Add support for @RequestMapping on Kotlin property accessors

This commit refines InvocableHandlerMethod (both Servlet and
Reactive variants) in order to support annotated property
accessors as they translate into regular Java methods, instead
of throwing a NullPointerException.

Closes gh-31856

spring-web/src/main/java/org/springframework/web/method/support/InvocableHandlerMethod.java

@@ -21,7 +21,6 @@ import java.lang.reflect.Method;
 import java.lang.reflect.Modifier;
 import java.util.Arrays;
 import java.util.Map;
-import java.util.Objects;
 
 import kotlin.Unit;
 import kotlin.jvm.JvmClassMappingKt;
@@ -306,8 +305,12 @@ public class InvocableHandlerMethod extends HandlerMethod {
 
 		@Nullable
 		@SuppressWarnings("deprecation")
-		public static Object invokeFunction(Method method, Object target, Object[] args) {
+		public static Object invokeFunction(Method method, Object target, Object[] args) throws InvocationTargetException, IllegalAccessException {
-			KFunction<?> function = Objects.requireNonNull(ReflectJvmMapping.getKotlinFunction(method));
+			KFunction<?> function = ReflectJvmMapping.getKotlinFunction(method);
+			// For property accessors
+			if (function == null) {
+				return method.invoke(target, args);
+			}
 			if (method.isAccessible() && !KCallablesJvm.isAccessible(function)) {
 				KCallablesJvm.setAccessible(function, true);
 			}

spring-web/src/test/kotlin/org/springframework/web/method/support/InvocableHandlerMethodKotlinTests.kt

@@ -98,6 +98,12 @@ class InvocableHandlerMethodKotlinTests {
 		Assertions.assertThat(value).isEqualTo(3.1)
 	}
 
+	@Test
+	fun propertyAccessor() {
+		val value = getInvocable(PropertyAccessorHandler::class.java).invokeForRequest(request, null)
+		Assertions.assertThat(value).isEqualTo("foo")
+	}
+
 	private fun getInvocable(clazz: Class<*>, vararg argTypes: Class<*>): InvocableHandlerMethod {
 		val method = ResolvableMethod.on(clazz).argTypes(*argTypes).resolveMethod()
 		val handlerMethod = InvocableHandlerMethod(clazz.constructors.first().newInstance(), method)
@@ -138,6 +144,12 @@ class InvocableHandlerMethodKotlinTests {
 			limit.value
 	}
 
+	private class PropertyAccessorHandler {
+
+		val prop: String
+			get() = "foo"
+	}
+
 	@JvmInline
 	value class LongValueClass(val value: Long)
 

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/InvocableHandlerMethod.java

@@ -24,7 +24,6 @@ import java.lang.reflect.Type;
 import java.util.ArrayList;
 import java.util.List;
 import java.util.Map;
-import java.util.Objects;
 import java.util.stream.Stream;
 
 import kotlin.Unit;
@@ -306,7 +305,7 @@ public class InvocableHandlerMethod extends HandlerMethod {
 		@Nullable
 		@SuppressWarnings("deprecation")
 		public static Object invokeFunction(Method method, Object target, Object[] args, boolean isSuspendingFunction,
-				ServerWebExchange exchange) {
+				ServerWebExchange exchange) throws InvocationTargetException, IllegalAccessException {
 
 			if (isSuspendingFunction) {
 				Object coroutineContext = exchange.getAttribute(COROUTINE_CONTEXT_ATTRIBUTE);
@@ -318,7 +317,11 @@ public class InvocableHandlerMethod extends HandlerMethod {
 				}
 			}
 			else {
-				KFunction<?> function = Objects.requireNonNull(ReflectJvmMapping.getKotlinFunction(method));
+				KFunction<?> function = ReflectJvmMapping.getKotlinFunction(method);
+				// For property accessors
+				if (function == null) {
+					return method.invoke(target, args);
+				}
 				if (method.isAccessible() && !KCallablesJvm.isAccessible(function)) {
 					KCallablesJvm.setAccessible(function, true);
 				}

spring-webflux/src/test/kotlin/org/springframework/web/reactive/result/InvocableHandlerMethodKotlinTests.kt

@@ -24,6 +24,7 @@ import org.junit.jupiter.api.Test
 import org.springframework.core.ReactiveAdapterRegistry
 import org.springframework.http.HttpStatus
 import org.springframework.http.server.reactive.ServerHttpResponse
+import org.springframework.web.bind.annotation.GetMapping
 import org.springframework.web.bind.annotation.RequestParam
 import org.springframework.web.bind.annotation.ResponseStatus
 import org.springframework.web.reactive.BindingContext
@@ -194,6 +195,14 @@ class InvocableHandlerMethodKotlinTests {
 		assertHandlerResultValue(result, "3.1")
 	}
 
+	@Test
+	fun propertyAccessor() {
+		this.resolvers.add(stubResolver(Mono.empty()))
+		val method = PropertyAccessorController::prop.getter.javaMethod!!
+		val result = invoke(PropertyAccessorController(), method)
+		assertHandlerResultValue(result, "foo")
+	}
+
 
 	private fun invokeForResult(handler: Any, method: Method, vararg providedArgs: Any): HandlerResult? {
 		return invoke(handler, method, *providedArgs).block(Duration.ofSeconds(5))
@@ -293,6 +302,13 @@ class InvocableHandlerMethodKotlinTests {
 
 	}
 
+	class PropertyAccessorController {
+
+		val prop: String
+			@GetMapping("/")
+			get() = "foo"
+	}
+
 	@JvmInline
 	value class LongValueClass(val value: Long)