GH-9436: Add support for SpEL IndexAccessor configuration (#9451)

Fixes: #9436
Issue link: https://github.com/spring-projects/spring-integration/issues/9436

* Expose `IndexAccessor` configuration options on the `AbstractEvaluationContextFactoryBean`
and `SpelPropertyAccessorRegistrar`
* Expose `<index-accessors>` sub-element for the `<spel-property-accessors>`
* Adjust tests
* Document the feature, including recently added `JsonIndexAccessor`

src/reference/antora/modules/ROOT/pages/spel.adoc

@@ -20,6 +20,11 @@ Starting with Spring Integration 3.0, you can add additional `PropertyAccessor`
 The framework provides the (read-only) `JsonPropertyAccessor`, which you can use to access fields from a `JsonNode` or JSON in a `String`.
 You can also create your own `PropertyAccessor` if you have specific needs.
 
+Starting with version 6.4, the `JsonIndexAccessor` implementation is provided that knows how to read indexes from JSON arrays, using Jackson's `ArrayNode` API.
+Supports indexes supplied as an integer literal, for example, `myJsonArray[1]`.
+Also supports negative indexes, for example, `myJsonArray[-1]` which equates to `myJsonArray[myJsonArray.length - 1]`.
+Furthermore, `null` is returned for any index that is out of bounds (see `ArrayNode.get(int)` for details).
+
 In addition, you can add custom functions.
 Custom functions are `static` methods declared on a class.
 Functions and property accessors are available in any SpEL expression used throughout the framework.
@@ -45,6 +50,9 @@ The following configuration shows how to directly configure the `IntegrationEval
 </bean>
 ----
 
+Starting with version 6.4, the `AbstractEvaluationContextFactoryBean` supports an injection of `IndexAccessor` instances.
+See `AbstractEvaluationContextFactoryBean` method JavaDocs for more information.
+
 For convenience, Spring Integration provides namespace support for both property accessors and functions, as described in the following sections.
 The framework automatically configures the factory bean on your behalf.
 
@@ -122,7 +130,7 @@ Each context has its own instance of the `integrationEvaluationContext` factory
 [[built-in-spel-functions]]
 === Built-in SpEL Functions
 
-Spring Integration provides the folloiwng  standard functions, which are registered with the application context automatically on start up:
+Spring Integration provides the following standard functions, which are registered with the application context automatically on start up:
 
 * `#jsonPath`: Evaluates a 'jsonPath' on a specified object.
 This function invokes `JsonPathUtils.evaluate(...)`, which delegates to the https://github.com/json-path/JsonPath[Jayway JsonPath library].
@@ -157,12 +165,16 @@ For more information regarding XML and XPath, see xref:xml.adoc[XML Support - De
 
 Spring Integration provides namespace support to let you create SpEL custom https://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/expression/PropertyAccessor.html[`PropertyAccessor`] implementations.
 You can use the `<spel-property-accessors/>` component to provide a list of custom `PropertyAccessor` instances to the `EvaluationContext` used throughout the framework.
-Instead of configuring the factory bean shown earlier, you can add one or more of these components, and the framework automatically adds the accessors to the default `integrationEvaluationContext` factory bean.
+Instead of configuring the factory bean shown earlier, you can add this component, and the framework automatically adds the accessors to the default `integrationEvaluationContext` factory bean.
+Also, starting with version 6.4, a dedicated `<index-accessors>` sub-element is provided to configure `IndexAccessor` beans similar way.
 The following example shows how to do so:
 
 [source,xml]
 ----
 <int:spel-property-accessors>
+    <index-accessors>
+        <beans:bean id="jsonIndex" class="org.springframework.integration.json.JsonIndexAccessor"/>
+    </index-accessors>
 	<bean id="jsonPA" class="org.springframework.integration.json.JsonPropertyAccessor"/>
 	<ref bean="fooPropertyAccessor"/>
 </int:spel-property-accessors>
@@ -170,15 +182,17 @@ The following example shows how to do so:
 
 In the preceding example, two custom `PropertyAccessor` instances are injected into the `EvaluationContext` (in the order in which they are declared).
 
+
 To provide `PropertyAccessor` instances by using Java Configuration, you should declare a `SpelPropertyAccessorRegistrar` bean with a name of `spelPropertyAccessorRegistrar` (dictated by the `IntegrationContextUtils.SPEL_PROPERTY_ACCESSOR_REGISTRAR_BEAN_NAME` constant).
-The following example shows how to configure two custom `PropertyAccessor` instances with Java:
+The following example shows how to configure two custom `PropertyAccessor` (and `IndexAccessor` starting with version 6.4) instances with Java:
 
 [source,java]
 ----
 @Bean
 public SpelPropertyAccessorRegistrar spelPropertyAccessorRegistrar() {
     return new SpelPropertyAccessorRegistrar(new JsonPropertyAccessor())
-                    .add(fooPropertyAccessor());
+                    .add(fooPropertyAccessor())
+                    .add(new JsonIndexAccessor());
 }
 ----
 

src/reference/antora/modules/ROOT/pages/whats-new.adoc

@@ -23,6 +23,10 @@ See xref:control-bus.adoc[Control Bus] for more information.
 Also, a `ControlBusController` (together with an `@EnableControlBusController`) is introduced for managing exposed commands by the mentioned `ControlBusCommandRegistry`.
 See xref:http.adoc[HTTP Support] for more information.
 
+The SpEL evaluation infrastructure now supports configuration for `IndexAccessor`.
+Also, an out-of-the-box `JsonIndexAccessor` is provided.
+See xref:spel.adoc[SpEL Support] for more information.
+
 [[x6.4-general]]
 === General Changes
 
@@ -74,4 +78,3 @@ See xref:sftp/session-factory.adoc[SFTP Session Factory] for more information.
 Multiple instances of `MqttPahoMessageDrivenChannelAdapter` and `Mqttv5PahoMessageDrivenChannelAdapter` can now be added at runtime using corresponding `ClientManager` through `IntegrationFlowContext`
 Also a `MqttMessageNotDeliveredEvent` event has been introduced to emit when action callback reacts to the delivery failure.
 See xref:mqtt.adoc[MQTT Support] for more information.
-