Support rewriting request parameters

Introduced a method to modify or replace request parameters during request processing, providing greater flexibility for parameter handling in routing.

Signed-off-by: raccoonback <[email protected]>

spring-cloud-gateway-server-mvc/src/main/java/org/springframework/cloud/gateway/server/mvc/filter/BeforeFilterFunctions.java

@@ -45,6 +45,7 @@
 import org.springframework.web.servlet.function.ServerRequest;
 import org.springframework.web.util.UriComponentsBuilder;
 import org.springframework.web.util.UriTemplate;
+import org.springframework.web.util.UriUtils;
 
 import static org.springframework.cloud.gateway.server.mvc.common.MvcUtils.CIRCUITBREAKER_EXECUTION_EXCEPTION_ATTR;
 import static org.springframework.util.CollectionUtils.unmodifiableMultiValueMap;
@@ -337,6 +338,30 @@ public static Function<ServerRequest, ServerRequest> rewritePath(String regexp,
 		};
 	}
 
+	public static Function<ServerRequest, ServerRequest> rewriteRequestParameter(String name, String replacement) {
+		return request -> {
+			MultiValueMap<String, String> queryParams = new LinkedMultiValueMap<>(request.params());
+			if (queryParams.containsKey(name)) {
+				queryParams.remove(name);
+				queryParams.add(name, replacement);
+			}
+
+			MultiValueMap<String, String> encodedQueryParams = UriUtils.encodeQueryParams(queryParams);
+			URI rewrittenUri = UriComponentsBuilder.fromUri(request.uri())
+				.replaceQueryParams(unmodifiableMultiValueMap(encodedQueryParams))
+				.build(true)
+				.toUri();
+
+			return ServerRequest.from(request)
+					.params(params -> {
+						params.remove(name);
+						params.add(name, replacement);
+					})
+					.uri(rewrittenUri)
+					.build();
+		};
+	}
+
 	public static Function<ServerRequest, ServerRequest> routeId(String routeId) {
 		return request -> {
 			MvcUtils.setRouteId(request, routeId);

spring-cloud-gateway-server-mvc/src/test/java/org/springframework/cloud/gateway/server/mvc/filter/BeforeFilterFunctionsTests.java

@@ -0,0 +1,110 @@
+/*
+ * Copyright 2013-2025 the original author or authors.
+ *
+ * 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 org.springframework.cloud.gateway.server.mvc.filter;
+
+import java.util.Collections;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.mock.web.MockHttpServletRequest;
+import org.springframework.test.web.servlet.request.MockMvcRequestBuilders;
+import org.springframework.web.servlet.function.ServerRequest;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * @author raccoonback
+ */
+class BeforeFilterFunctionsTests {
+
+	@Test
+	void rewriteRequestParameter() {
+		MockHttpServletRequest servletRequest = MockMvcRequestBuilders.get("http://localhost/path")
+			.param("foo", "bar")
+			.param("baz", "qux")
+			.buildRequest(null);
+
+		ServerRequest request = ServerRequest.create(servletRequest, Collections.emptyList());
+
+		ServerRequest result = BeforeFilterFunctions.rewriteRequestParameter("foo", "replacement").apply(request);
+
+		assertThat(result.param("foo")).isPresent().hasValue("replacement");
+		assertThat(result.uri().toString()).hasToString("http://localhost/path?baz=qux&foo=replacement");
+	}
+
+	@Test
+	void rewriteOnlyFirstRequestParameter() {
+		MockHttpServletRequest servletRequest = MockMvcRequestBuilders.get("http://localhost/path")
+			.param("foo", "bar_1")
+			.param("foo", "bar_2")
+			.param("foo", "bar_3")
+			.param("baz", "qux")
+			.buildRequest(null);
+
+		ServerRequest request = ServerRequest.create(servletRequest, Collections.emptyList());
+
+		ServerRequest result = BeforeFilterFunctions.rewriteRequestParameter("foo", "replacement").apply(request);
+
+		assertThat(result.param("foo")).isPresent().hasValue("replacement");
+		assertThat(result.uri().toString()).hasToString("http://localhost/path?baz=qux&foo=replacement");
+	}
+
+	@Test
+	void rewriteEncodedRequestParameter() {
+		MockHttpServletRequest servletRequest = MockMvcRequestBuilders.get("http://localhost/path")
+			.param("foo[]", "bar")
+			.param("baz", "qux")
+			.buildRequest(null);
+
+		ServerRequest request = ServerRequest.create(servletRequest, Collections.emptyList());
+
+		ServerRequest result = BeforeFilterFunctions.rewriteRequestParameter("foo[]", "replacement[]").apply(request);
+
+		assertThat(result.param("foo[]")).isPresent().hasValue("replacement[]");
+		assertThat(result.uri().toString()).hasToString("http://localhost/path?baz=qux&foo%5B%5D=replacement%5B%5D");
+	}
+
+	@Test
+	void rewriteRequestParameterWithEncodedRemainParameters() {
+		MockHttpServletRequest servletRequest = MockMvcRequestBuilders.get("http://localhost/path")
+			.param("foo", "bar")
+			.param("baz[]", "qux[]")
+			.buildRequest(null);
+
+		ServerRequest request = ServerRequest.create(servletRequest, Collections.emptyList());
+
+		ServerRequest result = BeforeFilterFunctions.rewriteRequestParameter("foo", "replacement").apply(request);
+
+		assertThat(result.param("foo")).isPresent().hasValue("replacement");
+		assertThat(result.uri().toString()).hasToString("http://localhost/path?baz%5B%5D=qux%5B%5D&foo=replacement");
+	}
+
+	@Test
+	void rewriteRequestParameterWithEncodedPath() {
+		MockHttpServletRequest servletRequest = MockMvcRequestBuilders.get("http://localhost/path/é/last")
+			.param("foo", "bar")
+			.buildRequest(null);
+
+		ServerRequest request = ServerRequest.create(servletRequest, Collections.emptyList());
+
+		ServerRequest result = BeforeFilterFunctions.rewriteRequestParameter("foo", "replacement").apply(request);
+
+		assertThat(result.param("foo")).isPresent().hasValue("replacement");
+		assertThat(result.uri().toString()).hasToString("http://localhost/path/%C3%A9/last?foo=replacement");
+	}
+
+}