Support for dynamic proxy configuration at the HTTP protocol level (#…

…3593)

Fixes #2746

Signed-off-by: Violeta Georgieva <[email protected]>
Co-authored-by: Violeta Georgieva <[email protected]>

docs/modules/ROOT/partials/proxy.adoc

@@ -19,3 +19,16 @@ The following example uses `ProxyProvider`:
 include::{examples-dir}/proxy/Application.java[lines=18..42]
 ----
 <1> Configures the connection establishment timeout to 20 seconds.
+
+
+// tag::proxy-when-method[]
+The `proxyWhen(...)` method allows for dynamic proxy configuration.
+
+- `proxyWhen(...)` takes an `HttpClientConfig` and enables setting a proxy for each individual HTTP request, providing flexibility to choose different proxies depending on the request.
+- Once the proxy is set using `proxyWhen(...)`, any previous proxy settings via `proxy(...)` or `noProxy()` will be ignored.
+// end::proxy-when-method[]
+{examples-link}/proxy/deferred/Application.java
+[%unbreakable]
+----
+include::{examples-dir}/proxy/deferred/Application.java[lines=18..53]
+----

reactor-netty-core/src/main/java/reactor/netty/transport/ClientTransport.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2020-2024 VMware, Inc. or its affiliates, All Rights Reserved.
+ * Copyright (c) 2020-2025 VMware, Inc. or its affiliates, All Rights Reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -252,7 +252,7 @@ public T proxy(Consumer<? super ProxyProvider.TypeSpec> proxyOptions) {
 		Objects.requireNonNull(proxyOptions, "proxyOptions");
 		ProxyProvider.Build builder = (ProxyProvider.Build) ProxyProvider.builder();
 		proxyOptions.accept(builder);
-		return proxyWithProxyProviderSupplier(() -> builder.build());
+		return proxyWithProxyProviderSupplier(builder::build);
 	}
 
 	final T proxyWithProxyProvider(ProxyProvider proxy) {

reactor-netty-examples/src/main/java/reactor/netty/examples/documentation/http/client/proxy/deferred/Application.java

@@ -0,0 +1,58 @@
+/*
+ * Copyright (c) 2025 VMware, Inc. or its affiliates, All Rights Reserved.
+ *
+ * Licensed 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
+ *
+ *   https://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.
+ */
+package reactor.netty.examples.documentation.http.client.proxy.deferred;
+
+import reactor.core.publisher.Mono;
+import reactor.netty.http.client.HttpClient;
+import reactor.netty.transport.ProxyProvider;
+
+public class Application {
+
+	public static void main(String[] args) {
+		HttpClient client =
+				HttpClient.create()
+						.proxyWhen((httpClientConfig, spec) -> {    // only applied
+							if (httpClientConfig.uri().startsWith("https://example.com")) {
+								return Mono.justOrEmpty(
+										spec.type(ProxyProvider.Proxy.HTTP)
+												.host("proxy")
+												.port(8080)
+												.nonProxyHosts("localhost")
+												.connectTimeoutMillis(20_000)
+								);
+							}
+
+							return Mono.empty();
+						})
+						.proxy(
+								spec -> spec.type(ProxyProvider.Proxy.HTTP)
+										.host("ignored-proxy-domain")
+										.port(9000)
+										.connectTimeoutMillis(20_000)
+						)   // ignored
+						.noProxy(); // ignored
+
+		String response =
+				client.get()
+						.uri("https://example.com/")
+						.responseContent()
+						.aggregate()
+						.asString()
+						.block();
+
+		System.out.println("Response " + response);
+	}
+}

reactor-netty-http/src/main/java/reactor/netty/http/client/HttpClient.java

@@ -66,6 +66,7 @@
 import reactor.netty.tcp.SslProvider;
 import reactor.netty.tcp.TcpClient;
 import reactor.netty.transport.ClientTransport;
+import reactor.netty.transport.ProxyProvider;
 import reactor.util.Logger;
 import reactor.util.Loggers;
 import reactor.util.annotation.Incubating;
@@ -1640,6 +1641,38 @@ public Mono<Void> warmup() {
 				Mono.fromRunnable(OpenSsl::version));
 	}
 
+	/**
+	 * Supports proxy configuration with a deferred approach.
+	 *
+	 * <p>When proxyWhen(...) is set, calls to proxy(...) and noProxy() methods are ignored.</p>
+	 * <p>This method allows dynamic determination of proxy settings, applying or skipping the proxy based on the configured conditions.</p>
+	 *
+	 * @param proxyBuilder a consumer for proxy configuration
+	 * @return a new {@link HttpClient} reference
+	 */
+	public final HttpClient proxyWhen(
+			BiFunction<HttpClientConfig, ? super ProxyProvider.TypeSpec, Mono<? extends ProxyProvider.Builder>> proxyBuilder) {
+		Objects.requireNonNull(proxyBuilder, "proxyBuilder");
+		HttpClient dup = duplicate();
+		dup.configuration()
+				.deferredConf(
+						config -> {
+							Mono<? extends ProxyProvider.Builder> mono = proxyBuilder.apply(config, ProxyProvider.builder());
+							if (mono == null || mono == Mono.<ProxyProvider.Builder>empty()) {
+								return Mono.just(config);
+							}
+
+							return mono.map(
+									builder -> {
+										config.proxyProvider(builder.build());
+										return config;
+									}
+							);
+						}
+				);
+		return dup;
+	}
+
 	/**
 	 * HTTP Websocket to connect the {@link HttpClient}.
 	 *