Class AsynchronousFileChannel
- All Implemented Interfaces:
Closeable
,AutoCloseable
,AsynchronousChannel
,Channel
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.
- Since:
- 1.7
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Constructor Summary
ModifierConstructorDescriptionprotected
Initializes a new instance of this class. -
Method Summary
Modifier and TypeMethodDescriptionabstract void
force
(boolean metaData) Forces any updates to this channel's file to be written to the storage device that contains it.lock()
Acquires an exclusive lock on this channel's file.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.final <A> void
lock
(A attachment, CompletionHandler<FileLock, ? super A> handler) 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.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.final 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.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.Methods declared in class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait
Methods declared in interface java.nio.channels.AsynchronousChannel
close
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Constructor Details
-
AsynchronousFileChannel
protected AsynchronousFileChannel()Initializes a new instance of this class.
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-
Method Details
-
open
public static AsynchronousFileChannel open(Path file, Set<? extends OpenOption> options, ExecutorService executor, FileAttribute<?>... attrs) throws IOException 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. TheREAD
andWRITE
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
andWRITE
, 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 close
method. If theclose
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 theExecutorService
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 theExecutor
. Minimally it should support an unbounded work queue and should not run tasks on the caller thread of theexecute
method. Shutting down the executor service while the channel is open results in unspecified behavior.The
attrs
parameter is an optional array of filefile-attributes
to set atomically when creating the file.The new channel is created by invoking the
newAsynchronousFileChannel
method on the provider that created thePath
.- Parameters:
file
- The path of the file to open or createoptions
- Options specifying how the file is openedexecutor
- The thread pool ornull
to associate the channel with the default thread poolattrs
- An optional list of file attributes to set atomically when creating the file- Returns:
- A new asynchronous file channel
- Throws:
IllegalArgumentException
- If the set contains an invalid combination of optionsUnsupportedOperationException
- If thefile
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 fileFileAlreadyExistsException
- If a file of that name already exists and theCREATE_NEW
option is specified and the file is being opened for writing (optional specific exception)IOException
- 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, theSecurityManager.checkRead(String)
method is invoked to check read access if the file is opened for reading. TheSecurityManager.checkWrite(String)
method is invoked to check write access if the file is opened for writing
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open
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]);opts
is aSet
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
- The path of the file to open or createoptions
- Options specifying how the file is opened- Returns:
- A new asynchronous file channel
- Throws:
IllegalArgumentException
- If the set contains an invalid combination of optionsUnsupportedOperationException
- If thefile
is associated with a provider that does not support creating file channels, or an unsupported open option is specifiedFileAlreadyExistsException
- If a file of that name already exists and theCREATE_NEW
option is specified and the file is being opened for writing (optional specific exception)IOException
- 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, theSecurityManager.checkRead(String)
method is invoked to check read access if the file is opened for reading. TheSecurityManager.checkWrite(String)
method is invoked to check write access if the file is opened for writing
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size
Returns the current size of this channel's file.- Returns:
- The current size of this channel's file, measured in bytes
- Throws:
ClosedChannelException
- If this channel is closedIOException
- If some other I/O error occurs
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truncate
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
- The new size, a non-negative byte count- Returns:
- This file channel
- Throws:
NonWritableChannelException
- 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 occurs
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force
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. Passingfalse
for this parameter indicates that only updates to the file's content need be written to storage; passingtrue
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
- Iftrue
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 closedIOException
- If some other I/O error occurs
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lock
public 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.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 resultingFileLock
.The region specified by the
position
andsize
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-argumentlock(Object,CompletionHandler)
method simply locks a region of sizeLong.MAX_VALUE
. If theposition
is non-negative and thesize
is zero, then a lock of sizeLong.MAX_VALUE - position
is returned. 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 throwsOverlappingFileLockException
.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.
- Type Parameters:
A
- The type of the attachment- Parameters:
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 sumposition
+size
must be non-negative. A value of zero means to lock all bytes from the specified starting position to the end of the file, regardless of whether the file is subsequently extended or truncatedshared
-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 benull
handler
- 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 regionIllegalArgumentException
- If the preconditions on the parameters do not holdNonReadableChannelException
- Ifshared
is true but this channel was not opened for readingNonWritableChannelException
- Ifshared
is false but this channel was not opened for writing
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lock
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 resultingFileLock
.An invocation of this method of the form
ch.lock(att,handler)
behaves in exactly the same way as the invocationch.
lock
(0L, Long.MAX_VALUE, false, att, handler)- Type Parameters:
A
- The type of the attachment- Parameters:
attachment
- The object to attach to the I/O operation; can benull
handler
- 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 regionNonWritableChannelException
- If this channel was not opened for writing
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lock
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 aFuture
representing the pending result. TheFuture
'sget
method returns theFileLock
on successful completion.- Parameters:
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 sumposition
+size
must be non-negative. A value of zero means to lock all bytes from the specified starting position to the end of the file, regardless of whether the file is subsequently extended or truncatedshared
-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:
- 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 regionIllegalArgumentException
- If the preconditions on the parameters do not holdNonReadableChannelException
- Ifshared
is true but this channel was not opened for readingNonWritableChannelException
- Ifshared
is false but this channel was not opened for writing
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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. TheFuture
'sget
method returns theFileLock
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:
- 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 regionNonWritableChannelException
- If this channel was not opened for writing
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tryLock
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. If theposition
is non-negative and thesize
is zero, then a lock of sizeLong.MAX_VALUE - position
is returned.- Parameters:
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 sumposition
+size
must be non-negative. A value of zero means to lock all bytes from the specified starting position to the end of the file, regardless of whether the file is subsequently extended or truncatedshared
-true
to request a shared lock,false
to request an exclusive lock- Returns:
- 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 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
- Ifshared
is true but this channel was not opened for readingNonWritableChannelException
- Ifshared
is false but this channel was not opened for writingIOException
- If some other I/O error occurs- See Also:
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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 invocationch.
tryLock
(0L, Long.MAX_VALUE, false)- Returns:
- 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 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
- Ifshared
is false but this channel was not opened for writingIOException
- If some other I/O error occurs- See Also:
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read
public 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.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.- Type Parameters:
A
- The type of the attachment- Parameters:
dst
- 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 benull
handler
- The handler for consuming the result- Throws:
IllegalArgumentException
- If the position is negative or the buffer is read-onlyNonReadableChannelException
- If this channel was not opened for reading
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read
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. TheFuture
'sget
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
- The buffer into which bytes are to be transferredposition
- The file position at which the transfer is to begin; must be non-negative- Returns:
- A
Future
object representing the pending result - Throws:
IllegalArgumentException
- If the position is negative or the buffer is read-onlyNonReadableChannelException
- If this channel was not opened for reading
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write
public 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.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.- Type Parameters:
A
- The type of the attachment- Parameters:
src
- 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 benull
handler
- The handler for consuming the result- Throws:
IllegalArgumentException
- If the position is negativeNonWritableChannelException
- If this channel was not opened for writing
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write
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. TheFuture
'sget
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
- The buffer from which bytes are to be transferredposition
- The file position at which the transfer is to begin; must be non-negative- Returns:
- A
Future
object representing the pending result - Throws:
IllegalArgumentException
- If the position is negativeNonWritableChannelException
- If this channel was not opened for writing
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