Support default methods in @HttpExchange interface

Closes gh-28491

Author: rstoyanchev

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

@@ -16,6 +16,7 @@
 
 package org.springframework.web.service.invoker;
 
+import java.lang.reflect.InvocationHandler;
 import java.lang.reflect.Method;
 import java.time.Duration;
 import java.util.ArrayList;
@@ -28,6 +29,7 @@
 import org.aopalliance.intercept.MethodInvocation;
 
 import org.springframework.aop.framework.ProxyFactory;
+import org.springframework.aop.framework.ReflectiveMethodInvocation;
 import org.springframework.beans.factory.InitializingBean;
 import org.springframework.context.EmbeddedValueResolverAware;
 import org.springframework.core.MethodIntrospector;
@@ -215,10 +217,19 @@ private HttpServiceMethodInterceptor(List<HttpServiceMethod> methods) {
 		}
 
 		@Override
-		public Object invoke(MethodInvocation invocation) {
+		public Object invoke(MethodInvocation invocation) throws Throwable {
 			Method method = invocation.getMethod();
 			HttpServiceMethod httpServiceMethod = this.httpServiceMethods.get(method);
-			return httpServiceMethod.invoke(invocation.getArguments());
+			if (httpServiceMethod != null) {
+				return httpServiceMethod.invoke(invocation.getArguments());
+			}
+			if (method.isDefault()) {
+				if (invocation instanceof ReflectiveMethodInvocation reflectiveMethodInvocation) {
+					Object proxy = reflectiveMethodInvocation.getProxy();
+					return InvocationHandler.invokeDefault(proxy, method, invocation.getArguments());
+				}
+			}
+			throw new IllegalStateException("Unexpected method invocation: " + method);
 		}
 	}
 

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

@@ -100,6 +100,8 @@ void reactorService() {
 		Mono<ResponseEntity<Flux<String>>> fluxEntity= service.getFluxEntity();
 		StepVerifier.create(fluxEntity.flatMapMany(HttpEntity::getBody)).expectNext("request", "To", "Entity", "Flux").verifyComplete();
 		verifyClientInvocation("requestToEntityFlux", BODY_TYPE);
+
+		assertThat(service.getDefaultMethodValue()).isEqualTo("default value");
 	}
 
 	@Test
@@ -228,6 +230,10 @@ private interface ReactorService {
 
 		@GetExchange
 		Mono<ResponseEntity<Flux<String>>> getFluxEntity();
+
+		default String getDefaultMethodValue() {
+			return "default value";
+		}
 	}