GH-9383: Introduce JsonIndexAccessor

Fixes: #9383

Issue link: https://github.com/spring-projects/spring-integration/issues/9383

* Polish `JsonPropertyAccessor[Tests]`
* Introduce `JsonIndexAccessor`

This commit introduces a `JsonIndexAccessor` as a complement to the
existing `JsonPropertyAccessor`.

When a `JsonIndexAccessor` is registered with the SpEL `EvaluationContext`,
JSON arrays can be consistently indexed via integer literals (e.g.,[1]) instead of string literals representing integers (e.g., ['1']).

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

@@ -0,0 +1,75 @@
+/*
+ * Copyright 2013-2024 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.integration.json;
+
+import com.fasterxml.jackson.databind.node.ArrayNode;
+
+import org.springframework.expression.AccessException;
+import org.springframework.expression.EvaluationContext;
+import org.springframework.expression.IndexAccessor;
+import org.springframework.expression.TypedValue;
+import org.springframework.lang.Nullable;
+
+/**
+ * A SpEL {@link IndexAccessor} that knows how to read indexes from JSON arrays, using
+ * Jackson's {@link ArrayNode} API.
+ *
+ * <p>Supports indexes supplied as an integer literal &mdash; for example, {@code myJsonArray[1]}.
+ * Also supports negative indexes &mdash; for example, {@code myJsonArray[-1]} which equates
+ * to {@code myJsonArray[myJsonArray.length - 1]}. Furthermore, {@code null} is returned for
+ * any index that is out of bounds (see {@link ArrayNode#get(int)} for details).
+ *
+ * @author Sam Brannen
+ * @since 6.4
+ * @see JsonPropertyAccessor
+ */
+public class JsonIndexAccessor implements IndexAccessor {
+
+	private static final Class<?>[] SUPPORTED_CLASSES = { ArrayNode.class };
+
+	@Override
+	public Class<?>[] getSpecificTargetClasses() {
+		return SUPPORTED_CLASSES;
+	}
+
+	@Override
+	public boolean canRead(EvaluationContext context, Object target, Object index) {
+		return (target instanceof ArrayNode && index instanceof Integer);
+	}
+
+	@Override
+	public TypedValue read(EvaluationContext context, Object target, Object index) throws AccessException {
+		ArrayNode arrayNode = (ArrayNode) target;
+		Integer intIndex = (Integer) index;
+		if (intIndex < 0) {
+			// negative index: get from the end of array, for compatibility with JsonPropertyAccessor.ArrayNodeAsList.
+			intIndex = arrayNode.size() + intIndex;
+		}
+		return JsonPropertyAccessor.typedValue(arrayNode.get(intIndex));
+	}
+
+	@Override
+	public boolean canWrite(EvaluationContext context, Object target, Object index) {
+		return false;
+	}
+
+	@Override
+	public void write(EvaluationContext context, Object target, Object index, @Nullable Object newValue) {
+		throw new UnsupportedOperationException("Write is not supported");
+	}
+
+}

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2013-2023 the original author or authors.
+ * Copyright 2013-2024 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.
@@ -24,6 +24,7 @@ import com.fasterxml.jackson.core.JsonProcessingException;
 import com.fasterxml.jackson.databind.JsonNode;
 import com.fasterxml.jackson.databind.ObjectMapper;
 import com.fasterxml.jackson.databind.node.ArrayNode;
+import com.fasterxml.jackson.databind.node.NullNode;
 
 import org.springframework.expression.AccessException;
 import org.springframework.expression.EvaluationContext;
@@ -35,7 +36,7 @@ import org.springframework.util.StringUtils;
 
 /**
  * A SpEL {@link PropertyAccessor} that knows how to read properties from JSON objects.
- * Uses Jackson {@link JsonNode} API for nested properties access.
+ * <p>Uses Jackson {@link JsonNode} API for nested properties access.
  *
  * @author Eric Bottard
  * @author Artem Bilan
@@ -43,8 +44,10 @@ import org.springframework.util.StringUtils;
  * @author Gary Russell
  * @author Pierre Lakreb
  * @author Vladislav Fefelov
+ * @author Sam Brannen
  *
  * @since 3.0
+ * @see JsonIndexAccessor
  */
 public class JsonPropertyAccessor implements PropertyAccessor {
 
@@ -80,23 +83,22 @@ public class JsonPropertyAccessor implements PropertyAccessor {
 			// Cannot parse - treat as not a JSON
 			return false;
 		}
-		Integer index = maybeIndex(name);
 		if (node instanceof ArrayNode) {
-			return index != null;
+			return maybeIndex(name) != null;
 		}
 		return true;
 	}
 
 	private JsonNode asJson(Object target) throws AccessException {
-		if (target instanceof JsonNode) {
-			return (JsonNode) target;
+		if (target instanceof JsonNode jsonNode) {
+			return jsonNode;
 		}
-		else if (target instanceof JsonNodeWrapper) {
-			return ((JsonNodeWrapper<?>) target).getRealNode();
+		else if (target instanceof JsonNodeWrapper<?> jsonNodeWrapper) {
+			return jsonNodeWrapper.getRealNode();
 		}
-		else if (target instanceof String) {
+		else if (target instanceof String content) {
 			try {
-				return this.objectMapper.readTree((String) target);
+				return this.objectMapper.readTree(content);
 			}
 			catch (JsonProcessingException e) {
 				throw new AccessException("Exception while trying to deserialize String", e);
@@ -160,8 +162,8 @@ public class JsonPropertyAccessor implements PropertyAccessor {
 		return true;
 	}
 
-	private static TypedValue typedValue(JsonNode json) throws AccessException {
-		if (json == null) {
+	static TypedValue typedValue(JsonNode json) throws AccessException {
+		if (json == null || json instanceof NullNode) {
 			return TypedValue.NULL;
 		}
 		else if (json.isValueNode()) {
@@ -199,8 +201,8 @@ public class JsonPropertyAccessor implements PropertyAccessor {
 		if (json == null) {
 			return null;
 		}
-		else if (json instanceof ArrayNode) {
-			return new ArrayNodeAsList((ArrayNode) json);
+		else if (json instanceof ArrayNode arrayNode) {
+			return new ArrayNodeAsList(arrayNode);
 		}
 		else if (json.isValueNode()) {
 			return getValue(json);
@@ -212,8 +214,6 @@ public class JsonPropertyAccessor implements PropertyAccessor {
 
 	interface JsonNodeWrapper<T> extends Comparable<T> {
 
-		String toString();
-
 		JsonNode getRealNode();
 
 	}
@@ -309,10 +309,8 @@ public class JsonPropertyAccessor implements PropertyAccessor {
 
 		@Override
 		public int compareTo(Object o) {
-			if (o instanceof JsonNodeWrapper<?>) {
-				return this.delegate.equals(((JsonNodeWrapper<?>) o).getRealNode()) ? 0 : 1;
-			}
-			return this.delegate.equals(o) ? 0 : 1;
+			Object that = (o instanceof JsonNodeWrapper<?> wrapper ? wrapper.getRealNode() : o);
+			return this.delegate.equals(that) ? 0 : 1;
 		}
 
 	}