Proper exception for controller method return types that do not work with MvcUriComponentsBuilder (e.g. final classes)

Includes direct use of ControllerMethodInvocationInterceptor for return type Object, avoiding the attempt to generate an Object subclass.

Issue: SPR-16710

spring-webmvc/src/main/java/org/springframework/web/servlet/mvc/method/annotation/MvcUriComponentsBuilder.java

@@ -253,10 +253,8 @@ public class MvcUriComponentsBuilder {
 	 * controller.getAddressesForCountry("US")
 	 * builder = MvcUriComponentsBuilder.fromMethodCall(controller);
 	 * </pre>
-	 *
 	 * <p><strong>Note:</strong> This method extracts values from "Forwarded"
 	 * and "X-Forwarded-*" headers if found. See class-level docs.
-	 *
 	 * @param info either the value returned from a "mock" controller
 	 * invocation or the "mock" controller itself after an invocation
 	 * @return a UriComponents instance
@@ -610,7 +608,11 @@ public class MvcUriComponentsBuilder {
 
 	@SuppressWarnings("unchecked")
 	private static <T> T initProxy(Class<?> type, ControllerMethodInvocationInterceptor interceptor) {
-		if (type.isInterface()) {
+		if (type == Object.class) {
+			return (T) interceptor;
+		}
+
+		else if (type.isInterface()) {
 			ProxyFactory factory = new ProxyFactory(EmptyTargetSource.INSTANCE);
 			factory.addInterface(type);
 			factory.addInterface(MethodInvocationInfo.class);
@@ -709,8 +711,18 @@ public class MvcUriComponentsBuilder {
 	}
 
 
+	public interface MethodInvocationInfo {
+
+		Class<?> getControllerType();
+
+		Method getControllerMethod();
+
+		Object[] getArgumentValues();
+	}
+
+
 	private static class ControllerMethodInvocationInterceptor
-			implements org.springframework.cglib.proxy.MethodInterceptor, MethodInterceptor {
+			implements org.springframework.cglib.proxy.MethodInterceptor, MethodInterceptor, MethodInvocationInfo {
 
 		private final Class<?> controllerType;
 
@@ -727,15 +739,15 @@ public class MvcUriComponentsBuilder {
 		@Override
 		@Nullable
 		public Object intercept(Object obj, Method method, Object[] args, @Nullable MethodProxy proxy) {
-			if (method.getName().equals("getControllerMethod")) {
+			if (method.getName().equals("getControllerType")) {
+				return this.controllerType;
+			}
+			else if (method.getName().equals("getControllerMethod")) {
 				return this.controllerMethod;
 			}
 			else if (method.getName().equals("getArgumentValues")) {
 				return this.argumentValues;
 			}
-			else if (method.getName().equals("getControllerType")) {
-				return this.controllerType;
-			}
 			else if (ReflectionUtils.isObjectMethod(method)) {
 				return ReflectionUtils.invokeMethod(method, obj, args);
 			}
@@ -743,7 +755,13 @@ public class MvcUriComponentsBuilder {
 				this.controllerMethod = method;
 				this.argumentValues = args;
 				Class<?> returnType = method.getReturnType();
-				return (void.class == returnType ? null : returnType.cast(initProxy(returnType, this)));
+				try {
+					return (returnType == void.class ? null : returnType.cast(initProxy(returnType, this)));
+				}
+				catch (Throwable ex) {
+					throw new IllegalStateException(
+							"Failed to create proxy for controller method return type: " + method, ex);
+				}
 			}
 		}
 
@@ -752,16 +770,23 @@ public class MvcUriComponentsBuilder {
 		public Object invoke(org.aopalliance.intercept.MethodInvocation inv) throws Throwable {
 			return intercept(inv.getThis(), inv.getMethod(), inv.getArguments(), null);
 		}
-	}
 
+		@Override
+		public Class<?> getControllerType() {
+			return this.controllerType;
+		}
 
-	public interface MethodInvocationInfo {
+		@Override
+		public Method getControllerMethod() {
+			Assert.state(this.controllerMethod != null, "Not initialized yet");
+			return this.controllerMethod;
+		}
 
-		Method getControllerMethod();
-
-		Object[] getArgumentValues();
-
-		Class<?> getControllerType();
+		@Override
+		public Object[] getArgumentValues() {
+			Assert.state(this.argumentValues != null, "Not initialized yet");
+			return this.argumentValues;
+		}
 	}