org.apache.sshd.client.subsystem.sftp

Interface SftpClient

All Superinterfaces:

java.lang.AutoCloseable, java.nio.channels.Channel, ChannelHolder, ClientChannelHolder, ClientSessionHolder, java.io.Closeable, NamedResource, SessionContextHolder, SessionHolder<ClientSession>, SubsystemClient

All Known Implementing Classes:

AbstractSftpClient, DefaultSftpClient, SftpFileSystem.Wrapper

public interface SftpClient
extends SubsystemClient

Nested Class Summary

Nested Classes

Modifier and Type	Interface and Description
static class	SftpClient.Attribute
static class	SftpClient.Attributes
static class	SftpClient.CloseableHandle
static class	SftpClient.CopyMode
static class	SftpClient.DirEntry
static class	SftpClient.Handle
static class	SftpClient.OpenMode

Field Summary

Fields

Modifier and Type	Field and Description
static java.util.Set<SftpClient.OpenMode>	DEFAULT_CHANNEL_MODES Default modes for opening a channel if no specific modes specified
static long	DEFAULT_CHANNEL_OPEN_TIMEOUT
static java.nio.charset.Charset	DEFAULT_NAME_DECODING_CHARSET Default value of "sftp-name-decoding-charset"
static long	DEFAULT_WAIT_TIMEOUT
static SftpClient.DirEntry[]	EMPTY_DIR_ENTRIES
static int	IO_BUFFER_SIZE
static int	MIN_BUFFER_SIZE
static int	MIN_READ_BUFFER_SIZE
static int	MIN_WRITE_BUFFER_SIZE
static java.lang.String	NAME_DECODING_CHARSET Used to indicate the Charset (or its name) for decoding referenced files/folders names - extracted from the client session when 1st initialized.
static java.lang.String	SFTP_CHANNEL_OPEN_TIMEOUT Property that can be used on the FactoryManager to control the internal timeout used by the client to open a channel.

Fields inherited from interface org.apache.sshd.common.NamedResource

BY_NAME_COMPARATOR, NAME_EXTRACTOR

Method Summary

Modifier and Type	Method and Description
java.lang.String	canonicalPath(java.lang.String path) The effective "normalized" remote path
void	close(SftpClient.Handle handle) Close the handle obtained from one of the open methods
<E extends SftpClientExtension> E	getExtension(java.lang.Class<? extends E> extensionType)
SftpClientExtension	getExtension(java.lang.String extensionName)
default java.lang.String	getName()
java.nio.charset.Charset	getNameDecodingCharset()
java.util.NavigableMap<java.lang.String,byte[]>	getServerExtensions()
int	getVersion()
boolean	isClosing()
void	link(java.lang.String linkPath, java.lang.String targetPath, boolean symbolic) Create a link
java.lang.Iterable<SftpClient.DirEntry>	listDir(SftpClient.Handle handle)
void	lock(SftpClient.Handle handle, long offset, long length, int mask)
SftpClient.Attributes	lstat(java.lang.String path) Retrieve remote path meta-data - do not follow symbolic links
void	mkdir(java.lang.String path) Create remote directory
default SftpClient.CloseableHandle	open(java.lang.String path) Opens a remote file for read
SftpClient.CloseableHandle	open(java.lang.String path, java.util.Collection<SftpClient.OpenMode> options) Opens a remote file with the specified mode(s)
default SftpClient.CloseableHandle	open(java.lang.String path, SftpClient.OpenMode... options) Opens a remote file with the specified mode(s)
SftpClient.CloseableHandle	openDir(java.lang.String path) Obtain a handle for a directory
java.nio.channels.FileChannel	openRemoteFileChannel(java.lang.String path, java.util.Collection<SftpClient.OpenMode> modes) Opens an FileChannel on the specified remote path
default java.nio.channels.FileChannel	openRemoteFileChannel(java.lang.String path, SftpClient.OpenMode... modes)
default java.nio.channels.FileChannel	openRemotePathChannel(java.lang.String path, java.util.Collection<? extends java.nio.file.OpenOption> options)
default java.nio.channels.FileChannel	openRemotePathChannel(java.lang.String path, java.nio.file.OpenOption... options)
default int	read(SftpClient.Handle handle, long fileOffset, byte[] dst) Reads data from the open (file) handle
default int	read(SftpClient.Handle handle, long fileOffset, byte[] dst, java.util.concurrent.atomic.AtomicReference<java.lang.Boolean> eofSignalled) Reads data from the open (file) handle
default int	read(SftpClient.Handle handle, long fileOffset, byte[] dst, int dstOffset, int len)
int	read(SftpClient.Handle handle, long fileOffset, byte[] dst, int dstOffset, int len, java.util.concurrent.atomic.AtomicReference<java.lang.Boolean> eofSignalled) Reads data from the open (file) handle
default java.io.InputStream	read(java.lang.String path)
default java.io.InputStream	read(java.lang.String path, java.util.Collection<SftpClient.OpenMode> mode)
default java.io.InputStream	read(java.lang.String path, int bufferSize)
java.io.InputStream	read(java.lang.String path, int bufferSize, java.util.Collection<SftpClient.OpenMode> mode) Read a remote file's data via an input stream
default java.io.InputStream	read(java.lang.String path, int bufferSize, SftpClient.OpenMode... mode)
default java.io.InputStream	read(java.lang.String path, SftpClient.OpenMode... mode)
default java.util.List<SftpClient.DirEntry>	readDir(SftpClient.Handle handle)
java.util.List<SftpClient.DirEntry>	readDir(SftpClient.Handle handle, java.util.concurrent.atomic.AtomicReference<java.lang.Boolean> eolIndicator)
java.lang.Iterable<SftpClient.DirEntry>	readDir(java.lang.String path)
java.lang.String	readLink(java.lang.String path) Retrieve target of a link
void	remove(java.lang.String path)
default void	rename(java.lang.String oldPath, java.lang.String newPath)
void	rename(java.lang.String oldPath, java.lang.String newPath, java.util.Collection<SftpClient.CopyMode> options)
default void	rename(java.lang.String oldPath, java.lang.String newPath, SftpClient.CopyMode... options)
void	rmdir(java.lang.String path) Remove remote directory
void	setNameDecodingCharset(java.nio.charset.Charset cs)
void	setStat(SftpClient.Handle handle, SftpClient.Attributes attributes) Update remote node meta-data
void	setStat(java.lang.String path, SftpClient.Attributes attributes) Update remote node meta-data
SftpClient.Attributes	stat(SftpClient.Handle handle) Retrieve file/directory handle meta-data
SftpClient.Attributes	stat(java.lang.String path) Retrieve remote path meta-data - follow symbolic links if encountered
default void	symLink(java.lang.String linkPath, java.lang.String targetPath) Create symbolic link
void	unlock(SftpClient.Handle handle, long offset, long length)
default void	write(SftpClient.Handle handle, long fileOffset, byte[] src)
void	write(SftpClient.Handle handle, long fileOffset, byte[] src, int srcOffset, int len) Write data to (open) file handle
default java.io.OutputStream	write(java.lang.String path)
default java.io.OutputStream	write(java.lang.String path, java.util.Collection<SftpClient.OpenMode> mode)
default java.io.OutputStream	write(java.lang.String path, int bufferSize)
java.io.OutputStream	write(java.lang.String path, int bufferSize, java.util.Collection<SftpClient.OpenMode> mode) Write to a remote file via an output stream
default java.io.OutputStream	write(java.lang.String path, int bufferSize, SftpClient.OpenMode... mode)
default java.io.OutputStream	write(java.lang.String path, SftpClient.OpenMode... mode)

Methods inherited from interface org.apache.sshd.client.subsystem.SubsystemClient

getSession

Methods inherited from interface org.apache.sshd.common.session.SessionHolder

getSessionContext

Methods inherited from interface org.apache.sshd.client.session.ClientSessionHolder

getClientSession

Methods inherited from interface org.apache.sshd.common.NamedResource

findByName, getNameList, getNames, ofName, removeByName

Methods inherited from interface java.nio.channels.Channel

close, isOpen

Methods inherited from interface org.apache.sshd.client.channel.ClientChannelHolder

getChannel, getClientChannel

Field Detail

NAME_DECODING_CHARSET

static final java.lang.String NAME_DECODING_CHARSET

Used to indicate the Charset (or its name) for decoding referenced files/folders names - extracted from the client session when 1st initialized.

See Also:

DEFAULT_NAME_DECODING_CHARSET, getNameDecodingCharset(), setNameDecodingCharset(Charset), Constant Field Values

DEFAULT_NAME_DECODING_CHARSET

static final java.nio.charset.Charset DEFAULT_NAME_DECODING_CHARSET

Default value of "sftp-name-decoding-charset"

EMPTY_DIR_ENTRIES

static final SftpClient.DirEntry[] EMPTY_DIR_ENTRIES

MIN_BUFFER_SIZE

static final int MIN_BUFFER_SIZE

See Also:

Constant Field Values

MIN_READ_BUFFER_SIZE

static final int MIN_READ_BUFFER_SIZE

See Also:

Constant Field Values

MIN_WRITE_BUFFER_SIZE

static final int MIN_WRITE_BUFFER_SIZE

See Also:

Constant Field Values

IO_BUFFER_SIZE

static final int IO_BUFFER_SIZE

See Also:

Constant Field Values

DEFAULT_WAIT_TIMEOUT

static final long DEFAULT_WAIT_TIMEOUT

SFTP_CHANNEL_OPEN_TIMEOUT

static final java.lang.String SFTP_CHANNEL_OPEN_TIMEOUT

Property that can be used on the FactoryManager to control the internal timeout used by the client to open a channel. If not specified then DEFAULT_CHANNEL_OPEN_TIMEOUT value is used

See Also:

Constant Field Values

DEFAULT_CHANNEL_OPEN_TIMEOUT

static final long DEFAULT_CHANNEL_OPEN_TIMEOUT

DEFAULT_CHANNEL_MODES

static final java.util.Set<SftpClient.OpenMode> DEFAULT_CHANNEL_MODES

Default modes for opening a channel if no specific modes specified

Method Detail

getVersion

int getVersion()

Returns:

The negotiated SFTP protocol version

getName

default java.lang.String getName()

Specified by:

getName in interface NamedResource

Returns:

The resource name

getNameDecodingCharset

java.nio.charset.Charset getNameDecodingCharset()

Returns:

The (never null) Charset used to decode referenced files/folders names

See Also:

NAME_DECODING_CHARSET

setNameDecodingCharset

void setNameDecodingCharset(java.nio.charset.Charset cs)

getServerExtensions

java.util.NavigableMap<java.lang.String,byte[]> getServerExtensions()

Returns:

An (unmodifiable) NavigableMap of the reported server extensions. where key=extension name (case insensitive)

isClosing

boolean isClosing()

open

default SftpClient.CloseableHandle open(java.lang.String path)
                                 throws java.io.IOException

Opens a remote file for read

Parameters:

path - The remote path

Returns:

The file's SftpClient.CloseableHandle

Throws:

java.io.IOException - If failed to open the remote file

See Also:

open(String, Collection)

open

default SftpClient.CloseableHandle open(java.lang.String path,
                                        SftpClient.OpenMode... options)
                                 throws java.io.IOException

Opens a remote file with the specified mode(s)

Parameters:

path - The remote path

options - The desired mode - if none specified then SftpClient.OpenMode.Read is assumed

Returns:

The file's SftpClient.CloseableHandle

Throws:

java.io.IOException - If failed to open the remote file

See Also:

open(String, Collection)

open

SftpClient.CloseableHandle open(java.lang.String path,
                                java.util.Collection<SftpClient.OpenMode> options)
                         throws java.io.IOException

Opens a remote file with the specified mode(s)

Parameters:

path - The remote path

options - The desired mode - if none specified then SftpClient.OpenMode.Read is assumed

Returns:

The file's SftpClient.CloseableHandle

Throws:

java.io.IOException - If failed to open the remote file

close

void close(SftpClient.Handle handle)
    throws java.io.IOException

Close the handle obtained from one of the open methods

Parameters:

handle - The Handle to close

Throws:

java.io.IOException - If failed to execute

remove

void remove(java.lang.String path)
     throws java.io.IOException

Parameters:

path - The remote path to remove

Throws:

java.io.IOException - If failed to execute

rename

default void rename(java.lang.String oldPath,
                    java.lang.String newPath)
             throws java.io.IOException

Throws:

java.io.IOException

rename

default void rename(java.lang.String oldPath,
                    java.lang.String newPath,
                    SftpClient.CopyMode... options)
             throws java.io.IOException

Throws:

java.io.IOException

rename

void rename(java.lang.String oldPath,
            java.lang.String newPath,
            java.util.Collection<SftpClient.CopyMode> options)
     throws java.io.IOException

Throws:

java.io.IOException

read

default int read(SftpClient.Handle handle,
                 long fileOffset,
                 byte[] dst)
          throws java.io.IOException

Reads data from the open (file) handle

Parameters:

handle - The file SftpClient.Handle to read from

fileOffset - The file offset to read from

dst - The destination buffer

Returns:

Number of read bytes - -1 if EOF reached

Throws:

java.io.IOException - If failed to read the data

See Also:

read(Handle, long, byte[], int, int)

read

default int read(SftpClient.Handle handle,
                 long fileOffset,
                 byte[] dst,
                 java.util.concurrent.atomic.AtomicReference<java.lang.Boolean> eofSignalled)
          throws java.io.IOException

Reads data from the open (file) handle

Parameters:

handle - The file SftpClient.Handle to read from

fileOffset - The file offset to read from

dst - The destination buffer

eofSignalled - If not null then upon return holds a value indicating whether EOF was reached due to the read. If null indicator value then this indication is not available

Returns:

Number of read bytes - -1 if EOF reached

Throws:

java.io.IOException - If failed to read the data

See Also:

read(Handle, long, byte[], int, int, AtomicReference), SFTP v6 - section 9.3

read

default int read(SftpClient.Handle handle,
                 long fileOffset,
                 byte[] dst,
                 int dstOffset,
                 int len)
          throws java.io.IOException

Throws:

java.io.IOException

read

int read(SftpClient.Handle handle,
         long fileOffset,
         byte[] dst,
         int dstOffset,
         int len,
         java.util.concurrent.atomic.AtomicReference<java.lang.Boolean> eofSignalled)
  throws java.io.IOException

Reads data from the open (file) handle

Parameters:

handle - The file SftpClient.Handle to read from

fileOffset - The file offset to read from

dst - The destination buffer

dstOffset - Offset in destination buffer to place the read data

len - Available destination buffer size to read

eofSignalled - If not null then upon return holds a value indicating whether EOF was reached due to the read. If null indicator value then this indication is not available

Returns:

Number of read bytes - -1 if EOF reached

Throws:

java.io.IOException - If failed to read the data

See Also:

SFTP v6 - section 9.3

write

default void write(SftpClient.Handle handle,
                   long fileOffset,
                   byte[] src)
            throws java.io.IOException

Throws:

java.io.IOException

write

void write(SftpClient.Handle handle,
           long fileOffset,
           byte[] src,
           int srcOffset,
           int len)
    throws java.io.IOException

Write data to (open) file handle

Parameters:

handle - The file SftpClient.Handle

fileOffset - Zero-based offset to write in file

src - Data buffer

srcOffset - Offset of valid data in buffer

len - Number of bytes to write

Throws:

java.io.IOException - If failed to write the data

mkdir

void mkdir(java.lang.String path)
    throws java.io.IOException

Create remote directory

Parameters:

path - Remote directory path

Throws:

java.io.IOException - If failed to execute

rmdir

void rmdir(java.lang.String path)
    throws java.io.IOException

Remove remote directory

Parameters:

path - Remote directory path

Throws:

java.io.IOException - If failed to execute

openDir

SftpClient.CloseableHandle openDir(java.lang.String path)
                            throws java.io.IOException

Obtain a handle for a directory

Parameters:

path - Remote directory path

Returns:

The associated directory SftpClient.Handle

Throws:

java.io.IOException - If failed to execute

readDir

default java.util.List<SftpClient.DirEntry> readDir(SftpClient.Handle handle)
                                             throws java.io.IOException

Parameters:

handle - Directory SftpClient.Handle to read from

Returns:

A List of entries - null to indicate no more entries Note: the list may be incomplete since the client and server have some internal imposed limit on the number of entries they can process. Therefore several calls to this method may be required (until null). In order to iterate over all the entries use readDir(String)

Throws:

java.io.IOException - If failed to access the remote site

readDir

java.util.List<SftpClient.DirEntry> readDir(SftpClient.Handle handle,
                                            java.util.concurrent.atomic.AtomicReference<java.lang.Boolean> eolIndicator)
                                     throws java.io.IOException

Parameters:

handle - Directory SftpClient.Handle to read from

eolIndicator - An indicator that can be used to get information whether end of list has been reached - ignored if null. Upon return, set value indicates whether all entries have been exhausted - a null value means that this information cannot be provided and another call to readDir is necessary in order to verify that no more entries are pending

Returns:

A List of entries - null to indicate no more entries

Throws:

java.io.IOException - If failed to access the remote site

See Also:

SFTP v6 - section 9.4

listDir

java.lang.Iterable<SftpClient.DirEntry> listDir(SftpClient.Handle handle)
                                         throws java.io.IOException

Parameters:

handle - A directory SftpClient.Handle

Returns:

An Iterable that can be used to iterate over all the directory entries (like readDir(String)). Note: the iterable instance is not re-usable - i.e., files can be iterated only once

Throws:

java.io.IOException - If failed to access the directory

canonicalPath

java.lang.String canonicalPath(java.lang.String path)
                        throws java.io.IOException

The effective "normalized" remote path

Parameters:

path - The requested path - may be relative, and/or contain dots - e.g., ".", "..", "./foo", "../bar"

Returns:

The effective "normalized" remote path

Throws:

java.io.IOException - If failed to execute

stat

SftpClient.Attributes stat(java.lang.String path)
                    throws java.io.IOException

Retrieve remote path meta-data - follow symbolic links if encountered

Parameters:

path - The remote path

Returns:

The associated SftpClient.Attributes

Throws:

java.io.IOException - If failed to execute

lstat

SftpClient.Attributes lstat(java.lang.String path)
                     throws java.io.IOException

Retrieve remote path meta-data - do not follow symbolic links

Parameters:

path - The remote path

Returns:

The associated SftpClient.Attributes

Throws:

java.io.IOException - If failed to execute

stat

SftpClient.Attributes stat(SftpClient.Handle handle)
                    throws java.io.IOException

Retrieve file/directory handle meta-data

Parameters:

handle - The SftpClient.Handle obtained via one of the open calls

Returns:

The associated SftpClient.Attributes

Throws:

java.io.IOException - If failed to execute

setStat

void setStat(java.lang.String path,
             SftpClient.Attributes attributes)
      throws java.io.IOException

Update remote node meta-data

Parameters:

path - The remote path

attributes - The SftpClient.Attributes to update

Throws:

java.io.IOException - If failed to execute

setStat

void setStat(SftpClient.Handle handle,
             SftpClient.Attributes attributes)
      throws java.io.IOException

Update remote node meta-data

Parameters:

handle - The SftpClient.Handle obtained via one of the open calls

attributes - The SftpClient.Attributes to update

Throws:

java.io.IOException - If failed to execute

readLink

java.lang.String readLink(java.lang.String path)
                   throws java.io.IOException

Retrieve target of a link

Parameters:

path - Remote path that represents a link

Returns:

The link target

Throws:

java.io.IOException - If failed to execute

symLink

default void symLink(java.lang.String linkPath,
                     java.lang.String targetPath)
              throws java.io.IOException

Create symbolic link

Parameters:

linkPath - The link location

targetPath - The referenced target by the link

Throws:

java.io.IOException - If failed to execute

See Also:

link(String, String, boolean)

link

void link(java.lang.String linkPath,
          java.lang.String targetPath,
          boolean symbolic)
   throws java.io.IOException

Create a link

Parameters:

linkPath - The link location

targetPath - The referenced target by the link

symbolic - If true then make this a symbolic link, otherwise a hard one

Throws:

java.io.IOException - If failed to execute

lock

void lock(SftpClient.Handle handle,
          long offset,
          long length,
          int mask)
   throws java.io.IOException

Throws:

java.io.IOException

unlock

void unlock(SftpClient.Handle handle,
            long offset,
            long length)
     throws java.io.IOException

Throws:

java.io.IOException

openRemotePathChannel

default java.nio.channels.FileChannel openRemotePathChannel(java.lang.String path,
                                                            java.nio.file.OpenOption... options)
                                                     throws java.io.IOException

Throws:

java.io.IOException

openRemotePathChannel

default java.nio.channels.FileChannel openRemotePathChannel(java.lang.String path,
                                                            java.util.Collection<? extends java.nio.file.OpenOption> options)
                                                     throws java.io.IOException

Throws:

java.io.IOException

openRemoteFileChannel

default java.nio.channels.FileChannel openRemoteFileChannel(java.lang.String path,
                                                            SftpClient.OpenMode... modes)
                                                     throws java.io.IOException

Throws:

java.io.IOException

openRemoteFileChannel

java.nio.channels.FileChannel openRemoteFileChannel(java.lang.String path,
                                                    java.util.Collection<SftpClient.OpenMode> modes)
                                             throws java.io.IOException

Opens an FileChannel on the specified remote path

Parameters:

path - The remote path

modes - The access mode(s) - if null/empty then the DEFAULT_CHANNEL_MODES are used

Returns:

The open FileChannel - Note: do not close this owner client instance until the channel is no longer needed since it uses the client for providing the channel's functionality.

Throws:

java.io.IOException - If failed to open the channel

See Also:

Channels.newInputStream(java.nio.channels.ReadableByteChannel), Channels.newOutputStream(java.nio.channels.WritableByteChannel)

readDir

java.lang.Iterable<SftpClient.DirEntry> readDir(java.lang.String path)
                                         throws java.io.IOException

Parameters:

path - The remote directory path

Returns:

An Iterable that can be used to iterate over all the directory entries (unlike readDir(Handle))

Throws:

java.io.IOException - If failed to access the remote site

See Also:

readDir(Handle)

read

default java.io.InputStream read(java.lang.String path)
                          throws java.io.IOException

Throws:

java.io.IOException

read

default java.io.InputStream read(java.lang.String path,
                                 int bufferSize)
                          throws java.io.IOException

Throws:

java.io.IOException

read

default java.io.InputStream read(java.lang.String path,
                                 SftpClient.OpenMode... mode)
                          throws java.io.IOException

Throws:

java.io.IOException

read

default java.io.InputStream read(java.lang.String path,
                                 int bufferSize,
                                 SftpClient.OpenMode... mode)
                          throws java.io.IOException

Throws:

java.io.IOException

read

default java.io.InputStream read(java.lang.String path,
                                 java.util.Collection<SftpClient.OpenMode> mode)
                          throws java.io.IOException

Throws:

java.io.IOException

read

java.io.InputStream read(java.lang.String path,
                         int bufferSize,
                         java.util.Collection<SftpClient.OpenMode> mode)
                  throws java.io.IOException

Read a remote file's data via an input stream

Parameters:

path - The remote file path

bufferSize - The internal read buffer size

mode - The remote file SftpClient.OpenModes

Returns:

An InputStream for reading the remote file data

Throws:

java.io.IOException - If failed to execute

write

default java.io.OutputStream write(java.lang.String path)
                            throws java.io.IOException

Throws:

java.io.IOException

write

default java.io.OutputStream write(java.lang.String path,
                                   int bufferSize)
                            throws java.io.IOException

Throws:

java.io.IOException

write

default java.io.OutputStream write(java.lang.String path,
                                   SftpClient.OpenMode... mode)
                            throws java.io.IOException

Throws:

java.io.IOException

write

default java.io.OutputStream write(java.lang.String path,
                                   int bufferSize,
                                   SftpClient.OpenMode... mode)
                            throws java.io.IOException

Throws:

java.io.IOException

write

default java.io.OutputStream write(java.lang.String path,
                                   java.util.Collection<SftpClient.OpenMode> mode)
                            throws java.io.IOException

Throws:

java.io.IOException

write

java.io.OutputStream write(java.lang.String path,
                           int bufferSize,
                           java.util.Collection<SftpClient.OpenMode> mode)
                    throws java.io.IOException

Write to a remote file via an output stream

Parameters:

path - The remote file path

bufferSize - The internal write buffer size

mode - The remote file SftpClient.OpenModes

Returns:

An OutputStream for writing the data

Throws:

java.io.IOException - If failed to execute

getExtension

<E extends SftpClientExtension> E getExtension(java.lang.Class<? extends E> extensionType)

Type Parameters:

E - The generic extension type

Parameters:

extensionType - The extension type

Returns:

The extension instance - Note: it is up to the caller to invoke OptionalFeature.isSupported() - null if this extension type is not implemented by the client

See Also:

getServerExtensions()

getExtension

SftpClientExtension getExtension(java.lang.String extensionName)

Parameters:

extensionName - The extension name

Returns:

The extension instance - Note: it is up to the caller to invoke OptionalFeature.isSupported() - null if this extension type is not implemented by the client

See Also:

getServerExtensions()