fix: preserve query string params for multipart requests in Gateway MVC

Fixes gh-3220

Signed-off-by: Arindam Singh <96876969+ar249@users.noreply.github.com>

Author: ar249

spring-cloud-gateway-server-webmvc/src/main/java/org/springframework/cloud/gateway/server/mvc/handler/GatewayMvcMultipartResolver.java

@@ -16,12 +16,14 @@
 
 package org.springframework.cloud.gateway.server.mvc.handler;
 
-import java.util.Collections;
+import java.nio.charset.StandardCharsets;
 import java.util.Map;
+import java.util.stream.Collectors;
 
 import jakarta.servlet.http.HttpServletRequest;
 
 import org.springframework.cloud.gateway.server.mvc.common.MvcUtils;
+import org.springframework.util.MultiValueMap;
 import org.springframework.web.multipart.MultipartException;
 import org.springframework.web.multipart.MultipartHttpServletRequest;
 import org.springframework.web.multipart.support.StandardMultipartHttpServletRequest;
@@ -67,7 +69,13 @@ protected void initializeMultipart() {
 		@Override
 		public Map<String, String[]> getParameterMap() {
 			if (isGatewayRequest(getRequest())) {
-				return Collections.emptyMap();
+				MultiValueMap<String, String> queryParams =
+						MvcUtils.decodeQueryString(getRequest().getQueryString(), StandardCharsets.UTF_8);
+
+				return queryParams.entrySet()
+						.stream()
+						.collect(Collectors.toMap(
+								Map.Entry::getKey, e -> e.getValue().toArray(new String[0])));
 			}
 			return super.getParameterMap();
 		}

spring-cloud-gateway-server-webmvc/src/test/java/org/springframework/cloud/gateway/server/mvc/handler/GatewayMvcMultipartResolverTest.java

@@ -0,0 +1,111 @@
+package org.springframework.cloud.gateway.server.mvc.handler;
+
+import org.apache.commons.lang3.StringUtils;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+import org.springframework.cloud.gateway.server.mvc.common.MvcUtils;
+import org.springframework.mock.web.MockHttpServletRequest;
+
+import java.nio.charset.StandardCharsets;
+import java.util.Map;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.never;
+import static org.mockito.Mockito.spy;
+import static org.mockito.Mockito.verify;
+
+public class GatewayMvcMultipartResolverTest {
+	private MockHttpServletRequest mockRequest;
+
+	@BeforeEach
+	public void setUp() {
+		mockRequest = new MockHttpServletRequest();
+		mockRequest.setContentType("multipart/form-data; boundary=----boundary");
+	}
+
+	private GatewayMvcMultipartResolver.GatewayMultipartHttpServletRequest buildWrapper() {
+		return new GatewayMvcMultipartResolver.GatewayMultipartHttpServletRequest(mockRequest);
+	}
+
+	private void makeGatewayRequest() {
+		mockRequest.setAttribute(MvcUtils.GATEWAY_ROUTE_ID_ATTR, "my-route");
+	}
+
+	@Test
+	public void getParameterMapGatewayRequestMultipleDistinctParams() {
+		makeGatewayRequest();
+		mockRequest.setQueryString("foo=bar&baz=qux");
+
+		Map<String, String[]> result = buildWrapper().getParameterMap();
+
+		assertThat(result).containsKeys("foo", "baz");
+		assertThat(result.get("foo")).containsExactly("bar");
+		assertThat(result.get("baz")).containsExactly("qux");
+	}
+
+	@Test
+	public void getParameterMapGatewayRequestNullQueryStringReturnsEmptyMap() {
+		makeGatewayRequest();
+		mockRequest.setQueryString(null);
+
+		Map<String, String[]> result = buildWrapper().getParameterMap();
+
+		assertThat(result).isEmpty();
+	}
+
+	@Test
+	public void getParameterMapGatewayRequestEmptyQueryStringReturnsEmptyMap() {
+		makeGatewayRequest();
+		mockRequest.setQueryString(StringUtils.EMPTY);
+
+		Map<String, String[]> result = buildWrapper().getParameterMap();
+
+		assertThat(result).isEmpty();
+	}
+
+	@Test
+	public void getParameterMap_gatewayRequest_multipartBodyIsNotParsed() {
+		makeGatewayRequest();
+
+		String body = """
+				------TestBoundary\r
+				Content-Disposition: form-data; name="file"; filename="test.txt"\r
+				Content-Type: text/plain\r
+				\r
+				file content here\r
+				------TestBoundary\r
+				Content-Disposition: form-data; name="field1"\r
+				\r
+				value1\r
+				------TestBoundary--\r
+				""";
+
+		mockRequest.setContentType("multipart/form-data; boundary=----TestBoundary");
+		mockRequest.setContent(body.getBytes(StandardCharsets.UTF_8));
+		mockRequest.setQueryString("queryParam=fromQuery");
+
+		GatewayMvcMultipartResolver.GatewayMultipartHttpServletRequest wrapper =
+				spy(new GatewayMvcMultipartResolver.GatewayMultipartHttpServletRequest(mockRequest));
+
+		Map<String, String[]> params = wrapper.getParameterMap();
+
+		// multipart parsing isn't triggered
+		verify(wrapper, never()).initializeMultipart();
+
+		// Body parts must not appear — only query string should be visible
+		assertThat(params).containsOnlyKeys("queryParam");
+		assertThat(params.get("queryParam")).containsExactly("fromQuery");
+		assertThat(params).doesNotContainKey("field1").doesNotContainKey("file");
+	}
+
+	@Test
+	public void getParameterMapNonGatewayRequestNoAttributesDelegatesToSuper() {
+		mockRequest.setParameter("superKey", "superValue");
+
+		Map<String, String[]> result = buildWrapper().getParameterMap();
+
+		assertThat(result).containsKey("superKey");
+		assertThat(result.get("superKey")).containsExactly("superValue");
+	}
+}