffers to get bytes from.
Returns
  • the number of bytes actually written to this channel.
Throws
AsynchronousCloseException if this channel is closed by another thread during this write operation.
ClosedByInterruptException if another thread interrupts the calling thread while this operation is in progress. The interrupt state of the calling thread is set and the channel is closed.
ClosedChannelException if this channel is closed.
IndexOutOfBoundsException if offset < 0 or length < 0, or if offset + length is greater than the size of buffers.
IOException if another I/O error occurs; details are in the message.
NonWritableChannelException if this channel was not opened for writing.

public abstract int write (ByteBuffer src)

Since: API Level 1

Writes bytes from the given byte buffer to this file channel.

The bytes are written starting at the current file position, and after some number of bytes are written (up to the remaining number of bytes in the buffer) the file position is increased by the number of bytes actually written.

Parameters
src the byte buffer containing the bytes to be written.
Returns
  • the number of bytes actually written.
Throws
NonWritableChannelException if the channel was not opened for writing.
ClosedChannelException if the channel was already closed.
AsynchronousCloseException if another thread closes the channel during the write.
ClosedByInterruptException if another thread interrupts the calling thread while this operation is in progress. The interrupt state of the calling thread is set and the channel is closed.
IOException if another I/O error occurs, details are in the message.
See Also

public abstract int write (ByteBuffer buffer, long position)

Since: API Level 1

Writes bytes from the given buffer to this file channel starting at the given file position.

The bytes are written starting at the given file position (up to the remaining number of bytes in the buffer). The number of bytes actually written is returned.

If the position is beyond the current end of file, then the file is first extended up to the given position by the required number of unspecified byte values.

Note that the file position is not modified by this method.

Parameters
buffer the buffer containing the bytes to be written.
position the (non-negative) position at which to write the bytes.
Returns
  • the number of bytes actually written.
Throws
IllegalArgumentException if position is less than 0.
ClosedChannelException if this channel is closed.
NonWritableChannelException if the channel was not opened in write-mode.
AsynchronousCloseException if this channel is closed by another thread while this method is executing.
ClosedByInterruptException if another thread interrupts the calling thread while this operation is in progress. The interrupt state of the calling thread is set and the channel is closed.
IOException if another I/O error occurs.