public abstract class AsynchronousFileChannel extends Object implements AsynchronousChannel
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.
| Modifier | Constructor and Description |
|---|---|
protected |
AsynchronousFileChannel()
Initializes a new instance of this class.
|
| Modifier and Type | Method and Description |
|---|---|
abstract void |
force(boolean metaData)
Forces any updates to this channel's file to be written to the storage
device that contains it.
|
Future<FileLock> |
lock()
Acquires an exclusive lock on this channel's file.
|
<A> void |
lock(A attachment,
CompletionHandler<FileLock,? super A> handler)
Acquires an exclusive lock on 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.
|
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.
|
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 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 <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 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.
|
FileLock |
tryLock()
Attempts to acquire an exclusive lock on this channel's file.
|
abstract FileLock |
tryLock(long position,
long size,
boolean shared)
Attempts to acquire a lock on the given region of 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.
|
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, waitcloseprotected AsynchronousFileChannel()
public static AsynchronousFileChannel open(Path file, Set<? extends OpenOption> options, ExecutorService executor, FileAttribute<?>... attrs) throws IOException
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.
file - The path of the file to open or createoptions - Options specifying how the file is openedexecutor - The thread pool or null to associate the channel with
the default thread poolattrs - An optional list of file attributes to set atomically when
creating the fileIllegalArgumentException - If the set contains an invalid combination of optionsUnsupportedOperationException - 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 fileIOException - If an I/O error occursSecurityException - 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 writingpublic static AsynchronousFileChannel open(Path file, OpenOption... options) throws IOException
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.
file - The path of the file to open or createoptions - Options specifying how the file is openedIllegalArgumentException - If the set contains an invalid combination of optionsUnsupportedOperationException - If the file is associated with a provider that does not
support creating file channels, or an unsupported open option is
specifiedIOException - If an I/O error occursSecurityException - 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 writingpublic abstract long size()
throws IOException
ClosedChannelException - If this channel is closedIOException - If some other I/O error occurspublic abstract AsynchronousFileChannel truncate(long size) throws IOException
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.
size - The new size, a non-negative byte countNonWritableChannelException - If this channel was not opened for writingClosedChannelException - If this channel is closedIllegalArgumentException - If the new size is negativeIOException - If some other I/O error occurspublic abstract void force(boolean metaData)
throws IOException
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.
metaData - 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
writtenClosedChannelException - If this channel is closedIOException - If some other I/O error occurspublic abstract <A> void lock(long position,
long size,
boolean shared,
A attachment,
CompletionHandler<FileLock,? super A> handler)
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.
A - The type of the attachmentposition - The position at which the locked region is to start; must be
non-negativesize - The size of the locked region; must be non-negative, and the sum
position + size must be non-negativeshared - 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 - The object to attach to the I/O operation; can be nullhandler - The handler for consuming the resultOverlappingFileLockException - 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 regionIllegalArgumentException - If the preconditions on the parameters do not holdNonReadableChannelException - If shared is true but this channel was not opened for readingNonWritableChannelException - If shared is false but this channel was not opened for writingpublic final <A> void lock(A attachment,
CompletionHandler<FileLock,? super A> handler)
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)
A - The type of the attachmentattachment - The object to attach to the I/O operation; can be nullhandler - The handler for consuming the resultOverlappingFileLockException - If a lock is already held by this Java virtual machine, or there
is already a pending attempt to lock a regionNonWritableChannelException - If this channel was not opened for writingpublic abstract Future<FileLock> lock(long position, long size, boolean shared)
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.
position - The position at which the locked region is to start; must be
non-negativesize - The size of the locked region; must be non-negative, and the sum
position + size must be non-negativeshared - 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)Future object representing the pending resultOverlappingFileLockException - If a lock is already held by this Java virtual machine, or there
is already a pending attempt to lock a regionIllegalArgumentException - If the preconditions on the parameters do not holdNonReadableChannelException - If shared is true but this channel was not opened for readingNonWritableChannelException - If shared is false but this channel was not opened for writingpublic final Future<FileLock> lock()
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)
Future object representing the pending resultOverlappingFileLockException - If a lock is already held by this Java virtual machine, or there
is already a pending attempt to lock a regionNonWritableChannelException - If this channel was not opened for writingpublic abstract FileLock tryLock(long position, long size, boolean shared) throws IOException
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.
position - The position at which the locked region is to start; must be
non-negativesize - The size of the locked region; must be non-negative, and the sum
position + size must be non-negativeshared - true to request a shared lock,
false to request an exclusive locknull if the lock could not be acquired
because another program holds an overlapping lockIllegalArgumentException - If the preconditions on the parameters do not holdClosedChannelException - If this channel is closedOverlappingFileLockException - 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 fileNonReadableChannelException - If shared is true but this channel was not opened for readingNonWritableChannelException - If shared is false but this channel was not opened for writingIOException - If some other I/O error occurslock(Object,CompletionHandler),
lock(long,long,boolean,Object,CompletionHandler),
tryLock()public final FileLock tryLock() throws IOException
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) null if the lock could not be acquired
because another program holds an overlapping lockClosedChannelException - If this channel is closedOverlappingFileLockException - 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
regionNonWritableChannelException - If shared is false but this channel was not opened for writingIOException - If some other I/O error occurslock(Object,CompletionHandler),
lock(long,long,boolean,Object,CompletionHandler),
tryLock(long,long,boolean)public abstract <A> void read(ByteBuffer dst, long position, A attachment, CompletionHandler<Integer,? super A> handler)
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.
A - The type of the attachmentdst - The buffer into which bytes are to be transferredposition - The file position at which the transfer is to begin;
must be non-negativeattachment - The object to attach to the I/O operation; can be nullhandler - The handler for consuming the resultIllegalArgumentException - If the position is negative or the buffer is read-onlyNonReadableChannelException - If this channel was not opened for readingpublic abstract Future<Integer> read(ByteBuffer dst, long 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.
dst - The buffer into which bytes are to be transferredposition - The file position at which the transfer is to begin;
must be non-negativeFuture object representing the pending resultIllegalArgumentException - If the position is negative or the buffer is read-onlyNonReadableChannelException - If this channel was not opened for readingpublic abstract <A> void write(ByteBuffer src, long position, A attachment, CompletionHandler<Integer,? super A> handler)
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.
A - The type of the attachmentsrc - The buffer from which bytes are to be transferredposition - The file position at which the transfer is to begin;
must be non-negativeattachment - The object to attach to the I/O operation; can be nullhandler - The handler for consuming the resultIllegalArgumentException - If the position is negativeNonWritableChannelException - If this channel was not opened for writingpublic abstract Future<Integer> write(ByteBuffer src, long 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.
src - The buffer from which bytes are to be transferredposition - The file position at which the transfer is to begin;
must be non-negativeFuture object representing the pending resultIllegalArgumentException - If the position is negativeNonWritableChannelException - If this channel was not opened for writing Submit a bug or feature
For further API reference and developer documentation, see Java SE Documentation. That documentation contains more detailed, developer-targeted descriptions, with conceptual overviews, definitions of terms, workarounds, and working code examples.
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