implements SeekableByteChannel GatheringByteChannel ScatteringByteChannel
A channel for reading, writing, mapping, and manipulating a file.
A file channel is a SeekableByteChannel
that is connected to
a file. It has a current position within its file which can
be both queried
and modified
. The file itself 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
. The
file may also have some associated metadata such as access
permissions, content type, and last-modification time; this class does not
define methods for metadata access.
In addition to the familiar read, write, and close operations of byte channels, this class defines the following file-specific operations:
Bytes may be
read
orwritten
at an absolute position in a file in a way that does not affect the channel's current position.A region of a file may be
mapped
directly into memory; for large files this is often much more efficient than invoking the usual read or write methods.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.Bytes can be transferred from a file
to some other channel
, andvice versa
, in a way that can be optimized by many operating systems into a very fast transfer directly to or from the filesystem cache.A region of a file may be
locked
against access by other programs.
File channels are safe for use by multiple concurrent threads. The
close
method may be invoked at any time, as specified
by the Channel
interface. Only one operation that involves the
channel's position or can change its file's size may be in progress at any
given time; attempts to initiate a second such operation while the first is
still in progress will block until the first operation completes. Other
operations, in particular those that take an explicit position, may proceed
concurrently; whether they in fact do so is dependent upon the underlying
implementation and is therefore unspecified.
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.
A file channel is created by invoking one of the open
methods defined by this class. A file channel can also be obtained from an
existing FileInputStream
, FileOutputStream
, or RandomAccessFile
object by invoking
that object's getChannel method, which returns a file channel that
is connected to the same underlying file. Where the file channel is obtained
from an existing stream or random access file then the state of the file
channel is intimately connected to that of the object whose getChannel
method returned the channel. Changing the channel's position, whether
explicitly or by reading or writing bytes, will change the file position of
the originating object, and vice versa. Changing the file's length via the
file channel will change the length seen via the originating object, and vice
versa. Changing the file's content by writing bytes will change the content
seen by the originating object, and vice versa.
At various points this class specifies that an
instance that is "open for reading," "open for writing," or "open for
reading and writing" is required. A channel obtained via the getChannel
method of a FileInputStream
instance will be open for reading. A channel
obtained via the getChannel
method of a FileOutputStream
instance will be open for
writing. Finally, a channel obtained via the getChannel
method of a RandomAccessFile
instance will be open for reading if the instance
was created with mode "r" and will be open for reading and writing
if the instance was created with mode "rw".
A file channel that is open for writing may be in
append mode, for example if it was obtained from a file-output stream
that was created by invoking the FileOutputStream(File,boolean)
constructor and passing true for
the second parameter. In this mode each invocation of a relative write
operation first advances the position to the end of the file and then writes
the requested data. Whether the advancement of the position and the writing
of the data are done in a single atomic operation is system-dependent and
therefore unspecified.
Nested Class Summary
class | FileChannel.MapMode | A typesafe enumeration for file-mapping modes. |
Protected Constructor Summary
FileChannel()
Initializes a new instance of this class.
|
Public Method Summary
abstract void |
force(boolean metaData)
Forces any updates to this channel's file to be written to the storage
device that contains it.
|
abstract FileLock |
lock(long position, long size, boolean shared)
Acquires a lock on the given region of this channel's file.
|
final FileLock |
lock()
Acquires an exclusive lock on this channel's file.
|
abstract MappedByteBuffer |
map(FileChannel.MapMode mode, long position, long size)
Maps a region of this channel's file directly into memory.
|
static FileChannel |
open(Path path, OpenOption... options)
Opens or creates a file, returning a file channel to access the file.
|
static FileChannel |
open(Path path, Set<? extends OpenOption> options, FileAttribute...<?> attrs)
Opens or creates a file, returning a file channel to access the file.
|
abstract FileChannel |
position(long newPosition)
Sets this channel's file position.
|
abstract long |
position()
Returns this channel's file position.
|
abstract long |
read(ByteBuffer[] dsts, int offset, int length)
Reads a sequence of bytes from this channel into a subsequence of the
given buffers.
|
abstract int | |
final long | |
abstract int |
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 long |
transferFrom(ReadableByteChannel src, long position, long count)
Transfers bytes into this channel's file from the given readable byte
channel.
|
abstract long |
transferTo(long position, long count, WritableByteChannel target)
Transfers bytes from this channel's file to the given writable byte
channel.
|
abstract FileChannel |
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 int | |
abstract int |
write(ByteBuffer src, long position)
Writes a sequence of bytes to this channel from the given buffer,
starting at the given file position.
|
final long | |
abstract long |
write(ByteBuffer[] srcs, int offset, int length)
Writes a sequence of bytes to this channel from a subsequence of the
given buffers.
|
Inherited Method Summary
Protected Constructors
protected FileChannel ()
Initializes a new instance of this class.
Public Methods
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. It may or
may not force changes that were made by modifying the content of a
mapped byte buffer
obtained by
invoking the map
method. Invoking the force
method of the mapped byte buffer will
force changes made to the buffer's content to be written.
Parameters
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 written |
---|
Throws
ClosedChannelException | If this channel is closed |
---|---|
IOException | If some other I/O error occurs |
public abstract FileLock lock (long position, long size, boolean shared)
Acquires a lock on the given region of this channel's file.
An invocation of this method will block until the region can be locked, this channel is closed, or the invoking thread is interrupted, whichever comes first.
If this channel is closed by another thread during an invocation of
this method then an AsynchronousCloseException
will be thrown.
If the invoking thread is interrupted while waiting to acquire the
lock then its interrupt status will be set and a FileLockInterruptionException
will be thrown. If the invoker's
interrupt status is set when this method is invoked then that exception
will be thrown immediately; the thread's interrupt status will not be
changed.
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 zero-argument
lock()
method simply locks a region of size Long.MAX_VALUE
.
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 | The position at which the locked region is to start; must be non-negative |
---|---|
size | The size of the locked region; must be non-negative, and the sum position + size must be non-negative |
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) |
Returns
- A lock object representing the newly-acquired lock
Throws
IllegalArgumentException | If the preconditions on the parameters do not hold |
---|---|
ClosedChannelException | If this channel is closed |
AsynchronousCloseException | If another thread closes this channel while the invoking thread is blocked in this method |
FileLockInterruptionException | If the invoking thread is interrupted while blocked in this method |
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 |
NonReadableChannelException | If shared is true 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 |
public final FileLock lock ()
Acquires an exclusive lock on this channel's file.
An invocation of this method of the form fc.lock() behaves in exactly the same way as the invocation
fc.lock
(0L, Long.MAX_VALUE, false)
Returns
- A lock object representing the newly-acquired lock
Throws
ClosedChannelException | If this channel is closed |
---|---|
AsynchronousCloseException | If another thread closes this channel while the invoking thread is blocked in this method |
FileLockInterruptionException | If the invoking thread is interrupted while blocked in this method |
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 |
NonWritableChannelException | If this channel was not opened for writing |
IOException | If some other I/O error occurs |
public abstract MappedByteBuffer map (FileChannel.MapMode mode, long position, long size)
Maps a region of this channel's file directly into memory.
A region of a file may be mapped into memory in one of three modes:
Read-only: Any attempt to modify the resulting buffer will cause a
ReadOnlyBufferException
to be thrown. (MapMode.READ_ONLY
)Read/write: Changes made to the resulting buffer will eventually be propagated to the file; they may or may not be made visible to other programs that have mapped the same file. (
MapMode.READ_WRITE
)Private: Changes made to the resulting buffer will not be propagated to the file and will not be visible to other programs that have mapped the same file; instead, they will cause private copies of the modified portions of the buffer to be created. (
MapMode.PRIVATE
)
For a read-only mapping, this channel must have been opened for reading; for a read/write or private mapping, this channel must have been opened for both reading and writing.
The mapped byte buffer
returned by this method will have a position of zero and a limit and
capacity of size; its mark will be undefined. The buffer and
the mapping that it represents will remain valid until the buffer itself
is garbage-collected.
A mapping, once established, is not dependent upon the file channel that was used to create it. Closing the channel, in particular, has no effect upon the validity of the mapping.
Many of the details of memory-mapped files are inherently dependent upon the underlying operating system and are therefore unspecified. The behavior of this method when the requested region is not completely contained within this channel's file is unspecified. Whether changes made to the content or size of the underlying file, by this program or another, are propagated to the buffer is unspecified. The rate at which changes to the buffer are propagated to the file is unspecified.
For most operating systems, mapping a file into memory is more
expensive than reading or writing a few tens of kilobytes of data via
the usual read
and write
methods. From the
standpoint of performance it is generally only worth mapping relatively
large files into memory.
Parameters
mode | One of the constants READ_ONLY , READ_WRITE , or PRIVATE defined in the FileChannel.MapMode class, according to
whether the file is to be mapped read-only, read/write, or
privately (copy-on-write), respectively |
---|---|
position | The position within the file at which the mapped region is to start; must be non-negative |
size | The size of the region to be mapped; must be non-negative and
no greater than Integer.MAX_VALUE |
Returns
- The mapped byte buffer
Throws
NonReadableChannelException | If the mode is READ_ONLY but
this channel was not opened for reading |
---|---|
NonWritableChannelException | If the mode is READ_WRITE or
PRIVATE but this channel was not opened
for both reading and writing |
IllegalArgumentException | If the preconditions on the parameters do not hold |
IOException | If some other I/O error occurs |
See Also
public static FileChannel open (Path path, OpenOption... options)
Opens or creates a file, returning a file channel to access the file.
An invocation of this method behaves in exactly the same way as the invocation
fc.open
(file, opts, new FileAttribute<?>[0]);
where opts
is a set of the options specified in the options
array.Parameters
path | The path of the file to open or create |
---|---|
options | Options specifying how the file is opened |
Returns
- A new file channel
Throws
IllegalArgumentException | If the set contains an invalid combination of options |
---|---|
UnsupportedOperationException | If the path 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 |
public static FileChannel open (Path path, Set<? extends OpenOption> options, FileAttribute...<?> attrs)
Opens or creates a file, returning a file channel to access the file.
The options
parameter determines how the file is opened.
The READ
and WRITE
options determine if the file should be opened for reading and/or
writing. If neither option (or the APPEND
option) is contained in the array then the file is opened for reading.
By default reading or writing commences at the beginning of the file.
In the addition to READ
and WRITE
, the following
options may be present:
Option | Description |
---|---|
APPEND |
If this option is present then the file is opened for writing and
each invocation of the channel's write method first advances
the position to the end of the file and then writes the requested
data. Whether the advancement of the position and the writing of the
data are done in a single atomic operation is system-dependent and
therefore unspecified. This option may not be used in conjunction
with the READ or TRUNCATE_EXISTING options. |
TRUNCATE_EXISTING |
If this option is present then the existing file is 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 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
path | The path of the file to open or create |
---|---|
options | Options specifying how the file is opened |
attrs | An optional list of file attributes to set atomically when creating the file |
Returns
- A new file channel
Throws
IllegalArgumentException | If the set contains an invalid combination of options |
---|---|
UnsupportedOperationException | If the path is associated with a provider that does not
support creating 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 |
public abstract FileChannel position (long newPosition)
Sets this channel's file position.
Setting the position to a value that is greater than the file's current size is legal but does not change the size of the file. A later attempt to read bytes at such a position will immediately return an end-of-file indication. A later attempt to write bytes at such a position will cause the file to 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
newPosition | The new position, a non-negative integer counting the number of bytes from the beginning of the file |
---|
Returns
- This file channel
Throws
ClosedChannelException | If this channel is closed |
---|---|
IllegalArgumentException | If the new position is negative |
IOException | If some other I/O error occurs |
public abstract long position ()
Returns this channel's file position.
Returns
- This channel's file position, a non-negative integer counting the number of bytes from the beginning of the file to the current position
Throws
ClosedChannelException | If this channel is closed |
---|---|
IOException | If some other I/O error occurs |
public abstract long read (ByteBuffer[] dsts, int offset, int length)
Reads a sequence of bytes from this channel into a subsequence of the given buffers.
Bytes are read starting at this channel's current file position, and
then the file position is updated with the number of bytes actually
read. Otherwise this method behaves exactly as specified in the ScatteringByteChannel
interface.
Parameters
dsts | The buffers into which bytes are to be transferred |
---|---|
offset | The offset within the buffer array of the first buffer into which bytes are to be transferred; must be non-negative and no larger than dsts.length |
length | The maximum number of buffers to be accessed; must be non-negative and no larger than dsts.length - offset |
Returns
- The number of bytes read, possibly zero, or -1 if the channel has reached end-of-stream
Throws
IOException |
---|
public abstract int read (ByteBuffer dst)
Reads a sequence of bytes from this channel into the given buffer.
Bytes are read starting at this channel's current file position, and
then the file position is updated with the number of bytes actually
read. Otherwise this method behaves exactly as specified in the ReadableByteChannel
interface.
Parameters
dst | The buffer into which bytes are to be transferred |
---|
Returns
- The number of bytes read, possibly zero, or -1 if the channel has reached end-of-stream
Throws
IOException |
---|
public final long read (ByteBuffer[] dsts)
Reads a sequence of bytes from this channel into the given buffers.
Bytes are read starting at this channel's current file position, and
then the file position is updated with the number of bytes actually
read. Otherwise this method behaves exactly as specified in the ScatteringByteChannel
interface.
Parameters
dsts | The buffers into which bytes are to be transferred |
---|
Returns
- The number of bytes read, possibly zero, or -1 if the channel has reached end-of-stream
Throws
IOException |
---|
public abstract int 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 works in the same manner as the read(ByteBuffer)
method, except that bytes are read starting at the
given file position rather than at the channel's current position. This
method does not modify this channel's position. If the given position
is greater than the file's current size then no bytes are read.
Parameters
dst | The buffer into which bytes are to be transferred |
---|---|
position | The file position at which the transfer is to begin; must be non-negative |
Returns
- The number of bytes read, possibly zero, or -1 if the given position is greater than or equal to the file's current size
Throws
IllegalArgumentException | If the position is negative |
---|---|
NonReadableChannelException | If this channel was not opened for reading |
ClosedChannelException | If this channel is closed |
AsynchronousCloseException | If another thread closes this channel while the read operation is in progress |
ClosedByInterruptException | If another thread interrupts the current thread while the read operation is in progress, thereby closing the channel and setting the current thread's interrupt status |
IOException | If some other I/O error occurs |
public abstract long 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 closed |
---|---|
IOException | If some other I/O error occurs |
public abstract long transferFrom (ReadableByteChannel src, long position, long count)
Transfers bytes into this channel's file from the given readable byte channel.
An attempt is made to read up to count bytes from the source channel and write them to this channel's file starting at the given position. An invocation of this method may or may not transfer all of the requested bytes; whether or not it does so depends upon the natures and states of the channels. Fewer than the requested number of bytes will be transferred if the source channel has fewer than count bytes remaining, or if the source channel is non-blocking and has fewer than count bytes immediately available in its input buffer.
This method does not modify this channel's position. If the given position is greater than the file's current size then no bytes are transferred. If the source channel has a position then bytes are read starting at that position and then the position is incremented by the number of bytes read.
This method is potentially much more efficient than a simple loop that reads from the source channel and writes to this channel. Many operating systems can transfer bytes directly from the source channel into the filesystem cache without actually copying them.
Parameters
src | The source channel |
---|---|
position | The position within the file at which the transfer is to begin; must be non-negative |
count | The maximum number of bytes to be transferred; must be non-negative |
Returns
- The number of bytes, possibly zero, that were actually transferred
Throws
IllegalArgumentException | If the preconditions on the parameters do not hold |
---|---|
NonReadableChannelException | If the source channel was not opened for reading |
NonWritableChannelException | If this channel was not opened for writing |
ClosedChannelException | If either this channel or the source channel is closed |
AsynchronousCloseException | If another thread closes either channel while the transfer is in progress |
ClosedByInterruptException | If another thread interrupts the current thread while the transfer is in progress, thereby closing both channels and setting the current thread's interrupt status |
IOException | If some other I/O error occurs |
public abstract long transferTo (long position, long count, WritableByteChannel target)
Transfers bytes from this channel's file to the given writable byte channel.
An attempt is made to read up to count bytes starting at the given position in this channel's file and write them to the target channel. An invocation of this method may or may not transfer all of the requested bytes; whether or not it does so depends upon the natures and states of the channels. Fewer than the requested number of bytes are transferred if this channel's file contains fewer than count bytes starting at the given position, or if the target channel is non-blocking and it has fewer than count bytes free in its output buffer.
This method does not modify this channel's position. If the given position is greater than the file's current size then no bytes are transferred. If the target channel has a position then bytes are written starting at that position and then the position is incremented by the number of bytes written.
This method is potentially much more efficient than a simple loop that reads from this channel and writes to the target channel. Many operating systems can transfer bytes directly from the filesystem cache to the target channel without actually copying them.
Parameters
position | The position within the file at which the transfer is to begin; must be non-negative |
---|---|
count | The maximum number of bytes to be transferred; must be non-negative |
target | The target channel |
Returns
- The number of bytes, possibly zero, that were actually transferred
Throws
IllegalArgumentException | If the preconditions on the parameters do not hold |
---|---|
NonReadableChannelException | If this channel was not opened for reading |
NonWritableChannelException | If the target channel was not opened for writing |
ClosedChannelException | If either this channel or the target channel is closed |
AsynchronousCloseException | If another thread closes either channel while the transfer is in progress |
ClosedByInterruptException | If another thread interrupts the current thread while the transfer is in progress, thereby closing both channels and setting the current thread's interrupt status |
IOException | If some other I/O error occurs |
public abstract FileChannel 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. In either case, if this channel's file position is greater than the given size then it is set to that size.
Parameters
size | The new size, a non-negative byte count |
---|
Returns
- 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 |
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.
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 zero-argument
tryLock()
method simply locks a region of size Long.MAX_VALUE
.
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 | The position at which the locked region is to start; must be non-negative |
---|---|
size | The size of the locked region; must be non-negative, and the sum position + size must be non-negative |
shared | 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 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 |
IOException | If some other I/O error occurs |
public final FileLock tryLock ()
Attempts to acquire an exclusive lock on this channel's file.
An invocation of this method of the form fc.tryLock() behaves in exactly the same way as the invocation
fc.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 closed |
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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 |
IOException | If some other I/O error occurs |
public abstract int write (ByteBuffer src)
Writes a sequence of bytes to this channel from the given buffer.
Bytes are written starting at this channel's current file position
unless the channel is in append mode, in which case the position is
first advanced to the end of the file. The file is grown, if necessary,
to accommodate the written bytes, and then the file position is updated
with the number of bytes actually written. Otherwise this method
behaves exactly as specified by the WritableByteChannel
interface.
Parameters
src | The buffer from which bytes are to be retrieved |
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Returns
- The number of bytes written, possibly zero
Throws
IOException |
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public abstract int 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 works in the same manner as the write(ByteBuffer)
method, except that bytes are written starting at
the given file position rather than at the channel's current position.
This method does not modify this channel's position. If the given
position is greater than the file's current size 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 transferred |
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position | The file position at which the transfer is to begin; must be non-negative |
Returns
- The number of bytes written, possibly zero
Throws
IllegalArgumentException | If the position is negative |
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NonWritableChannelException | If this channel was not opened for writing |
ClosedChannelException | If this channel is closed |
AsynchronousCloseException | If another thread closes this channel while the write operation is in progress |
ClosedByInterruptException | If another thread interrupts the current thread while the write operation is in progress, thereby closing the channel and setting the current thread's interrupt status |
IOException | If some other I/O error occurs |
public final long write (ByteBuffer[] srcs)
Writes a sequence of bytes to this channel from the given buffers.
Bytes are written starting at this channel's current file position
unless the channel is in append mode, in which case the position is
first advanced to the end of the file. The file is grown, if necessary,
to accommodate the written bytes, and then the file position is updated
with the number of bytes actually written. Otherwise this method
behaves exactly as specified in the GatheringByteChannel
interface.
Parameters
srcs | The buffers from which bytes are to be retrieved |
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Returns
- The number of bytes written, possibly zero
Throws
IOException |
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public abstract long write (ByteBuffer[] srcs, int offset, int length)
Writes a sequence of bytes to this channel from a subsequence of the given buffers.
Bytes are written starting at this channel's current file position
unless the channel is in append mode, in which case the position is
first advanced to the end of the file. The file is grown, if necessary,
to accommodate the written bytes, and then the file position is updated
with the number of bytes actually written. Otherwise this method
behaves exactly as specified in the GatheringByteChannel
interface.
Parameters
srcs | The buffers from which bytes are to be retrieved |
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offset | The offset within the buffer array of the first buffer from which bytes are to be retrieved; must be non-negative and no larger than srcs.length |
length | The maximum number of buffers to be accessed; must be non-negative and no larger than srcs.length - offset |
Returns
- The number of bytes written, possibly zero
Throws
IOException |
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