GH-3395: Fix XML expression default for ARFOGateway

Resolves #3395

Outbound remote file gateway parser requires `expression` even though some
commands don't need or use it.

* Propagate the empty string for `expression` attribute
* Fix default (`payload`) expression logic in the `AbstractRemoteFileOutboundGateway`

**Cherry-pick to `5.3.x`**

Author: garyrussell

spring-integration-file/src/main/java/org/springframework/integration/file/config/AbstractRemoteFileOutboundGatewayParser.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2020 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.
@@ -58,9 +58,7 @@ protected BeanDefinitionBuilder parseHandler(Element element, ParserContext pars
 		}
 		else {
 			builder.addConstructorArgValue(element.getAttribute("command"));
-			if (element.hasAttribute(EXPRESSION_ATTRIBUTE)) {
-				builder.addConstructorArgValue(element.getAttribute(EXPRESSION_ATTRIBUTE));
-			}
+			builder.addConstructorArgValue(element.getAttribute(EXPRESSION_ATTRIBUTE));
 		}
 		IntegrationNamespaceUtils.setValueIfAttributeDefined(builder, element, "command-options", "options");
 		IntegrationNamespaceUtils.setValueIfAttributeDefined(builder, element, "reply-timeout", "sendTimeout");

spring-integration-file/src/main/java/org/springframework/integration/file/remote/gateway/AbstractRemoteFileOutboundGateway.java

@@ -187,21 +187,23 @@ public AbstractRemoteFileOutboundGateway(RemoteFileTemplate<F> remoteFileTemplat
 	 * 'get' etc), and an expression to determine the filename.
 	 * @param remoteFileTemplate the remote file template.
 	 * @param command the command.
-	 * @param expression the filename expression.
+	 * @param expressionArg the filename expression.
 	 */
 	public AbstractRemoteFileOutboundGateway(RemoteFileTemplate<F> remoteFileTemplate, Command command,
-			@Nullable String expression) {
+			@Nullable String expressionArg) {
 
 		Assert.notNull(remoteFileTemplate, "'remoteFileTemplate' cannot be null");
 		this.remoteFileTemplate = remoteFileTemplate;
 		this.command = command;
-		if (expression == null) {
-			Assert.state(Command.LS.equals(this.command)
-							|| Command.NLST.equals(this.command)
-							|| Command.PUT.equals(this.command)
-							|| Command.MPUT.equals(this.command),
-					"Only LS, NLST, PUT and MPUT commands can rely on the working directory.\n" +
-							"All other commands must be supplied with the filename expression");
+		String expression = expressionArg;
+		boolean expressionNeeded = !(Command.LS.equals(this.command)
+				|| Command.NLST.equals(this.command)
+				|| Command.PUT.equals(this.command)
+				|| Command.MPUT.equals(this.command));
+		if (!StringUtils.hasText(expression) && expressionNeeded) {
+			expression = "payload";
+		}
+		if (!StringUtils.hasText(expression)) {
 			this.fileNameProcessor = null;
 		}
 		else {

spring-integration-sftp/src/test/java/org/springframework/integration/sftp/config/SftpOutboundGatewayParserTests-context.xml

@@ -113,4 +113,26 @@
 
 	<int:channel id="outbound"/>
 
+	<int-sftp:outbound-gateway id="noExpressionLS"
+		request-channel="nullChannel"
+		reply-channel="nullChannel"
+		session-factory="sf"
+		remote-directory="."
+		command="ls" />
+
+	<int-sftp:outbound-gateway id="noExpressionPUT"
+		request-channel="nullChannel"
+		reply-channel="nullChannel"
+		session-factory="sf"
+		remote-directory="."
+		command="put" />
+
+	<int-sftp:outbound-gateway id="noExpressionGET"
+		request-channel="nullChannel"
+		reply-channel="nullChannel"
+		session-factory="sf"
+		remote-directory="."
+		local-directory="."
+		command="get" />
+
 </beans>

spring-integration-sftp/src/test/java/org/springframework/integration/sftp/config/SftpOutboundGatewayParserTests.java

@@ -69,6 +69,15 @@ public class SftpOutboundGatewayParserTests {
 	@Autowired
 	AbstractEndpoint advised;
 
+	@Autowired
+	AbstractEndpoint noExpressionLS;
+
+	@Autowired
+	AbstractEndpoint noExpressionPUT;
+
+	@Autowired
+	AbstractEndpoint noExpressionGET;
+
 	@Autowired
 	FileNameGenerator generator;
 
@@ -165,6 +174,14 @@ public void advised() {
 		assertThat(adviceCalled).isEqualTo(1);
 	}
 
+	@Test
+	void noExpression() {
+		assertThat(TestUtils.getPropertyValue(this.noExpressionLS, "handler.fileNameProcessor")).isNull();
+		assertThat(TestUtils.getPropertyValue(this.noExpressionPUT, "handler.fileNameProcessor")).isNull();
+		assertThat(TestUtils.getPropertyValue(this.noExpressionGET,
+				"handler.fileNameProcessor.expression.expression")).isEqualTo("payload");
+	}
+
 	public static class FooAdvice extends AbstractRequestHandlerAdvice {
 
 		@Override