INT-4241: Wrap ArrayNodes with ArrayNodeAsList

JIRA: https://jira.spring.io/browse/INT-4241

To allow `ArrayNode` to be treated as `List` in SpEL expressions, wrap them to the internal `ArrayNodeAsList` implementation

Fix Checkstyle errors

* Add `iterator()` support to the `ArrayNodeAsList`

spring-integration-core/src/main/java/org/springframework/integration/json/JsonPropertyAccessor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2013-2016 the original author or authors.
+ * Copyright 2013-2017 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.
@@ -17,6 +17,8 @@
 package org.springframework.integration.json;
 
 import java.io.IOException;
+import java.util.AbstractList;
+import java.util.Iterator;
 
 import org.springframework.expression.AccessException;
 import org.springframework.expression.EvaluationContext;
@@ -36,6 +38,7 @@ import com.fasterxml.jackson.databind.node.ObjectNode;
  *
  * @author Eric Bottard
  * @author Artem Bilan
+ * @author Paul Martin
  * @since 3.0
  */
 public class JsonPropertyAccessor implements PropertyAccessor {
@@ -44,7 +47,7 @@ public class JsonPropertyAccessor implements PropertyAccessor {
 	 * The kind of types this can work with.
 	 */
 	private static final Class<?>[] SUPPORTED_CLASSES = new Class<?>[] { String.class, ToStringFriendlyJsonNode.class,
-			ObjectNode.class, ArrayNode.class };
+			ArrayNodeAsList.class, ObjectNode.class, ArrayNode.class };
 
 	// Note: ObjectMapper is thread-safe
 	private ObjectMapper objectMapper = new ObjectMapper();
@@ -58,7 +61,12 @@ public class JsonPropertyAccessor implements PropertyAccessor {
 	public boolean canRead(EvaluationContext context, Object target, String name) throws AccessException {
 		ContainerNode<?> container = asJson(target);
 		Integer index = maybeIndex(name);
-		return ((index != null && container.has(index)) || container.has(name));
+		if (container instanceof ArrayNode) {
+			return index != null;
+		}
+		else {
+			return ((index != null && container.has(index)) || container.has(name));
+		}
 	}
 
 	private ContainerNode<?> assertContainerNode(JsonNode json) throws AccessException {
@@ -79,6 +87,10 @@ public class JsonPropertyAccessor implements PropertyAccessor {
 			ToStringFriendlyJsonNode wrapper = (ToStringFriendlyJsonNode) target;
 			return assertContainerNode(wrapper.node);
 		}
+		else if (target instanceof ArrayNodeAsList) {
+			ArrayNodeAsList wrapper = (ArrayNodeAsList) target;
+			return assertContainerNode(wrapper.node);
+		}
 		else if (target instanceof String) {
 			try {
 				JsonNode json = this.objectMapper.readTree((String) target);
@@ -113,16 +125,10 @@ public class JsonPropertyAccessor implements PropertyAccessor {
 		ContainerNode<?> container = asJson(target);
 		Integer index = maybeIndex(name);
 		if (index != null && container.has(index)) {
-			return new TypedValue(wrap(container.get(index)));
+			return typedValue(container.get(index));
 		}
 		else {
-			JsonNode json = container.get(name);
-			if (json != null) {
-				return new TypedValue(wrap(json));
-			}
-			else {
-				return TypedValue.NULL;
-			}
+			return typedValue(container.get(name));
 		}
 	}
 
@@ -141,15 +147,43 @@ public class JsonPropertyAccessor implements PropertyAccessor {
 		this.objectMapper = objectMapper;
 	}
 
-	private ToStringFriendlyJsonNode wrap(JsonNode json) {
-		return new ToStringFriendlyJsonNode(json);
+	private static TypedValue typedValue(JsonNode json) {
+		if (json == null) {
+			return TypedValue.NULL;
+		}
+		else {
+			return new TypedValue(wrap(json));
+		}
 	}
 
-	public static class ToStringFriendlyJsonNode {
+	public static WrappedJsonNode wrap(JsonNode json) {
+		if (json == null) {
+			return null;
+		}
+		else if (json instanceof ArrayNode) {
+			return new ArrayNodeAsList((ArrayNode) json);
+		}
+		else {
+			return new ToStringFriendlyJsonNode(json);
+		}
+	}
+
+	/**
+	 * The base interface for wrapped {@link JsonNode}.
+	 * @since 5.0
+	 */
+	public interface WrappedJsonNode {
+
+	}
+
+	/**
+	 * A {@link WrappedJsonNode} implementation to represent {@link JsonNode} as string.
+	 */
+	public static class ToStringFriendlyJsonNode implements WrappedJsonNode {
 
 		private final JsonNode node;
 
-		public ToStringFriendlyJsonNode(JsonNode node) {
+		ToStringFriendlyJsonNode(JsonNode node) {
 			this.node = node;
 		}
 
@@ -186,4 +220,48 @@ public class JsonPropertyAccessor implements PropertyAccessor {
 
 	}
 
+	/**
+	 * An {@link AbstractList} implementation around {@link ArrayNode} with {@link WrappedJsonNode} aspect.
+	 * @since 5.0
+	 */
+	public static class ArrayNodeAsList extends AbstractList<WrappedJsonNode> implements WrappedJsonNode {
+
+		private final ArrayNode node;
+
+		ArrayNodeAsList(ArrayNode node) {
+			this.node = node;
+		}
+
+		@Override
+		public WrappedJsonNode get(int index) {
+			return wrap(this.node.get(index));
+		}
+
+		@Override
+		public int size() {
+			return this.node.size();
+		}
+
+		@Override
+		public Iterator<WrappedJsonNode> iterator() {
+
+			return new Iterator<WrappedJsonNode>() {
+
+				private final Iterator<JsonNode> delegate = ArrayNodeAsList.this.node.iterator();
+
+				@Override
+				public boolean hasNext() {
+					return this.delegate.hasNext();
+				}
+
+				@Override
+				public WrappedJsonNode next() {
+					return wrap(this.delegate.next());
+				}
+
+			};
+		}
+
+	}
+
 }