Add @ModelAttribute support for HTTP interface clients

spring-web/src/main/java/org/springframework/web/service/invoker/HttpServiceProxyFactory.java

@@ -242,6 +242,7 @@ private List<HttpServiceArgumentResolver> initArgumentResolvers() {
 			resolvers.add(new RequestBodyArgumentResolver(this.exchangeAdapter));
 			resolvers.add(new PathVariableArgumentResolver(service));
 			resolvers.add(new RequestParamArgumentResolver(service));
+			resolvers.add(new ModelAttributeArgumentResolver(service));
 			resolvers.add(new RequestPartArgumentResolver(this.exchangeAdapter));
 			resolvers.add(new CookieValueArgumentResolver(service));
 			if (this.exchangeAdapter.supportsRequestAttributes()) {

spring-web/src/main/java/org/springframework/web/service/invoker/ModelAttributeArgumentResolver.java

@@ -0,0 +1,124 @@
+/*
+ * Copyright 2002-present 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.web.service.invoker;
+
+import org.jspecify.annotations.Nullable;
+import org.springframework.beans.BeanWrapper;
+import org.springframework.beans.PropertyAccessorFactory;
+import org.springframework.core.MethodParameter;
+import org.springframework.core.convert.ConversionService;
+import org.springframework.util.ObjectUtils;
+import org.springframework.web.bind.annotation.BindParam;
+import org.springframework.web.bind.annotation.ModelAttribute;
+
+import java.beans.PropertyDescriptor;
+import java.lang.reflect.Field;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.HashMap;
+import java.util.Map;
+
+/**
+ * Resolves {@link ModelAttribute}-annotated method parameters by expanding a bean
+ * into request parameters for an HTTP client.
+ *
+ * <p>Behavior:
+ * <ul>
+ *   <li>Each readable bean property yields a request parameter named after the property.</li>
+ *   <li>{@link BindParam} can override the parameter name. It is supported on both fields and
+ *       getter methods; if both are present, the getter annotation wins.</li>
+ *   <li>Null property values are skipped.</li>
+ *   <li>Values are converted to strings via the configured {@link ConversionService} when
+ *       possible; otherwise, {@code toString()} is used as a fallback.</li>
+ * </ul>
+ *
+ * @author Hermann Pencole
+ * @since 7.0
+ */
+public class ModelAttributeArgumentResolver extends AbstractNamedValueArgumentResolver {
+	private final ConversionService conversionService;
+
+	/**
+	 * Constructor for a resolver to a String value.
+	 * @param conversionService the {@link ConversionService} to use to format
+	 * Object to String values
+	 */
+	public ModelAttributeArgumentResolver(ConversionService conversionService) {
+		super();
+		this.conversionService = conversionService;
+	}
+
+
+	@Override
+	protected @Nullable NamedValueInfo createNamedValueInfo(MethodParameter parameter) {
+		ModelAttribute annot = parameter.getParameterAnnotation(ModelAttribute.class);
+		if (annot == null) {
+			return null;
+		}
+		return new NamedValueInfo(
+				annot.name(), false, null, "model attribute",
+				true);
+	}
+
+	@Override
+	protected void addRequestValue(String name, Object argument, MethodParameter parameter, HttpRequestValues.Builder requestValues) {
+		// Create a map to store custom parameter names
+		Map<String, String> customParamNames = new HashMap<>();
+
+		// Retrieve all @BindParam annotations
+		Class<?> clazz = argument.getClass();
+		for (Field field : clazz.getDeclaredFields()) {
+			BindParam bindParam = field.getAnnotation(BindParam.class);
+			if (bindParam != null) {
+				customParamNames.put(field.getName(), bindParam.value());
+			}
+		}
+
+		// Convert object to query parameters
+		BeanWrapper wrapper = PropertyAccessorFactory.forBeanPropertyAccess(argument);
+		for (PropertyDescriptor descriptor : wrapper.getPropertyDescriptors()) {
+			String propertyName = descriptor.getName();
+			if (!"class".equals(propertyName)) {
+				Object value = wrapper.getPropertyValue(propertyName);
+				if (value != null) {
+					// Use a custom name if it exists, otherwise use the property name
+					String paramName = customParamNames.getOrDefault(propertyName, propertyName);
+					requestValues.addRequestParameter(paramName, convertSingleToString(value));
+				}
+			}
+		}
+	}
+
+	/**
+	 * Convert an arbitrary value to a string using the configured {@link ConversionService}
+	 * when possible, otherwise falls back to {@code toString()}.
+	 */
+	private String convertSingleToString(Object value) {
+		try {
+			if (this.conversionService.canConvert(value.getClass(), String.class)) {
+				String converted = this.conversionService.convert(value, String.class);
+				return converted != null ? converted : "";
+			}
+		} catch (Exception ignore) {
+			// Fallback to toString below
+		}
+		return String.valueOf(value);
+	}
+
+
+
+}

spring-web/src/test/java/org/springframework/web/service/invoker/ModelAttributeArgumentResolverTests.java

@@ -0,0 +1,98 @@
+/*
+ * Copyright 2002-present 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.web.service.invoker;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.core.convert.support.DefaultConversionService;
+import org.springframework.util.MultiValueMap;
+import org.springframework.web.bind.annotation.BindParam;
+import org.springframework.web.bind.annotation.ModelAttribute;
+import org.springframework.web.bind.annotation.RequestParam;
+import org.springframework.web.service.annotation.GetExchange;
+import org.springframework.web.service.annotation.PostExchange;
+import org.springframework.web.util.UriComponents;
+import org.springframework.web.util.UriComponentsBuilder;
+
+import java.util.List;
+import java.util.Map;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link ModelAttributeArgumentResolver}.
+ *
+ * <p>Additional tests for this resolver:
+ * <ul>
+ * <li>Base class functionality in {@link NamedValueArgumentResolverTests}
+ * <li>Form data vs query params in {@link HttpRequestValuesTests}
+ * </ul>
+ *
+ * @author Hermann Pencole
+ */
+class ModelAttributeArgumentResolverTests {
+
+	private final TestExchangeAdapter client = new TestExchangeAdapter();
+
+	private final HttpServiceProxyFactory.Builder builder = HttpServiceProxyFactory.builderFor(this.client);
+
+
+	@Test
+	void requestParam() {
+		Service service = builder.build().createClient(Service.class);
+		service.postForm(new MyBean1_2("value 1", true),new MyBean3_4("value 3", List.of("1", "2", "3")));
+
+		Object body = this.client.getRequestValues().getBodyValue();
+		assertThat(body).isInstanceOf(MultiValueMap.class);
+		assertThat((MultiValueMap<String, String>) body).hasSize(4)
+				.containsEntry("param.1", List.of("value 1"))
+				.containsEntry("param2", List.of("true"))
+				.containsEntry("param.3", List.of("value 3"))
+				.containsEntry("param4", List.of("1,2,3"));
+	}
+
+	@Test
+	void requestParamWithDisabledFormattingCollectionValue() {
+		Service service = builder.build().createClient(Service.class);
+		service.getWithParams(new MyBean1_2("value 1", true),new MyBean3_4("value 3",  List.of("1", "2", "3")));
+
+		HttpRequestValues values = this.client.getRequestValues();
+		String uriTemplate = values.getUriTemplate();
+		Map<String, String> uriVariables = values.getUriVariables();
+		UriComponents uri = UriComponentsBuilder.fromUriString(uriTemplate).buildAndExpand(uriVariables).encode();
+		assertThat(uri.getQuery()).isEqualTo("param.1=value%201&param2=true&param.3=value%203&param4=1,2,3");
+	}
+
+	private interface Service {
+
+		@PostExchange(contentType = "application/x-www-form-urlencoded")
+		void postForm(@ModelAttribute MyBean1_2 param1_2, @ModelAttribute MyBean3_4 param3_4);
+
+		@GetExchange
+		void getWithParams(@ModelAttribute MyBean1_2 param1_2, @ModelAttribute MyBean3_4 param3_4);
+	}
+
+	private record MyBean1_2 (
+			@BindParam("param.1") String param1,
+			Boolean param2
+	){}
+
+	private record MyBean3_4 (
+			@BindParam("param.3") String param3,
+			List<String> param4
+	){}
+
+}