jdk.nio.channels.AsynchronousFileChannel — clojure-interop/java.nio 1.0.5

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.

*open clj

(*open file options)

(*open file options executor 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.

file - The path of the file to open or create - java.nio.file.Path options - Options specifying how the file is opened - java.util.Set executor - The thread pool or null to associate the channel with the default thread pool - java.util.concurrent.ExecutorService attrs - An optional list of file attributes to set atomically when creating the file - java.nio.file.attribute.FileAttribute

returns: A new asynchronous file channel - java.nio.channels.AsynchronousFileChannel

throws: java.lang.IllegalArgumentException - If the set contains an invalid combination of options

force clj

(force this meta-data)

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.

meta-data - 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 - boolean

throws: java.nio.channels.ClosedChannelException - If this channel is closed

lock clj

(lock this)

(lock this attachment handler)

(lock this position size shared)

(lock this position size shared attachment 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.

position - The position at which the locked region is to start; must be non-negative - long size - The size of the locked region; must be non-negative, and the sum position size must be non-negative - long shared - 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) - boolean attachment - The object to attach to the I/O operation; can be null - A handler - The handler for consuming the result - java.nio.channels.CompletionHandler

returns: <A> void

throws: java.nio.channels.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

read clj

(read this dst position)

(read this dst position attachment 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.

dst - The buffer into which bytes are to be transferred - java.nio.ByteBuffer position - The file position at which the transfer is to begin; must be non-negative - long attachment - The object to attach to the I/O operation; can be null - A handler - The handler for consuming the result - java.nio.channels.CompletionHandler

returns: <A> void

throws: java.lang.IllegalArgumentException - If the position is negative or the buffer is read-only

size clj

(size this)

Returns the current size of this channel's file.

returns: The current size of this channel's file, measured in bytes - long

throws: java.nio.channels.ClosedChannelException - If this channel is closed

truncate clj

(truncate this 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.

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

returns: This file channel - java.nio.channels.AsynchronousFileChannel

throws: java.nio.channels.NonWritableChannelException - If this channel was not opened for writing

try-lock clj

(try-lock this)

(try-lock this position size 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.

position - The position at which the locked region is to start; must be non-negative - long size - The size of the locked region; must be non-negative, and the sum position size must be non-negative - long shared - true to request a shared lock, false to request an exclusive lock - boolean

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 - java.nio.channels.FileLock

throws: java.lang.IllegalArgumentException - If the preconditions on the parameters do not hold

write clj

(write this src position)

(write this src position attachment 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.

src - The buffer from which bytes are to be transferred - java.nio.ByteBuffer position - The file position at which the transfer is to begin; must be non-negative - long attachment - The object to attach to the I/O operation; can be null - A handler - The handler for consuming the result - java.nio.channels.CompletionHandler

returns: <A> void

throws: java.lang.IllegalArgumentException - If the position is negative