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AsynchronousFileChannel

public abstract class AsynchronousFileChannel
extends Object implements AsynchronousChannel

java.lang.Object
   ↳ java.nio.channels.AsynchronousFileChannel


An asynchronous channel for reading, writing, and manipulating a file.

An asynchronous file channel is created when a file is opened by invoking one of the open methods defined by this class. The file 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.

An asynchronous file channel does not have a current position within the file. Instead, the file position is specified to each read and write method that initiates asynchronous operations. A CompletionHandler is specified as a parameter and is invoked to consume the result of the I/O operation. This class also defines read and write methods that initiate asynchronous operations, returning a Future to represent the pending result of the operation. The Future may be used to check if the operation has completed, wait for its completion, and retrieve the result.

In addition to read and write operations, this class defines the following operations:

  • Updates made to a file may be forced out to the underlying storage device, ensuring that data are not lost in the event of a system crash.

  • A region of a file may be locked against access by other programs.

An AsynchronousFileChannel is associated with a thread pool to which tasks are submitted to handle I/O events and dispatch to completion handlers that consume the results of I/O operations on the channel. The completion handler for an I/O operation initiated on a channel is guaranteed to be invoked by one of the threads in the thread pool (This ensures that the completion handler is run by a thread with the expected identity). Where an I/O operation completes immediately, and the initiating thread is itself a thread in the thread pool, then the completion handler may be invoked directly by the initiating thread. When an AsynchronousFileChannel is created without specifying a thread pool then the channel is associated with a system-dependent default thread pool that may be shared with other channels. The default thread pool is configured by the system properties defined by the AsynchronousChannelGroup class.

Channels of this type are safe for use by multiple concurrent threads. The close method may be invoked at any time, as specified by the Channel interface. This causes all outstanding asynchronous operations on the channel to complete with the exception AsynchronousCloseException. Multiple read and write operations may be outstanding at the same time. When multiple read and write operations are outstanding then the ordering of the I/O operations, and the order that the completion handlers are invoked, is not specified; they are not, in particular, guaranteed to execute in the order that the operations were initiated. The ByteBuffers used when reading or writing are not safe for use by multiple concurrent I/O operations. Furthermore, after an I/O operation is initiated then care should be taken to ensure that the buffer is not accessed until after the operation has completed.

As with FileChannel, 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.

Summary

Protected constructors

AsynchronousFileChannel()

Initializes a new instance of this class.

Public methods

abstract void force(boolean metaData)

Forces any updates to this channel's file to be written to the storage device that contains it.

abstract <A> void lock(long position, long size, boolean shared, A attachment, CompletionHandler<FileLock, ? super A> handler)

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

abstract Future<FileLock> lock(long position, long size, boolean shared)

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

final <A> void lock(A attachment, CompletionHandler<FileLock, ? super A> handler)

Acquires an exclusive lock on this channel's file.

final Future<FileLock> lock()

Acquires an exclusive lock on this channel's file.

static AsynchronousFileChannel open(Path file, OpenOption... options)

Opens or creates a file for reading and/or writing, returning an asynchronous file channel to access the file.

static AsynchronousFileChannel open(Path file, Set<? extends OpenOption> options, ExecutorService executor, FileAttribute...<?> attrs)

Opens or creates a file for reading and/or writing, returning an asynchronous file channel to access the file.

abstract <A> void read(ByteBuffer dst, long position, A attachment, CompletionHandler<Integer, ? super A> handler)

Reads a sequence of bytes from this channel into the given buffer, starting at the given file position.

abstract Future<Integer> read(ByteBuffer dst, long position)

Reads a sequence of bytes from this channel into the given buffer, starting at the given file position.

abstract long size()

Returns the current size of this channel's file.

abstract AsynchronousFileChannel truncate(long size)

Truncates this channel's file to the given size.

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

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

final FileLock tryLock()

Attempts to acquire an exclusive lock on this channel's file.

abstract Future<Integer> write(ByteBuffer src, long position)

Writes a sequence of bytes to this channel from the given buffer, starting at the given file position.

abstract <A> void write(ByteBuffer src, long position, A attachment, CompletionHandler<Integer, ? super A> handler)

Writes a sequence of bytes to this channel from the given buffer, starting at the given file position.

Inherited methods

Protected constructors

AsynchronousFileChannel

added in API level 26
protected AsynchronousFileChannel ()

Initializes a new instance of this class.

Public methods

force

added in API level 26
public abstract void force (boolean metaData)

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.

Parameters
metaData boolean: If true then this method is required to force changes to both the file's content and metadata to be written to storage; otherwise, it need only force content changes to be written

Throws
ClosedChannelException If this channel is closed
IOException If some other I/O error occurs

lock

added in API level 26
public abstract void lock (long position, 
                long size, 
                boolean shared, 
                A attachment, 
                CompletionHandler<FileLock, ? super A> handler)

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

This method initiates an operation to acquire a lock on the given region of this channel's file. The handler parameter is a completion handler that is invoked when the lock is acquired (or the operation fails). The result passed to the completion handler is the resulting FileLock.

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 two-argument lock(Object, CompletionHandler) method simply locks a region of size Long.MAX_VALUE. If a lock that overlaps the requested region is already held by this Java virtual machine, or this method has been invoked to lock an overlapping region and that operation has not completed, then this method throws OverlappingFileLockException.

Some operating systems do not support a mechanism to acquire a file lock in an asynchronous manner. Consequently an implementation may acquire the file lock in a background thread or from a task executed by a thread in the associated thread pool. If there are many lock operations outstanding then it may consume threads in the Java virtual machine for indefinite periods.

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.

Parameters
position long: The position at which the locked region is to start; must be non-negative

size long: The size of the locked region; must be non-negative, and the sum position + size must be non-negative

shared boolean: true to request a shared lock, in which case this channel must be open for reading (and possibly writing); false to request an exclusive lock, in which case this channel must be open for writing (and possibly reading)

attachment A: The object to attach to the I/O operation; can be null

handler CompletionHandler: The handler for consuming the result

Throws
OverlappingFileLockException If a lock that overlaps the requested region is already held by this Java virtual machine, or there is already a pending attempt to lock an overlapping region
IllegalArgumentException If the preconditions on the parameters do not hold
NonReadableChannelException If shared is true but this channel was not opened for reading
NonWritableChannelException If shared is false but this channel was not opened for writing

lock

added in API level 26
public abstract Future<FileLock> lock (long position, 
                long size, 
                boolean shared)

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

This method initiates an operation to acquire a lock on the given region of this channel's file. The method behaves in exactly the same manner as the lock(long, long, boolean, Object, CompletionHandler) method except that instead of specifying a completion handler, this method returns a Future representing the pending result. The Future's get method returns the FileLock on successful completion.

Parameters
position long: The position at which the locked region is to start; must be non-negative

size long: The size of the locked region; must be non-negative, and the sum position + size must be non-negative

shared boolean: true to request a shared lock, in which case this channel must be open for reading (and possibly writing); false to request an exclusive lock, in which case this channel must be open for writing (and possibly reading)

Returns
Future<FileLock> a Future object representing the pending result

Throws
OverlappingFileLockException If a lock is already held by this Java virtual machine, or there is already a pending attempt to lock a region
IllegalArgumentException If the preconditions on the parameters do not hold
NonReadableChannelException If shared is true but this channel was not opened for reading
NonWritableChannelException If shared is false but this channel was not opened for writing

lock

added in API level 26
public final void lock (A attachment, 
                CompletionHandler<FileLock, ? super A> handler)

Acquires an exclusive lock on this channel's file.

This method initiates an operation to acquire a lock on the given region of this channel's file. The handler parameter is a completion handler that is invoked when the lock is acquired (or the operation fails). The result passed to the completion handler is the resulting FileLock.

An invocation of this method of the form ch.lock(att,handler) behaves in exactly the same way as the invocation

     ch.lock(0L, Long.MAX_VALUE, false, att, handler)
 

Parameters
attachment A: The object to attach to the I/O operation; can be null

handler CompletionHandler: The handler for consuming the result

Throws
OverlappingFileLockException If a lock is already held by this Java virtual machine, or there is already a pending attempt to lock a region
NonWritableChannelException If this channel was not opened for writing

lock

added in API level 26
public final Future<FileLock> lock ()

Acquires an exclusive lock on this channel's file.

This method initiates an operation to acquire an exclusive lock on this channel's file. The method returns a Future representing the pending result of the operation. The Future's get method returns the FileLock on successful completion.

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

     ch.lock(0L, Long.MAX_VALUE, false)
 

Returns
Future<FileLock> a Future object representing the pending result

Throws
OverlappingFileLockException If a lock is already held by this Java virtual machine, or there is already a pending attempt to lock a region
NonWritableChannelException If this channel was not opened for writing

open

added in API level 26
public static AsynchronousFileChannel open (Path file, 
                OpenOption... options)

Opens or creates a file for reading and/or writing, returning an asynchronous file channel to access the file.

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

     ch.open(file, opts, null, new FileAttribute<?>[0]);
 
where opts is a Set containing the options specified to this method.

The resulting channel is associated with default thread pool to which tasks are submitted to handle I/O events and dispatch to completion handlers that consume the result of asynchronous operations performed on the resulting channel.

Parameters
file Path: The path of the file to open or create

options OpenOption: Options specifying how the file is opened

Returns
AsynchronousFileChannel A new asynchronous file channel

Throws
IllegalArgumentException If the set contains an invalid combination of options
UnsupportedOperationException If the file is associated with a provider that does not support creating file channels, or an unsupported open option is specified
IOException If an I/O error occurs
SecurityException If a security manager is installed and it denies an unspecified permission required by the implementation. In the case of the default provider, the SecurityManager.checkRead(String) method is invoked to check read access if the file is opened for reading. The SecurityManager.checkWrite(String) method is invoked to check write access if the file is opened for writing

open

added in API level 26
public static AsynchronousFileChannel open (Path file, 
                Set<? extends OpenOption> options, 
                ExecutorService executor, 
                FileAttribute...<?> attrs)

Opens or creates a file for reading and/or writing, returning an asynchronous file channel to access the file.

The options parameter determines how the file is opened. The READ and WRITE options determines if the file should be opened for reading and/or writing. If neither option is contained in the array then an existing file is opened for reading.

In addition to READ and WRITE, the following options may be present:

Option Description
TRUNCATE_EXISTING When opening an existing file, the file is first 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 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 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 executor parameter is the ExecutorService to which tasks are submitted to handle I/O events and dispatch completion results for operations initiated on resulting channel. The nature of these tasks is highly implementation specific and so care should be taken when configuring the Executor. Minimally it should support an unbounded work queue and should not run tasks on the caller thread of the execute method. Shutting down the executor service while the channel is open results in unspecified behavior.

The 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 Path.

Parameters
file Path: The path of the file to open or create

options Set: Options specifying how the file is opened

executor ExecutorService: The thread pool or null to associate the channel with the default thread pool

attrs FileAttribute: An optional list of file attributes to set atomically when creating the file

Returns
AsynchronousFileChannel A new asynchronous file channel

Throws
IllegalArgumentException If the set contains an invalid combination of options
UnsupportedOperationException If the file is associated with a provider that does not support creating asynchronous file channels, or an unsupported open option is specified, or the array contains an attribute that cannot be set atomically when creating the file
IOException If an I/O error occurs
SecurityException If a security manager is installed and it denies an unspecified permission required by the implementation. In the case of the default provider, the SecurityManager.checkRead(String) method is invoked to check read access if the file is opened for reading. The SecurityManager.checkWrite(String) method is invoked to check write access if the file is opened for writing

read

added in API level 26
public abstract void read (ByteBuffer dst, 
                long position, 
                A attachment, 
                CompletionHandler<Integer, ? super A> handler)

Reads a sequence of bytes from this channel into the given buffer, starting at the given file position.

This method initiates the reading of a sequence of bytes from this channel into the given buffer, starting at the given file position. The result of the read is the number of bytes read or -1 if the given position is greater than or equal to the file's size at the time that the read is attempted.

This method works in the same manner as the AsynchronousByteChannel.read(ByteBuffer, Object, CompletionHandler) method, except that bytes are read starting at the given file position. If the given file position is greater than the file's size at the time that the read is attempted then no bytes are read.

Parameters
dst ByteBuffer: The buffer into which bytes are to be transferred

position long: The file position at which the transfer is to begin; must be non-negative

attachment A: The object to attach to the I/O operation; can be null

handler CompletionHandler: The handler for consuming the result

Throws
IllegalArgumentException If the position is negative or the buffer is read-only
NonReadableChannelException If this channel was not opened for reading

read

added in API level 26
public abstract Future<Integer> read (ByteBuffer dst, 
                long position)

Reads a sequence of bytes from this channel into the given buffer, starting at the given file position.

This method initiates the reading of a sequence of bytes from this channel into the given buffer, starting at the given file position. This method returns a Future representing the pending result of the operation. The Future's get method returns the number of bytes read or -1 if the given position is greater than or equal to the file's size at the time that the read is attempted.

This method works in the same manner as the AsynchronousByteChannel.read(ByteBuffer) method, except that bytes are read starting at the given file position. If the given file position is greater than the file's size at the time that the read is attempted then no bytes are read.

Parameters
dst ByteBuffer: The buffer into which bytes are to be transferred

position long: The file position at which the transfer is to begin; must be non-negative

Returns
Future<Integer> A Future object representing the pending result

Throws
IllegalArgumentException If the position is negative or the buffer is read-only
NonReadableChannelException If this channel was not opened for reading

size

added in API level 26
public abstract long size ()

Returns the current size of this channel's file.

Returns
long The current size of this channel's file, measured in bytes

Throws
ClosedChannelException If this channel is closed
IOException If some other I/O error occurs

truncate

added in API level 26
public abstract AsynchronousFileChannel truncate (long size)

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.

Parameters
size long: The new size, a non-negative byte count

Returns
AsynchronousFileChannel This file channel

Throws
NonWritableChannelException If this channel was not opened for writing
ClosedChannelException If this channel is closed
IllegalArgumentException If the new size is negative
IOException If some other I/O error occurs

tryLock

added in API level 26
public abstract FileLock tryLock (long position, 
                long size, 
                boolean shared)

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.

Parameters
position long: The position at which the locked region is to start; must be non-negative

size long: The size of the locked region; must be non-negative, and the sum position + size must be non-negative

shared boolean: true to request a shared lock, false to request an exclusive lock

Returns
FileLock 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
IllegalArgumentException If the preconditions on the parameters do not hold
ClosedChannelException If this channel is closed
OverlappingFileLockException If a lock that overlaps the requested region is already held by this Java virtual machine, or if another thread is already blocked in this method and is attempting to lock an overlapping region of the same file
NonReadableChannelException If shared is true but this channel was not opened for reading
NonWritableChannelException If shared is false but this channel was not opened for writing
IOException If some other I/O error occurs

tryLock

added in API level 26
public final FileLock tryLock ()

Attempts to acquire an exclusive lock on this channel's file.

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

     ch.tryLock(0L, Long.MAX_VALUE, false) 

Returns
FileLock A lock object representing the newly-acquired lock, or null if the lock could not be acquired because another program holds an overlapping lock

Throws
ClosedChannelException If this channel is closed
OverlappingFileLockException If a lock that overlaps the requested region is already held by this Java virtual machine, or if another thread is already blocked in this method and is attempting to lock an overlapping region
NonWritableChannelException If shared is false but this channel was not opened for writing
IOException If some other I/O error occurs

write

added in API level 26
public abstract Future<Integer> write (ByteBuffer src, 
                long position)

Writes a sequence of bytes to this channel from the given buffer, starting at the given file position.

This method initiates the writing of a sequence of bytes to this channel from the given buffer, starting at the given file position. The method returns a Future representing the pending result of the write operation. The Future's get method returns the number of bytes written.

This method works in the same manner as the AsynchronousByteChannel.write(ByteBuffer) method, except that bytes are written starting at the given file position. If the given position is greater than the file's size, at the time that the write is attempted, 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.

Parameters
src ByteBuffer: The buffer from which bytes are to be transferred

position long: The file position at which the transfer is to begin; must be non-negative

Returns
Future<Integer> A Future object representing the pending result

Throws
IllegalArgumentException If the position is negative
NonWritableChannelException If this channel was not opened for writing

write

added in API level 26
public abstract void write (ByteBuffer src, 
                long position, 
                A attachment, 
                CompletionHandler<Integer, ? super A> handler)

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 AsynchronousByteChannel.write(ByteBuffer, Object, CompletionHandler) method, except that bytes are written starting at the given file position. If the given position is greater than the file's size, at the time that the write is attempted, 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.

Parameters
src ByteBuffer: The buffer from which bytes are to be transferred

position long: The file position at which the transfer is to begin; must be non-negative

attachment A: The object to attach to the I/O operation; can be null

handler CompletionHandler: The handler for consuming the result

Throws
IllegalArgumentException If the position is negative
NonWritableChannelException If this channel was not opened for writing