The Secure File Transfer Protocol (SFTP) is a network protocol which allows you to transfer files between two computers on the Internet over any reliable stream.

The SFTP protocol requires a secure channel, such as SSH, as well as visibility to a client's identity throughout the SFTP session.

Spring Integration supports sending and receiving files over SFTP by providing two types of clients - Inbound Channel Adapters and Outbound Channel Adapters as well as convenient namespace configuration to define these clients.

xmlns:sftp="http://www.springframework.org/schema/integration/sftp"
xsi:schemaLocation="http://www.springframework.org/schema/integration/sftp 
	http://www.springframework.org/schema/integration/sftp/spring-integration-sftp-2.0.xsd" 

Before configuring SFTP adapters you must configure an SFTP Session Factory. You can configure the SFTP Session Factory via a regular bean definition: Below is a basic configuration:

<beans:bean id="sftpSessionFactory" class="org.springframework.integration.sftp.session.DefaultSftpSessionFactory">
		<beans:property name="host" value="loclahost"/>
		<beans:property name="privateKey" value="classpath:META-INF/keys/sftpTest"/>
		<beans:property name="privateKeyPassphrase" value="springIntegration"/>
		<beans:property name="port" value="22"/>
		<beans:property name="user" value="kermit"/>
</beans:bean>

Every time an adapter requests a session object from its SessionFactory the session is returned from a session pool maintained by a caching wrapper around the factory. A Session in the session pool might go stale (if it has been disconnected by the server due to inactivity) so the SessionFactory will perform validation to make sure that it never returns a stale session to the adapter. If a stale session was encountered, it will be removed from the pool, and a new one will be created.

	Note
	If you experience connectivity problems and would like to trace Session creation as well as see which Sessions are polled you may enable it by setting the logger to TRACE level (e.g., log4j.category.org.springframework.integration.file=TRACE)

Now all you need to do is inject this SFTP Session Factory into your adapters.

	Note
	A more practical way to provide values for the SFTP Session Factory would be via Spring's property placeholder support (http://static.springsource.org/spring/docs/3.0.x/spring-framework-reference/html/beans.html#beans-factory-placeholderconfigurer)

The SFTP Inbound Channel Adapter is a special listener that will connect to the server and listen for the remote directory events (e.g., new file created) at which point it will initiate a file transfer.

<sftp:inbound-channel-adapter id="sftpAdapterAutoCreate"
  			session-factory="sftpSessionFactory"
			channel="requestChannel"
			filename-pattern="*.txt"
			remote-directory="/foo/bar"
			local-directory="file:target/foo"
			auto-create-local-directory="true"
			delete-remote-files="false">
		<poller fixed-rate="1000"/>
</sftp:inbound-channel-adapter>

As you can see from the configuration above you can configure the SFTP Inbound Channel Adapter via the inbound-channel-adapter element while also providing values for various attributes such as local-directory - where files are going to be transferred TO and remote-directory - the remote source directory where files are going to be transferred FROM - as well as other attributes including a session-factory reference to the bean we configured earlier.

Some times file filtering based on the simple pattern specified via filename-pattern attribute might not be sufficient. If this is the case, you can use the filename-regex attribute to specify a Regular Expression (e.g. filename-regex=".*\.test$"). And of course if you need complete control you can use the filter attribute to provide a reference to a custom implementation of the org.springframework.integration.file.filters.FileListFilter - a strategy interface for filtering a list of files.

Please refer to the schema for more detail on these attributes.

It is also important to understand that SFTP Inbound Channel Adapter is a Polling Consumer and therefore you must configure a poller (either a global default or a local sub-element). Once the file has been transferred to a local directory, a Message with java.io.File as its payload type will be generated and sent to the channel identified by the channel attribute.

More on File Filtering and Large Files

Some times a file that just appeared in the monitored (remote) directory is not complete. Typically such a file will be written with some temporary extension (e.g., foo.txt.writing) and then renamed after the writing process completes. As a user in most cases you are only interested in files that are complete and would like to filter only those files. To handle these scenarios, use filtering support provided via the filename-pattern, filename-regex and filter attributes. If you need a custom filter implementation simply include a reference in your adapter via the filter attribute.

<int-sftp:inbound-channel-adapter id="sftpInbondAdapter"
			channel="receiveChannel" 
			session-factory="sftpSessionFactory"
			filter="customFilter"
			local-directory="file:/local-test-dir"
			remote-directory="/remote-test-dir">
		<int:poller fixed-rate="1000" max-messages-per-poll="10" task-executor="executor"/>
</int-sftp:inbound-channel-adapter>
	
<bean id="customFilter" class="org.foo.CustomFilter"/>

The SFTP Outbound Channel Adapteris a special MessageHandler that will connect to the remote directory and will initiate a file transfer for every file it will receive as the payload of an incoming Message. It also supports several representations of the File so you are not limited to the File object. Similar to the FTP outbound adapter, the SFTP Outbound Channel Adapter supports the following payloads: 1) java.io.File - the actual file object; 2) byte[] - byte array that represents the file contents; 3) java.lang.String - text that represents the file contents.

<int-sftp:outbound-channel-adapter id="sftpOutboundAdapter"
				session-factory="sftpSessionFactory"
				channel="inputChannel"
				charset="UTF-8"
				remote-directory="foo/bar"
				remote-filename-generator-expression="payload.getName() + '-foo'"/>

As you can see from the configuration above you can configure the SFTP Outbound Channel Adapter via the outbound-channel-adapter element. Please refer to the schema for more detail on these attributes.

SpEL and the SFTP Outbound Adapter

As with many other components in Spring Integration, you can benefit from the Spring Expression Language (SpEL) support when configuring an SFTP Outbound Channel Adapter, by specifying two attributes remote-directory-expression and remote-filename-generator-expression (see above). The expression evaluation context will have the Message as its root object, thus allowing you to provide expressions which can dynamically compute the file name or the existing directory path based on the data in the Message (either from 'payload' or 'headers'). In the example above we are defining the remote-filename-generator-expression attribute with an expression value that computes the file name based on its original name while also appending a suffix: '-foo'.

Since we use JSch libraries (http://www.jcraft.com/jsch/) to provide SFTP support, at times you may require more information from the JSch API itself, especially if something is not working properly (e.g., Authentication exceptions). Unfortunately JSch does not use commons-logging but instead relies on custom implementations of their com.jcraft.jsch.Logger interface. As of Spring Integration 2.0.1, we have implemented this interface. So, now all you need to do to enable JSch logging is to configure your logger the way you usually do. For example, here is valid configuration of a logger using Log4J.

log4j.category.com.jcraft.jsch=DEBUG

One of the optimizations implemented by the SFTP adapters is session caching. Similar to JDBC pooling of Connections, the SFTP Adapters maintain a pool of Sessions by default. However there are times when this behavior is not desired (e.g., security etc.). To disable session caching you can set the cache-sessions attribute to false (the default value is true).

<int-sftp:inbound-channel-adapter id="ftpInbound"
				channel="sftpChannel" 
				. . .
				cache-sessions="false"
                . . . 
</int-sftp:inbound-channel-adapter>

The same attribute can also be used with Outbound Channel Adapters.