Added a new listener for events related to Client

jaxrs-api/src/main/java/jakarta/ws/rs/client/ClientBuilder.java

@@ -272,4 +272,17 @@ public ClientBuilder keyStore(final KeyStore keyStore, final String password) {
      * @return a new client instance.
      */
     public abstract Client build();
+
+    /**
+     * Register a {@link ClientListener} to receive client lifecycle events.
+     * <p>
+     * The listener will be notified of events such as connection establishment,
+     * closure, failures, and reconnection attempts.
+     * </p>
+     *
+     * @param listener the client listener to register
+     * @return an updated client builder instance
+     * @since 4.0
+     */
+    public abstract ClientBuilder listener(ClientListener listener);
 }

jaxrs-api/src/main/java/jakarta/ws/rs/client/ClientListener.java

@@ -0,0 +1,85 @@
+/*
+ * Copyright (c) 2026 Contributors to the Eclipse Foundation
+ *
+ * This program and the accompanying materials are made available under the
+ * terms of the Eclipse Public License v. 2.0, which is available at
+ * http://www.eclipse.org/legal/epl-2.0.
+ *
+ * This Source Code may also be made available under the following Secondary
+ * Licenses when the conditions for such availability set forth in the
+ * Eclipse Public License v. 2.0 are satisfied: GNU General Public License,
+ * version 2 with the GNU Classpath Exception, which is available at
+ * https://www.gnu.org/software/classpath/license.html.
+ *
+ * SPDX-License-Identifier: EPL-2.0 OR GPL-2.0 WITH Classpath-exception-2.0
+ */
+
+package jakarta.ws.rs.client;
+
+/**
+ * A listener interface for receiving client lifecycle events.
+ * <p>
+ * Implementations of this interface can be registered with a {@link ClientBuilder}
+ * to receive notifications about client lifecycle events such as connection,
+ * disconnection, and failures.
+ * </p>
+ * <p>
+ * All methods in this interface have default implementations that do nothing,
+ * allowing implementations to override only the events they are interested in.
+ * </p>
+ */
+public interface ClientListener {
+
+    /**
+     * Called when the client successfully establishes a connection.
+     * <p>
+     * This method is invoked after the client has been created and is ready
+     * to process requests.
+     * </p>
+     *
+     * @param client the client that connected
+     */
+    default void connected(Client client) {
+    }
+
+    /**
+     * Called when the client is closed.
+     * <p>
+     * This method is invoked when {@link Client#close()} is called or when
+     * the client is closed by the container (e.g., during CDI shutdown).
+     * Implementations can use this to perform cleanup operations.
+     * </p>
+     *
+     * @param client the client that was closed
+     */
+    default void closed(Client client) {
+    }
+
+    /**
+     * Called when a connection failure occurs.
+     * <p>
+     * This method is invoked when the client encounters a connection error
+     * that prevents it from communicating with the server. This could be
+     * due to network issues, server unavailability, or other connection
+     * problems.
+     * </p>
+     *
+     * @param client the client that experienced the failure
+     * @param cause  the exception that caused the failure
+     */
+    default void connectionFailed(Client client, Throwable cause) {
+    }
+
+    /**
+     * Called when the client attempts to reconnect after a connection failure.
+     * <p>
+     * This method is invoked when the client detects a connection failure
+     * and is attempting to re-establish the connection. This can be used
+     * to notify users or log reconnection attempts.
+     * </p>
+     *
+     * @param client the client that is reconnecting
+     */
+    default void reconnecting(Client client) {
+    }
+}