GH-3372: Expose (S)FTP remoteComparator for DSL

Fixes https://github.com/spring-projects/spring-integration/issues/3372

**Cherry-pick to 5.3.x & 5.2.x**

spring-integration-file/src/main/java/org/springframework/integration/file/dsl/RemoteFileInboundChannelAdapterSpec.java

@@ -17,6 +17,7 @@
 package org.springframework.integration.file.dsl;
 
 import java.io.File;
+import java.util.Comparator;
 import java.util.LinkedHashMap;
 import java.util.Map;
 import java.util.function.Function;
@@ -282,6 +283,18 @@ public abstract class RemoteFileInboundChannelAdapterSpec<F, S extends RemoteFil
 		return _this();
 	}
 
+	/**
+	 * Set a comparator to sort the retrieved list of {@code F} (the Type that represents
+	 * the remote file) prior to applying filters and max fetch size.
+	 * @param remoteComparator the {@link Comparator} for remote files.
+	 * @return the spec.
+	 * @since 5.2.9
+	 */
+	public S remoteComparator(Comparator<F> remoteComparator) {
+		this.synchronizer.setComparator(remoteComparator);
+		return _this();
+	}
+
 	@Override
 	public Map<Object, String> getComponentsToRegister() {
 		Map<Object, String> componentsToRegister = new LinkedHashMap<>();

spring-integration-sftp/src/test/java/org/springframework/integration/sftp/dsl/SftpTests.java

@@ -20,6 +20,7 @@ import static org.assertj.core.api.Assertions.assertThat;
 
 import java.io.File;
 import java.io.InputStream;
+import java.util.Comparator;
 import java.util.List;
 import java.util.regex.Matcher;
 
@@ -76,7 +77,8 @@ public class SftpTests extends SftpTestSupport {
 								.remoteDirectory("sftpSource")
 								.regexFilter(".*\\.txt$")
 								.localFilenameExpression("#this.toUpperCase() + '.a'")
-								.localDirectory(getTargetLocalDirectory()),
+								.localDirectory(getTargetLocalDirectory())
+						.remoteComparator(Comparator.naturalOrder()),
 						e -> e.id("sftpInboundAdapter").poller(Pollers.fixedDelay(100)))
 				.channel(out)
 				.get();
@@ -86,13 +88,13 @@ public class SftpTests extends SftpTestSupport {
 		Object payload = message.getPayload();
 		assertThat(payload).isInstanceOf(File.class);
 		File file = (File) payload;
-		assertThat(file.getName()).isIn(" SFTPSOURCE1.TXT.a", "SFTPSOURCE2.TXT.a");
+		assertThat(file.getName()).isEqualTo(" SFTPSOURCE1.TXT.a");
 		assertThat(file.getAbsolutePath()).contains("localTarget");
 
 		message = out.receive(10_000);
 		assertThat(message).isNotNull();
 		file = (File) message.getPayload();
-		assertThat(file.getName()).isIn(" SFTPSOURCE1.TXT.a", "SFTPSOURCE2.TXT.a");
+		assertThat(file.getName()).isIn("SFTPSOURCE2.TXT.a");
 		assertThat(file.getAbsolutePath()).contains("localTarget");
 
 		registration.destroy();