INT-4046: Add FtpRemoteFileTemplate.ExistsMode

JIRA: https://jira.spring.io/browse/INT-4046

Since not all FTP servers provide proper `STAT` command implementation,
plus the `NLIST` doesn't work properly for directories cases, introduce the `FtpRemoteFileTemplate.ExistsMode`
to let:
* to perform `STAT` by default (previous) behavior;
* to switch to `NLIST` for `FtpRemoteFileTemplate` internal use;
* perform the full `NLIST` and `FTPClient.changeWorkingDirectory()` algorithm if needed.

* Improve (S)Ftp components to use proper `RemoteFileTemplate` for internal instantiation
* Introduce `FtpMessageHandler` to wrap `FtpRemoteFileTemplate` with the proper `NLIST` `ExistsMode`
* Cover `NLIST` switching from the `FtpOutboundChannelAdapterParser` and `FtpOutboundGatewayParser`
* Document the `FtpRemoteFileTemplate.ExistsMode`

* Fix typo in the recently introduced `RemoteFileOperations.getSession()` method name
* Add JavaDoc to `Session.exists()`
* Add `NLIST` support for the `FtpSession.exists()` to meet the API requirements

**Cherry-pick to 4.2.x and 4.1.x**

Addressing PR comments

Doc Polishing

Conflicts:
	spring-integration-file/src/main/java/org/springframework/integration/file/remote/AbstractRemoteFileStreamingMessageSource.java
	spring-integration-file/src/main/java/org/springframework/integration/file/remote/RemoteFileOperations.java
	spring-integration-file/src/main/java/org/springframework/integration/file/remote/RemoteFileTemplate.java
	spring-integration-ftp/src/main/java/org/springframework/integration/ftp/session/FtpSession.java
	spring-integration-ftp/src/test/java/org/springframework/integration/ftp/config/FtpOutboundChannelAdapterParserTests.java
	spring-integration-sftp/src/main/java/org/springframework/integration/sftp/gateway/SftpOutboundGateway.java
	spring-integration-sftp/src/main/java/org/springframework/integration/sftp/outbound/SftpMessageHandler.java
	spring-integration-sftp/src/main/java/org/springframework/integration/sftp/outbound/package-info.java

spring-integration-sftp/src/main/java/org/springframework/integration/sftp/config/SftpOutboundChannelAdapterParser.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2014 the original author or authors.
+ * Copyright 2014-2016 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,6 +17,7 @@ package org.springframework.integration.sftp.config;
 
 import org.springframework.integration.file.config.RemoteFileOutboundChannelAdapterParser;
 import org.springframework.integration.file.remote.RemoteFileOperations;
+import org.springframework.integration.sftp.outbound.SftpMessageHandler;
 import org.springframework.integration.sftp.session.SftpRemoteFileTemplate;
 
 /**
@@ -28,6 +29,12 @@ import org.springframework.integration.sftp.session.SftpRemoteFileTemplate;
  */
 public class SftpOutboundChannelAdapterParser extends RemoteFileOutboundChannelAdapterParser {
 
+
+	@Override
+	protected Class<?> handlerClass() {
+		return SftpMessageHandler.class;
+	}
+
 	@Override
 	protected Class<? extends RemoteFileOperations<?>> getTemplateClass() {
 		return SftpRemoteFileTemplate.class;

spring-integration-sftp/src/main/java/org/springframework/integration/sftp/gateway/SftpOutboundGateway.java

@@ -27,6 +27,7 @@ import org.springframework.integration.file.remote.RemoteFileTemplate;
 import org.springframework.integration.file.remote.gateway.AbstractRemoteFileOutboundGateway;
 import org.springframework.integration.file.remote.session.SessionFactory;
 import org.springframework.integration.sftp.session.SftpFileInfo;
+import org.springframework.integration.sftp.session.SftpRemoteFileTemplate;
 
 import com.jcraft.jsch.ChannelSftp.LsEntry;
 
@@ -47,7 +48,7 @@ public class SftpOutboundGateway extends AbstractRemoteFileOutboundGateway<LsEnt
 	 */
 	public SftpOutboundGateway(SessionFactory<LsEntry> sessionFactory,
 			MessageSessionCallback<LsEntry, ?> messageSessionCallback) {
-		super(sessionFactory, messageSessionCallback);
+		this(new SftpRemoteFileTemplate(sessionFactory), messageSessionCallback);
 	}
 
 	/**
@@ -69,7 +70,7 @@ public class SftpOutboundGateway extends AbstractRemoteFileOutboundGateway<LsEnt
 	 * @param expression the filename expression.
 	 */
 	public SftpOutboundGateway(SessionFactory<LsEntry> sessionFactory, String command, String expression) {
-		super(sessionFactory, command, expression);
+		this(new SftpRemoteFileTemplate(sessionFactory), command, expression);
 	}
 
 	/**

spring-integration-sftp/src/main/java/org/springframework/integration/sftp/outbound/SftpMessageHandler.java

@@ -0,0 +1,64 @@
+/*
+ * Copyright 2016 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.springframework.integration.sftp.outbound;
+
+import org.springframework.integration.file.remote.handler.FileTransferringMessageHandler;
+import org.springframework.integration.file.remote.session.SessionFactory;
+import org.springframework.integration.file.support.FileExistsMode;
+import org.springframework.integration.sftp.session.SftpRemoteFileTemplate;
+
+import com.jcraft.jsch.ChannelSftp.LsEntry;
+
+/**
+ * Subclass of {@link FileTransferringMessageHandler} for SFTP.
+ *
+ * @author Gary Russell
+ * @since 4.3
+ *
+ */
+public class SftpMessageHandler extends FileTransferringMessageHandler<LsEntry> {
+
+	/**
+	 * @param remoteFileTemplate the template.
+	 * @see FileTransferringMessageHandler#FileTransferringMessageHandler
+	 * (org.springframework.integration.file.remote.RemoteFileTemplate)
+	 */
+	public SftpMessageHandler(SftpRemoteFileTemplate remoteFileTemplate) {
+		super(remoteFileTemplate);
+	}
+
+	/**
+	 *
+	 * @param remoteFileTemplate the template.
+	 * @param mode the file exists mode.
+	 * @see FileTransferringMessageHandler#FileTransferringMessageHandler
+	 * (org.springframework.integration.file.remote.RemoteFileTemplate, FileExistsMode)
+	 */
+	public SftpMessageHandler(SftpRemoteFileTemplate remoteFileTemplate, FileExistsMode mode) {
+		super(remoteFileTemplate, mode);
+	}
+
+	/**
+	 * @param sessionFactory the session factory.
+	 * @see FileTransferringMessageHandler#FileTransferringMessageHandler
+	 * (SessionFactory)
+	 */
+	public SftpMessageHandler(SessionFactory<LsEntry> sessionFactory) {
+		this(new SftpRemoteFileTemplate(sessionFactory));
+	}
+
+}

spring-integration-sftp/src/main/java/org/springframework/integration/sftp/outbound/package-info.java

@@ -0,0 +1,4 @@
+/**
+ * Provides classes for the SFTP outbound channel adapter.
+ */
+package org.springframework.integration.sftp.outbound;

spring-integration-sftp/src/main/java/org/springframework/integration/sftp/session/SftpRemoteFileTemplate.java

@@ -25,7 +25,6 @@ import org.springframework.integration.file.remote.session.SessionFactory;
 
 import com.jcraft.jsch.ChannelSftp;
 import com.jcraft.jsch.ChannelSftp.LsEntry;
-import com.jcraft.jsch.SftpException;
 
 /**
  * SFTP version of {@code RemoteFileTemplate} providing type-safe access to
@@ -57,22 +56,4 @@ public class SftpRemoteFileTemplate extends RemoteFileTemplate<LsEntry> {
 		});
 	}
 
-	@Override
-	public boolean exists(final String path) {
-		return executeWithClient(new ClientCallback<ChannelSftp, Boolean>() {
-
-			@Override
-			public Boolean doWithClient(ChannelSftp client) {
-				try {
-					return client.stat(path) != null;
-				}
-				catch (SftpException e) {
-					return false;
-				}
-			}
-		});
-	}
-
-
-
 }