Support trailing comment in DTD declaration in XML config

Prior to this commit, Spring failed to determine that an XML config file
was DTD-based if the DTD declaration was followed by a comment.

This commit fixes this by modifying the consumeCommentTokens(String)
algorithm in XmlValidationModeDetector so that both leading and trailing
comments are properly consumed without losing any XML content.

Closes gh-23605

spring-core/src/main/java/org/springframework/util/xml/XmlValidationModeDetector.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2019 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.
@@ -30,6 +30,7 @@ import org.springframework.util.StringUtils;
  *
  * @author Rob Harrop
  * @author Juergen Hoeller
+ * @author Sam Brannen
  * @since 2.0
  */
 public class XmlValidationModeDetector {
@@ -142,20 +143,27 @@ public class XmlValidationModeDetector {
 	}
 
 	/**
-	 * Consumes all the leading comment data in the given String and returns the remaining content, which
-	 * may be empty since the supplied content might be all comment data. For our purposes it is only important
-	 * to strip leading comment content on a line since the first piece of non comment content will be either
-	 * the DOCTYPE declaration or the root element of the document.
+	 * Consume all leading and trailing comments in the given String and return
+	 * the remaining content, which may be empty since the supplied content might
+	 * be all comment data.
 	 */
 	@Nullable
 	private String consumeCommentTokens(String line) {
-		if (!line.contains(START_COMMENT) && !line.contains(END_COMMENT)) {
+		int indexOfStartComment = line.indexOf(START_COMMENT);
+		if (indexOfStartComment == -1 && !line.contains(END_COMMENT)) {
 			return line;
 		}
+
+		String result = "";
 		String currLine = line;
+		if (indexOfStartComment >= 0) {
+			result = line.substring(0, indexOfStartComment);
+			currLine = line.substring(indexOfStartComment);
+		}
+
 		while ((currLine = consume(currLine)) != null) {
 			if (!this.inComment && !currLine.trim().startsWith(START_COMMENT)) {
-				return currLine;
+				return result + currLine;
 			}
 		}
 		return null;

spring-core/src/test/java/org/springframework/util/xml/XmlValidationModeDetectorTests.java

@@ -0,0 +1,62 @@
+/*
+ * Copyright 2002-2019 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
+ *
+ *      https://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.util.xml;
+
+import java.io.InputStream;
+
+import org.junit.Test;
+
+import static org.junit.Assert.assertEquals;
+import static org.springframework.util.xml.XmlValidationModeDetector.VALIDATION_DTD;
+
+/**
+ * Unit tests for {@link XmlValidationModeDetector}.
+ *
+ * @author Sam Brannen
+ * @since 5.1.10
+ */
+public class XmlValidationModeDetectorTests {
+
+	private final XmlValidationModeDetector xmlValidationModeDetector = new XmlValidationModeDetector();
+
+
+	@Test
+	public void dtdWithTrailingComment() throws Exception {
+		dtdDetection("dtdWithTrailingComment.xml");
+	}
+
+	@Test
+	public void dtdWithLeadingComment() throws Exception {
+		dtdDetection("dtdWithLeadingComment.xml");
+	}
+
+	@Test
+	public void dtdWithCommentOnNextLine() throws Exception {
+		dtdDetection("dtdWithCommentOnNextLine.xml");
+	}
+
+	@Test
+	public void dtdWithMultipleComments() throws Exception {
+		dtdDetection("dtdWithMultipleComments.xml");
+	}
+
+	private void dtdDetection(String fileName) throws Exception {
+		InputStream inputStream = getClass().getResourceAsStream(fileName);
+		assertEquals(VALIDATION_DTD, xmlValidationModeDetector.detectValidationMode(inputStream));
+	}
+
+}

spring-core/src/test/resources/org/springframework/util/xml/dtdWithCommentOnNextLine.xml

@@ -0,0 +1,7 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN 2.0//EN" "https://www.springframework.org/dtd/spring-beans-2.0.dtd">
+<!-- comment on next line -->
+
+<beans>
+
+</beans>

spring-core/src/test/resources/org/springframework/util/xml/dtdWithLeadingComment.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- leading comment --><!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN 2.0//EN" "https://www.springframework.org/dtd/spring-beans-2.0.dtd">
+
+<beans>
+
+</beans>

spring-core/src/test/resources/org/springframework/util/xml/dtdWithMultipleComments.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!-- comment #1 -->  <!-- comment #2 -->  <!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN 2.0//EN" "https://www.springframework.org/dtd/spring-beans-2.0.dtd"><!--
+trailing  
+      comment
+-->
+
+<beans>
+
+</beans>

spring-core/src/test/resources/org/springframework/util/xml/dtdWithTrailingComment.xml

@@ -0,0 +1,6 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE beans PUBLIC "-//SPRING//DTD BEAN 2.0//EN" "https://www.springframework.org/dtd/spring-beans-2.0.dtd"><!-- trailing comment -->
+
+<beans>
+
+</beans>