Enhance ActionListener documentation (#104239)

Improve comments for ActionListener#delegateFailure and
SubscribableListener.

Closes #103829

server/src/main/java/org/elasticsearch/action/ActionListener.java

@@ -89,19 +89,21 @@ default ActionListener<Response> delegateResponse(BiConsumer<ActionListener<Resp
     }
 
     /**
-     * Creates a listener that delegates all exceptions it receives to another listener.
+     * Creates a new listener, wrapping this one, that overrides {@link #onResponse} handling with the given {@code bc} consumer.
+     * {@link #onFailure(Exception)} handling is delegated to the original listener. Exceptions in {@link #onResponse} are forbidden.
      *
-     * @param bc BiConsumer invoked with delegate listener and response
+     * @param bc {@link BiConsumer} invoked via {@link #onResponse} with the original listener and the response with which the new listener
+     * was completed.
      * @param <T> Type of the delegating listener's response
-     * @return Delegating listener
+     * @return a new listener that delegates failures to this listener and runs {@code bc} on a response.
      */
     default <T> ActionListener<T> delegateFailure(BiConsumer<ActionListener<Response>, T> bc) {
         return new ActionListenerImplementations.DelegatingFailureActionListener<>(this, bc);
     }
 
     /**
-     * Same as {@link #delegateFailure(BiConsumer)} except that any failure thrown by {@code bc} or the delegate listener's
-     * {@link #onResponse} will be passed to the delegate listeners {@link #onFailure(Exception)}.
+     * Same as {@link #delegateFailure(BiConsumer)} except that any failure thrown by {@code bc} or the original listener's
+     * {@link #onResponse} will be passed to the original listener's {@link #onFailure(Exception)}.
      */
     default <T> ActionListener<T> delegateFailureAndWrap(CheckedBiConsumer<ActionListener<Response>, T, ? extends Exception> bc) {
         return new ActionListenerImplementations.ResponseWrappingActionListener<>(this, bc);
@@ -150,7 +152,7 @@ public String toString() {
      * the sense that an exception from the {@code onResponse} consumer is passed into the {@code onFailure} consumer.
      * <p>
      * If the {@code onFailure} argument is {@code listener::onFailure} for some other {@link ActionListener}, prefer to use
-     * {@link #delegateFailureAndWrap} instead.
+     * {@link #delegateFailureAndWrap} instead for performance reasons.
      * @param onResponse the checked consumer of the response, executed when the listener is completed successfully. If it throws an
      *                   exception, the exception is passed to the {@code onFailure} consumer.
      * @param onFailure the consumer of the failure, executed when the listener is completed with an exception (or it is completed

server/src/main/java/org/elasticsearch/action/ActionListenerImplementations.java

@@ -197,6 +197,11 @@ public String toString() {
         }
     }
 
+    /**
+     * Replaces the onResponse handling of a given ActionListener with a lambda that receives both the original listener and a response.
+     * This is useful when a listener is needed to do some additional work with a response before passing a response on to the original
+     * listener.
+     */
     static final class DelegatingFailureActionListener<T, R> extends DelegatingActionListener<T, R> {
 
         private final BiConsumer<ActionListener<R>, T> bc;
@@ -221,6 +226,10 @@ public String toString() {
         }
     }
 
+    /**
+     * The same as {@link DelegatingFailureActionListener} with the addition of exception handling in {@link #onResponse(Object)} to forward
+     * any exceptions to {@link #onFailure(Exception)}.
+     */
     static final class ResponseWrappingActionListener<T, R> extends DelegatingActionListener<T, R> {
 
         private final CheckedBiConsumer<ActionListener<R>, T, ? extends Exception> bc;

server/src/main/java/org/elasticsearch/action/DelegatingActionListener.java

@@ -11,8 +11,12 @@
 import static org.elasticsearch.action.ActionListenerImplementations.safeOnFailure;
 
 /**
- * A wrapper around an {@link ActionListener} which delegates failures safely to the inner listener's {@link ActionListener#onFailure}
- * method and which has a {@link #toString()} implementation which describes this class and the delegate.
+ * A wrapper around an {@link ActionListener} {@code L} that by default delegates failures to {@code L}'s {@link ActionListener#onFailure}
+ * method. The wrapper also provides a {@link #toString()} implementation that describes this class and the delegate.
+ * <p>
+ * This is a useful base class for creating ActionListener wrappers that override the {@link #onResponse} handling, with access to
+ * {@code L}, while retaining all of {@code L}'s other handling. It can also be useful to override other methods to do new work with access
+ * to {@code L}.
  */
 public abstract class DelegatingActionListener<Response, DelegateResponse> implements ActionListener<Response> {
 

server/src/main/java/org/elasticsearch/action/support/SubscribableListener.java

@@ -34,11 +34,12 @@
  * An {@link ActionListener} to which other {@link ActionListener} instances can subscribe, such that when this listener is completed it
  * fans-out its result to the subscribed listeners.
  * <p>
- * Similar to {@link ListenableActionFuture} and {@link ListenableFuture} except for its handling of exceptions: if this listener is
- * completed exceptionally then the exception is passed to subscribed listeners without modification.
+ * Exceptions are passed to subscribed listeners without modification. {@link ListenableActionFuture} and {@link ListenableFuture} are child
+ * classes that provide additional exception handling.
  * <p>
- * Often this will be used to chain together a sequence of async actions, similarly to {@link CompletionStage} (without the
- * {@code catch (Throwable t)}), such as in the following example:
+ * A sequence of async steps can be chained together using a series of {@link SubscribableListener}s, similar to {@link CompletionStage}
+ * (without the {@code catch (Throwable t)}). Listeners can be created for each step, where the next step subscribes to the result of the
+ * previous, using utilities like {@link #andThen(CheckedBiConsumer)}. The following example demonstrates how this might be used:
  * <pre>{@code
  * private void exampleAsyncMethod(String request, List<Long> items, ActionListener<Boolean> finalListener) {
  *     SubscribableListener