YamlProcessor embraces SnakeYAML 1.18+ duplicate key handling

Includes removal of StrictMapAppenderConstructor for compatibility with SnakeYAML 1.21.

Issue: SPR-16791

spring-beans/src/main/java/org/springframework/beans/factory/config/YamlMapFactoryBean.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -64,6 +64,8 @@ import org.springframework.lang.Nullable;
  * Note that the value of "foo" in the first document is not simply replaced
  * with the value in the second, but its nested values are merged.
  *
+ * <p>Requires SnakeYAML 1.18 or higher, as of Spring Framework 5.0.6.
+ *
  * @author Dave Syer
  * @author Juergen Hoeller
  * @since 4.1

spring-beans/src/main/java/org/springframework/beans/factory/config/YamlProcessor.java

@@ -18,7 +18,6 @@ package org.springframework.beans.factory.config;
 
 import java.io.IOException;
 import java.io.Reader;
-import java.util.AbstractMap;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
@@ -26,14 +25,11 @@ import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.Properties;
-import java.util.Set;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
+import org.yaml.snakeyaml.LoaderOptions;
 import org.yaml.snakeyaml.Yaml;
-import org.yaml.snakeyaml.constructor.Constructor;
-import org.yaml.snakeyaml.nodes.MappingNode;
-import org.yaml.snakeyaml.parser.ParserException;
 import org.yaml.snakeyaml.reader.UnicodeReader;
 
 import org.springframework.core.CollectionFactory;
@@ -45,6 +41,8 @@ import org.springframework.util.StringUtils;
 /**
  * Base class for YAML factories.
  *
+ * <p>Requires SnakeYAML 1.18 or higher, as of Spring Framework 5.0.6.
+ *
  * @author Dave Syer
  * @author Juergen Hoeller
  * @since 4.1
@@ -144,9 +142,14 @@ public abstract class YamlProcessor {
 
 	/**
 	 * Create the {@link Yaml} instance to use.
+	 * <p>The default implementation sets the "allowDuplicateKeys" flag to {@code false},
+	 * enabling built-in duplicate key handling in SnakeYAML 1.18+.
+	 * @see LoaderOptions#setAllowDuplicateKeys(boolean)
 	 */
 	protected Yaml createYaml() {
-		return new Yaml(new StrictMapAppenderConstructor());
+		LoaderOptions options = new LoaderOptions();
+		options.setAllowDuplicateKeys(false);
+		return new Yaml(options);
 	}
 
 	private boolean process(MatchCallback callback, Yaml yaml, Resource resource) {
@@ -390,45 +393,4 @@ public abstract class YamlProcessor {
 		FIRST_FOUND
 	}
 
-
-	/**
-	 * A specialized {@link Constructor} that checks for duplicate keys.
-	 */
-	protected static class StrictMapAppenderConstructor extends Constructor {
-
-		// Declared as public for use in subclasses
-		public StrictMapAppenderConstructor() {
-			super();
-		}
-
-		@Override
-		protected Map<Object, Object> constructMapping(MappingNode node) {
-			try {
-				return super.constructMapping(node);
-			}
-			catch (IllegalStateException ex) {
-				throw new ParserException("while parsing MappingNode",
-						node.getStartMark(), ex.getMessage(), node.getEndMark());
-			}
-		}
-
-		@Override
-		protected Map<Object, Object> createDefaultMap() {
-			final Map<Object, Object> delegate = super.createDefaultMap();
-			return new AbstractMap<Object, Object>() {
-				@Override
-				public Object put(Object key, Object value) {
-					if (delegate.containsKey(key)) {
-						throw new IllegalStateException("Duplicate key: " + key);
-					}
-					return delegate.put(key, value);
-				}
-				@Override
-				public Set<Entry<Object, Object>> entrySet() {
-					return delegate.entrySet();
-				}
-			};
-		}
-	}
-
 }

spring-beans/src/main/java/org/springframework/beans/factory/config/YamlPropertiesFactoryBean.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -74,6 +74,8 @@ import org.springframework.lang.Nullable;
  * servers[1]=foo.bar.com
  * </pre>
  *
+ * <p>Requires SnakeYAML 1.18 or higher, as of Spring Framework 5.0.6.
+ *
  * @author Dave Syer
  * @author Stephane Nicoll
  * @author Juergen Hoeller