Destination address ordering + Happy Eyeballs v2 (concurrent multihome dialing).

Add Rfc6724AddressSelectingDnsResolver; refactor MultihomeIOSessionRequester; wire via ConnectionConfig.
New AsyncClientHappyEyeballs example (URI args, System.out trace) and JUnit tests; Java 8 compatible.

Author: fitekonea

httpclient5/src/main/java/org/apache/hc/client5/http/Rfc6724AddressSelectingDnsResolver.java

@@ -44,30 +44,64 @@ public class ConnectionConfig implements Cloneable {
 
     private static final Timeout DEFAULT_CONNECT_TIMEOUT = Timeout.ofMinutes(3);
 
+    /**
+     * @since 5.6
+     */
+    private static final TimeValue DEFAULT_HE_ATTEMPT_DELAY = TimeValue.ofMilliseconds(250);
+    /**
+     * @since 5.6
+     */
+    private static final TimeValue DEFAULT_HE_OTHER_FAMILY_DELAY = TimeValue.ofMilliseconds(50);
+
     public static final ConnectionConfig DEFAULT = new Builder().build();
 
     private final Timeout connectTimeout;
     private final Timeout socketTimeout;
     private final TimeValue validateAfterInactivity;
     private final TimeValue timeToLive;
 
+    /**
+     * @since 5.6
+     */
+    private final boolean staggeredConnectEnabled;
+    /**
+     * @since 5.6
+     */
+    private final TimeValue happyEyeballsAttemptDelay;
+    /**
+     * @since 5.6
+     */
+    private final TimeValue happyEyeballsOtherFamilyDelay;
+    /**
+     * @since 5.6
+     */
+    private final ProtocolFamilyPreference protocolFamilyPreference;
+
     /**
      * Intended for CDI compatibility
      */
     protected ConnectionConfig() {
-        this(DEFAULT_CONNECT_TIMEOUT, null, null, null);
+        this(DEFAULT_CONNECT_TIMEOUT, null, null, null, false, DEFAULT_HE_ATTEMPT_DELAY, DEFAULT_HE_OTHER_FAMILY_DELAY, ProtocolFamilyPreference.INTERLEAVE);
     }
 
     ConnectionConfig(
             final Timeout connectTimeout,
             final Timeout socketTimeout,
             final TimeValue validateAfterInactivity,
-            final TimeValue timeToLive) {
+            final TimeValue timeToLive,
+            final boolean staggeredConnectEnabled,
+            final TimeValue happyEyeballsAttemptDelay,
+            final TimeValue happyEyeballsOtherFamilyDelay,
+            final ProtocolFamilyPreference protocolFamilyPreference) {
         super();
         this.connectTimeout = connectTimeout;
         this.socketTimeout = socketTimeout;
         this.validateAfterInactivity = validateAfterInactivity;
         this.timeToLive = timeToLive;
+        this.staggeredConnectEnabled = staggeredConnectEnabled;
+        this.happyEyeballsAttemptDelay = happyEyeballsAttemptDelay != null ? happyEyeballsAttemptDelay : DEFAULT_HE_ATTEMPT_DELAY;
+        this.happyEyeballsOtherFamilyDelay = happyEyeballsOtherFamilyDelay != null ? happyEyeballsOtherFamilyDelay : DEFAULT_HE_OTHER_FAMILY_DELAY;
+        this.protocolFamilyPreference = protocolFamilyPreference != null ? protocolFamilyPreference : ProtocolFamilyPreference.INTERLEAVE;
     }
 
     /**
@@ -98,6 +132,46 @@ public TimeValue getTimeToLive() {
         return timeToLive;
     }
 
+    /**
+     * Whether staggered (Happy Eyeballs–style) connection attempts are enabled.
+     *
+     * @see Builder#setStaggeredConnectEnabled(boolean)
+     * @since 5.6
+     */
+    public boolean isStaggeredConnectEnabled() {
+        return staggeredConnectEnabled;
+    }
+
+    /**
+     * Delay between subsequent staggered connection attempts.
+     *
+     * @see Builder#setHappyEyeballsAttemptDelay(TimeValue)
+     * @since 5.6
+     */
+    public TimeValue getHappyEyeballsAttemptDelay() {
+        return happyEyeballsAttemptDelay;
+    }
+
+    /**
+     * Initial delay before launching the first address of the other protocol family.
+     *
+     * @see Builder#setHappyEyeballsOtherFamilyDelay(TimeValue)
+     * @since 5.6
+     */
+    public TimeValue getHappyEyeballsOtherFamilyDelay() {
+        return happyEyeballsOtherFamilyDelay;
+    }
+
+    /**
+     * Protocol family preference controlling address selection and ordering.
+     *
+     * @see Builder#setProtocolFamilyPreference(ProtocolFamilyPreference)
+     * @since 5.6
+     */
+    public ProtocolFamilyPreference getProtocolFamilyPreference() {
+        return protocolFamilyPreference;
+    }
+
     @Override
     protected ConnectionConfig clone() throws CloneNotSupportedException {
         return (ConnectionConfig) super.clone();
@@ -111,6 +185,10 @@ public String toString() {
         builder.append(", socketTimeout=").append(socketTimeout);
         builder.append(", validateAfterInactivity=").append(validateAfterInactivity);
         builder.append(", timeToLive=").append(timeToLive);
+        builder.append(", staggeredConnectEnabled=").append(staggeredConnectEnabled);
+        builder.append(", happyEyeballsAttemptDelay=").append(happyEyeballsAttemptDelay);
+        builder.append(", happyEyeballsOtherFamilyDelay=").append(happyEyeballsOtherFamilyDelay);
+        builder.append(", protocolFamilyPreference=").append(protocolFamilyPreference);
         builder.append("]");
         return builder.toString();
     }
@@ -124,7 +202,11 @@ public static ConnectionConfig.Builder copy(final ConnectionConfig config) {
                 .setConnectTimeout(config.getConnectTimeout())
                 .setSocketTimeout(config.getSocketTimeout())
                 .setValidateAfterInactivity(config.getValidateAfterInactivity())
-                .setTimeToLive(config.getTimeToLive());
+                .setTimeToLive(config.getTimeToLive())
+                .setStaggeredConnectEnabled(config.isStaggeredConnectEnabled())
+                .setHappyEyeballsAttemptDelay(config.getHappyEyeballsAttemptDelay())
+                .setHappyEyeballsOtherFamilyDelay(config.getHappyEyeballsOtherFamilyDelay())
+                .setProtocolFamilyPreference(config.getProtocolFamilyPreference());
     }
 
     public static class Builder {
@@ -134,6 +216,12 @@ public static class Builder {
         private TimeValue validateAfterInactivity;
         private TimeValue timeToLive;
 
+        // New fields (defaults)
+        private boolean staggeredConnectEnabled = false; // disabled by default
+        private TimeValue happyEyeballsAttemptDelay = DEFAULT_HE_ATTEMPT_DELAY;
+        private TimeValue happyEyeballsOtherFamilyDelay = DEFAULT_HE_OTHER_FAMILY_DELAY;
+        private ProtocolFamilyPreference protocolFamilyPreference = ProtocolFamilyPreference.INTERLEAVE;
+
         Builder() {
             super();
             this.connectTimeout = DEFAULT_CONNECT_TIMEOUT;
@@ -232,12 +320,62 @@ public Builder setTimeToLive(final long timeToLive, final TimeUnit timeUnit) {
             return this;
         }
 
+        /**
+         * Enables or disables staggered (Happy Eyeballs–style) connection attempts.
+         *
+         * @since 5.6
+         * @return this instance.
+         */
+        public Builder setStaggeredConnectEnabled(final boolean enabled) {
+            this.staggeredConnectEnabled = enabled;
+            return this;
+        }
+
+        /**
+         * Sets the delay between staggered connection attempts.
+         *
+         * @since 5.6
+         * @return this instance.
+         */
+        public Builder setHappyEyeballsAttemptDelay(final TimeValue delay) {
+            this.happyEyeballsAttemptDelay = delay;
+            return this;
+        }
+
+        /**
+         * Sets the initial delay before launching the first address of the other
+         * protocol family (IPv6 vs IPv4) when interleaving attempts.
+         *
+         * @since 5.6
+         * @return this instance.
+         */
+        public Builder setHappyEyeballsOtherFamilyDelay(final TimeValue delay) {
+            this.happyEyeballsOtherFamilyDelay = delay;
+            return this;
+        }
+
+        /**
+         * Sets the protocol family preference that guides address selection and ordering.
+         *
+         * @since 5.6
+         * @return this instance.
+         */
+        public Builder setProtocolFamilyPreference(final ProtocolFamilyPreference preference) {
+            this.protocolFamilyPreference = preference;
+            return this;
+        }
+
         public ConnectionConfig build() {
             return new ConnectionConfig(
                     connectTimeout != null ? connectTimeout : DEFAULT_CONNECT_TIMEOUT,
                     socketTimeout,
                     validateAfterInactivity,
-                    timeToLive);
+                    timeToLive,
+                    staggeredConnectEnabled,
+                    happyEyeballsAttemptDelay != null ? happyEyeballsAttemptDelay : DEFAULT_HE_ATTEMPT_DELAY,
+                    happyEyeballsOtherFamilyDelay != null ? happyEyeballsOtherFamilyDelay : DEFAULT_HE_OTHER_FAMILY_DELAY,
+                    protocolFamilyPreference != null ? protocolFamilyPreference : ProtocolFamilyPreference.INTERLEAVE
+            );
         }
 
     }

httpclient5/src/main/java/org/apache/hc/client5/http/config/ConnectionConfig.java

@@ -0,0 +1,67 @@
+/*
+ * ====================================================================
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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
+ *
+ *   http://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.
+ * ====================================================================
+ *
+ * This software consists of voluntary contributions made by many
+ * individuals on behalf of the Apache Software Foundation.  For more
+ * information on the Apache Software Foundation, please see
+ * <http://www.apache.org/>.
+ *
+ */
+package org.apache.hc.client5.http.config;
+
+/**
+ * Protocol family preference for outbound connections.
+ *
+ * <p>Used by connection initiation code to filter or order destination
+ * addresses and, when enabled, to interleave families during staggered attempts.
+ *
+ * @since 5.6
+ */
+public enum ProtocolFamilyPreference {
+    /** Keep families as returned (or RFC 6724 ordered). */
+    DEFAULT,
+    /**
+     * Prefer IPv4 addresses but allow IPv6 as a fallback.
+     */
+    PREFER_IPV4,
+
+    /**
+     * Prefer IPv6 addresses but allow IPv4 as a fallback.
+     */
+    PREFER_IPV6,
+
+    /**
+     * Use only IPv4 addresses.
+     */
+    IPV4_ONLY,
+
+    /**
+     * Use only IPv6 addresses.
+     */
+    IPV6_ONLY,
+
+    /**
+     * Interleave address families (v6, then v4, then v6, …) when multiple
+     * addresses are available. When staggered connects are enabled, the first
+     * address of the other family is delayed by a small offset.
+     */
+    INTERLEAVE
+}
+

httpclient5/src/main/java/org/apache/hc/client5/http/config/ProtocolFamilyPreference.java

@@ -37,6 +37,7 @@
 import org.apache.hc.client5.http.DnsResolver;
 import org.apache.hc.client5.http.SchemePortResolver;
 import org.apache.hc.client5.http.UnsupportedSchemeException;
+import org.apache.hc.client5.http.config.ConnectionConfig;
 import org.apache.hc.client5.http.config.TlsConfig;
 import org.apache.hc.client5.http.impl.ConnPoolSupport;
 import org.apache.hc.client5.http.impl.DefaultSchemePortResolver;
@@ -71,21 +72,14 @@ public class DefaultAsyncClientConnectionOperator implements AsyncClientConnecti
     private final MultihomeIOSessionRequester sessionRequester;
     private final Lookup<TlsStrategy> tlsStrategyLookup;
 
-    /**
-     * Constructs a new {@code DefaultAsyncClientConnectionOperator}.
-     *
-     * <p><strong>Note:</strong> this class is marked {@code @Internal}; rely on it
-     * only if you are prepared for incompatible changes in a future major
-     * release.  Typical client code should use the high-level builders in
-     * {@code HttpAsyncClients} instead.</p>
-     */
-    protected DefaultAsyncClientConnectionOperator(
+    DefaultAsyncClientConnectionOperator(
             final Lookup<TlsStrategy> tlsStrategyLookup,
             final SchemePortResolver schemePortResolver,
-            final DnsResolver dnsResolver) {
+            final DnsResolver dnsResolver,
+            final ConnectionConfig defaultConnectionConfig) {
         this.tlsStrategyLookup = Args.notNull(tlsStrategyLookup, "TLS strategy lookup");
         this.schemePortResolver = schemePortResolver != null ? schemePortResolver : DefaultSchemePortResolver.INSTANCE;
-        this.sessionRequester = new MultihomeIOSessionRequester(dnsResolver);
+        this.sessionRequester = new MultihomeIOSessionRequester(dnsResolver, defaultConnectionConfig);
     }
 
     @Override