apache · arturobernalg · Oct 2, 2025
diff --git a/httpclient5/src/main/java/org/apache/hc/client5/http/ConnectAlpnProvider.java b/httpclient5/src/main/java/org/apache/hc/client5/http/ConnectAlpnProvider.java
@@ -0,0 +1,59 @@
+/*
+ * ====================================================================
+ * 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;
+
+import java.util.List;
+
+import org.apache.hc.core5.http.HttpHost;
+
+/**
+ * Supplies the Application-Layer Protocol Negotiation (ALPN) protocol IDs
+ * to advertise in the HTTP {@code ALPN} header on a {@code CONNECT} request
+ * (RFC 7639).
+ *
+ * <p>If this method returns {@code null} or an empty list, the client will
+ * not add the {@code ALPN} header.</p>
+ *
+ * <p>Implementations should be fast and side-effect free; it may be invoked
+ * for each CONNECT attempt.</p>
+ *
+ * @since 5.6
+ */
+@FunctionalInterface
+public interface ConnectAlpnProvider {
+
+    /**
+     * Returns the ALPN protocol IDs to advertise for a tunnel to {@code target}
+     * over the given {@code route}.
+     *
+     * @param target the origin server the tunnel will connect to (non-null)
+     * @param route  the planned connection route, including proxy info (non-null)
+     * @return list of protocol IDs (e.g., {@code "h2"}, {@code "http/1.1"});
+     * {@code null} or empty to omit the header
+     */
+    List<String> getAlpnForTunnel(HttpHost target, HttpRoute route);
+}
diff --git a/httpclient5/src/main/java/org/apache/hc/client5/http/impl/AlpnHeaderSupport.java b/httpclient5/src/main/java/org/apache/hc/client5/http/impl/AlpnHeaderSupport.java
@@ -0,0 +1,111 @@
+/*
+ * ====================================================================
+ * 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.impl;
+
+import java.nio.charset.StandardCharsets;
+import java.util.ArrayList;
+import java.util.List;
+
+import org.apache.hc.core5.annotation.Contract;
+import org.apache.hc.core5.annotation.Internal;
+import org.apache.hc.core5.annotation.ThreadingBehavior;
+import org.apache.hc.core5.http.Header;
+import org.apache.hc.core5.http.HttpHeaders;
+import org.apache.hc.core5.http.ProtocolException;
+import org.apache.hc.core5.http.message.MessageSupport;
+import org.apache.hc.core5.net.PercentCodec;
+import org.apache.hc.core5.util.Args;
+
+/**
+ * Codec for the HTTP {@code ALPN} header field (RFC 7639).
+ *
+ * @since 5.7
+ */
+@Contract(threading = ThreadingBehavior.IMMUTABLE)
+@Internal
+public final class AlpnHeaderSupport {
+
+    private AlpnHeaderSupport() {
+    }
+
+    /**
+     * Formats a list of raw ALPN protocol IDs into a single {@code ALPN} header.
+     */
+    public static Header formatValue(final List<String> protocolIds) {
+        Args.notEmpty(protocolIds, "protocolIds");
+        return MessageSupport.headerOfTokens(HttpHeaders.ALPN, protocolIds, AlpnHeaderSupport::encodeId);
+    }
+
+    /**
+     * Parses an {@code ALPN} header into decoded protocol IDs.
+     *
+     * @throws ProtocolException if a token is not a well-formed percent-encoded protocol ID.
+     */
+    public static List<String> parseValue(final Header header) throws ProtocolException {
+        final List<String> tokens = new ArrayList<>();
+        MessageSupport.parseTokens(header, tokens::add);
+        final List<String> out = new ArrayList<>(tokens.size());
+        for (final String token : tokens) {
+            out.add(decodeId(token));
+        }
+        return out;
+    }
+
+    /**
+     * Encodes a single raw protocol ID to canonical token form using the HTTP token codec
+     * from core, which keeps RFC 7230 {@code tchar} octets literal and percent-encodes the
+     * rest (including {@code '%'}) with uppercase hexadecimal.
+     */
+    public static String encodeId(final String id) {
+        Args.notBlank(id, "id");
+        return PercentCodec.HTTP_TOKEN.encode(id);
+    }
+
+    /**
+     * Decodes a percent-encoded token to a raw protocol ID using UTF-8.
+     * <p>
+     * A {@code '%'} that is not followed by two hexadecimal digits is a malformed
+     * token and is rejected as a protocol error.
+     *
+     * @throws ProtocolException if the token contains malformed percent-encoding.
+     */
+    public static String decodeId(final String token) throws ProtocolException {
+        Args.notBlank(token, "token");
+        for (int i = 0; i < token.length(); i++) {
+            if (token.charAt(i) == '%') {
+                if (i + 2 >= token.length()
+                        || Character.digit(token.charAt(i + 1), 16) < 0
+                        || Character.digit(token.charAt(i + 2), 16) < 0) {
+                    throw new ProtocolException("Malformed percent-encoding in ALPN protocol id: " + token);
+                }
+                i += 2;
+            }
+        }
+        return PercentCodec.decode(token, StandardCharsets.UTF_8);
+    }
+
+}
diff --git a/httpclient5/src/main/java/org/apache/hc/client5/http/impl/async/AsyncConnectExec.java b/httpclient5/src/main/java/org/apache/hc/client5/http/impl/async/AsyncConnectExec.java
@@ -34,6 +34,7 @@
 import java.util.concurrent.atomic.AtomicReference;
 
 import org.apache.hc.client5.http.AuthenticationStrategy;
+import org.apache.hc.client5.http.ConnectAlpnProvider;
 import org.apache.hc.client5.http.EndpointInfo;
 import org.apache.hc.client5.http.HttpRoute;
 import org.apache.hc.client5.http.RouteTracker;
@@ -47,6 +48,7 @@
 import org.apache.hc.client5.http.auth.ChallengeType;
 import org.apache.hc.client5.http.auth.MalformedChallengeException;
 import org.apache.hc.client5.http.config.RequestConfig;
+import org.apache.hc.client5.http.impl.AlpnHeaderSupport;
 import org.apache.hc.client5.http.impl.auth.AuthCacheKeeper;
 import org.apache.hc.client5.http.impl.auth.AuthenticationHandler;
 import org.apache.hc.client5.http.impl.routing.BasicRouteDirector;
@@ -99,18 +101,31 @@ public final class AsyncConnectExec implements AsyncExecChainHandler {
     private final AuthCacheKeeper authCacheKeeper;
     private final HttpRouteDirector routeDirector;
 
+    private final ConnectAlpnProvider alpnProvider;
+
+
     public AsyncConnectExec(
             final HttpProcessor proxyHttpProcessor,
             final AuthenticationStrategy proxyAuthStrategy,
             final SchemePortResolver schemePortResolver,
             final boolean authCachingDisabled) {
+       this(proxyHttpProcessor, proxyAuthStrategy, schemePortResolver, authCachingDisabled, null);
+    }
+
+    public AsyncConnectExec(
+            final HttpProcessor proxyHttpProcessor,
+            final AuthenticationStrategy proxyAuthStrategy,
+            final SchemePortResolver schemePortResolver,
+            final boolean authCachingDisabled,
+            final ConnectAlpnProvider alpnProvider) {
         Args.notNull(proxyHttpProcessor, "Proxy HTTP processor");
         Args.notNull(proxyAuthStrategy, "Proxy authentication strategy");
         this.proxyHttpProcessor = proxyHttpProcessor;
         this.proxyAuthStrategy = proxyAuthStrategy;
         this.authenticator = new AuthenticationHandler();
         this.authCacheKeeper = authCachingDisabled ? null : new AuthCacheKeeper(schemePortResolver);
         this.routeDirector = BasicRouteDirector.INSTANCE;
+        this.alpnProvider = alpnProvider;
     }
 
     static class State {
@@ -275,7 +290,7 @@ public void cancelled() {
                 if (LOG.isDebugEnabled()) {
                     LOG.debug("{} create tunnel", exchangeId);
                 }
-                createTunnel(state, proxy, target, scope, new AsyncExecCallback() {
+                createTunnel(state, proxy, target, route, scope, new AsyncExecCallback() {
 
                     @Override
                     public AsyncDataConsumer handleResponse(final HttpResponse response, final EntityDetails entityDetails) throws HttpException, IOException {
@@ -380,6 +395,7 @@ private void createTunnel(
             final State state,
             final HttpHost proxy,
             final HttpHost nextHop,
+            final HttpRoute route,
             final AsyncExecChain.Scope scope,
             final AsyncExecCallback asyncExecCallback) {
 
@@ -426,6 +442,13 @@ public void produceRequest(final RequestChannel requestChannel,
                 final HttpRequest connect = new BasicHttpRequest(Method.CONNECT, nextHop, nextHop.toHostString());
                 connect.setVersion(HttpVersion.HTTP_1_1);
 
+                // --- RFC 7639: inject ALPN header (if provided) ----------------
+                if (alpnProvider != null) {
+                    final List<String> alpn = alpnProvider.getAlpnForTunnel(nextHop, route);
+                    if (alpn != null && !alpn.isEmpty()) {
+                        connect.setHeader(AlpnHeaderSupport.formatValue(alpn));
+                    }
+                }
                 proxyHttpProcessor.process(connect, null, clientContext);
                 authenticator.addAuthResponse(proxy, ChallengeType.PROXY, connect, proxyAuthExchange, clientContext);
 

diff --git a/httpclient5/src/main/java/org/apache/hc/client5/http/impl/async/HttpAsyncClientBuilder.java b/httpclient5/src/main/java/org/apache/hc/client5/http/impl/async/HttpAsyncClientBuilder.java
@@ -39,6 +39,7 @@
 import java.util.function.UnaryOperator;
 
 import org.apache.hc.client5.http.AuthenticationStrategy;
+import org.apache.hc.client5.http.ConnectAlpnProvider;
 import org.apache.hc.client5.http.ConnectionKeepAliveStrategy;
 import org.apache.hc.client5.http.EarlyHintsListener;
 import org.apache.hc.client5.http.HttpRequestRetryStrategy;
@@ -257,6 +258,8 @@ private ExecInterceptorEntry(
     private boolean tlsRequired;
 
 
+    private ConnectAlpnProvider connectAlpnProvider;
+
     /**
      * Maps {@code Content-Encoding} tokens to decoder factories in insertion order.
      */
@@ -936,6 +939,19 @@ public final HttpAsyncClientBuilder disableRequestPriority() {
         return this;
     }
 
+    /**
+     * Sets the {@link ConnectAlpnProvider} used to populate the {@code ALPN} header
+     * (RFC 7639) on {@code CONNECT} requests when establishing an HTTP tunnel through a proxy.
+     *
+     * @param connectAlpnProvider the provider, or {@code null} to not send an {@code ALPN} header.
+     * @return this builder.
+     * @since 5.7
+     */
+    public HttpAsyncClientBuilder setConnectAlpnProvider(final ConnectAlpnProvider connectAlpnProvider) {
+        this.connectAlpnProvider = connectAlpnProvider;
+        return this;
+    }
+
     /**
      * Registers a global {@link org.apache.hc.client5.http.EarlyHintsListener}
      * that will be notified when the client receives {@code 103 Early Hints}
@@ -1086,7 +1102,8 @@ public CloseableHttpAsyncClient build() {
                         new DefaultHttpProcessor(new RequestTargetHost(), new RequestUserAgent(userAgentCopy)),
                         proxyAuthStrategyCopy,
                         schemePortResolver != null ? schemePortResolver : DefaultSchemePortResolver.INSTANCE,
-                        authCachingDisabled),
+                        authCachingDisabled,
+                        connectAlpnProvider),
                 ChainElement.CONNECT.name());
 
         if (earlyHintsListener != null) {