Check for duplicate keys in YAML map nodes

Snake YAML allows for duplicate keys in map nodes. See
https://code.google.com/p/snakeyaml/issues/detail?id=199

This commit uses a dedicated Constructor extension that explicitly
checks for such duplicate keys.

Issue: SPR-12318

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

@@ -17,6 +17,7 @@
 package org.springframework.beans.factory.config;
 
 import java.io.IOException;
+import java.util.AbstractMap;
 import java.util.Arrays;
 import java.util.Collection;
 import java.util.Collections;
@@ -25,10 +26,14 @@ import java.util.List;
 import java.util.Map;
 import java.util.Map.Entry;
 import java.util.Properties;
+import java.util.Set;
 
 import org.apache.commons.logging.Log;
 import org.apache.commons.logging.LogFactory;
 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.springframework.core.io.Resource;
 import org.springframework.util.Assert;
@@ -126,7 +131,7 @@ public abstract class YamlProcessor {
 	 * @param callback a callback to delegate to once matching documents are found
 	 */
 	protected void process(MatchCallback callback) {
-		Yaml yaml = new Yaml();
+		Yaml yaml = new Yaml(new StrictMapAppenderConstructor());
 		for (Resource resource : this.resources) {
 			boolean found = process(callback, yaml, resource);
 			if (this.resolutionMethod == ResolutionMethod.FIRST_FOUND && found) {
@@ -351,4 +356,44 @@ public abstract class YamlProcessor {
 		FIRST_FOUND
 	}
 
+
+	/**
+	 * A specialized {@link Constructor} that checks for duplicate keys.
+	 */
+	private static class StrictMapAppenderConstructor extends Constructor {
+
+		public 	StrictMapAppenderConstructor() {
+			super();
+		}
+
+		@Override
+		protected Map<Object, Object> constructMapping(MappingNode node) {
+			try {
+				return super.constructMapping(node);
+			} catch (IllegalStateException e) {
+				throw new ParserException("while parsing MappingNode",
+						node.getStartMark(), e.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();
+				}
+			};
+		}
+
+	}
+
 }