Refine MethodParameter#isOptional Kotlin implementation

This commit adds support for Continuation parameter that is now
considered as an optional parameter since it is never provided by
the user.

It also simplifies and optimizes the implementation.

Closes gh-23991

spring-core/src/main/java/org/springframework/core/MethodParameter.java

@@ -26,11 +26,9 @@ import java.lang.reflect.Parameter;
 import java.lang.reflect.ParameterizedType;
 import java.lang.reflect.Type;
 import java.util.HashMap;
-import java.util.List;
 import java.util.Map;
 import java.util.Optional;
 import java.util.function.Predicate;
-import java.util.stream.Collectors;
 
 import kotlin.reflect.KFunction;
 import kotlin.reflect.KParameter;
@@ -398,7 +396,7 @@ public class MethodParameter {
 	 * either in the form of Java 8's {@link java.util.Optional}, any variant
 	 * of a parameter-level {@code Nullable} annotation (such as from JSR-305
 	 * or the FindBugs set of annotations), or a language-level nullable type
-	 * declaration in Kotlin.
+	 * declaration or {@code Continuation} parameter in Kotlin.
 	 * @since 4.3
 	 */
 	public boolean isOptional() {
@@ -867,37 +865,39 @@ public class MethodParameter {
 	private static class KotlinDelegate {
 
 		/**
-		 * Check whether the specified {@link MethodParameter} represents a nullable Kotlin type
-		 * or an optional parameter (with a default value in the Kotlin declaration).
+		 * Check whether the specified {@link MethodParameter} represents a nullable Kotlin type,
+		 * an optional parameter (with a default value in the Kotlin declaration) or a {@code Continuation} parameter
+		 * used in suspending functions.
 		 */
 		public static boolean isOptional(MethodParameter param) {
 			Method method = param.getMethod();
-			Constructor<?> ctor = param.getConstructor();
 			int index = param.getParameterIndex();
 			if (method != null && index == -1) {
 				KFunction<?> function = ReflectJvmMapping.getKotlinFunction(method);
 				return (function != null && function.getReturnType().isMarkedNullable());
 			}
-			else {
-				KFunction<?> function = null;
-				Predicate<KParameter> predicate = null;
-				if (method != null) {
-					function = ReflectJvmMapping.getKotlinFunction(method);
-					predicate = p -> KParameter.Kind.VALUE.equals(p.getKind());
-				}
-				else if (ctor != null) {
-					function = ReflectJvmMapping.getKotlinFunction(ctor);
-					predicate = p -> KParameter.Kind.VALUE.equals(p.getKind()) ||
-							KParameter.Kind.INSTANCE.equals(p.getKind());
+			KFunction<?> function;
+			Predicate<KParameter> predicate;
+			if (method != null) {
+				if (param.parameterType.getName().equals("kotlin.coroutines.Continuation")) {
+					return true;
 				}
-				if (function != null) {
-					List<KParameter> parameters = function.getParameters();
-					KParameter parameter = parameters
-							.stream()
-							.filter(predicate)
-							.collect(Collectors.toList())
-							.get(index);
-					return (parameter.getType().isMarkedNullable() || parameter.isOptional());
+				function = ReflectJvmMapping.getKotlinFunction(method);
+				predicate = p -> KParameter.Kind.VALUE.equals(p.getKind());
+			}
+			else {
+				function = ReflectJvmMapping.getKotlinFunction(param.getConstructor());
+				predicate = p -> KParameter.Kind.VALUE.equals(p.getKind()) ||
+						KParameter.Kind.INSTANCE.equals(p.getKind());
+			}
+			if (function != null) {
+				int i = 0;
+				for (KParameter kParameter : function.getParameters()) {
+					if (predicate.test(kParameter)) {
+						if (index == i++) {
+							return (kParameter.getType().isMarkedNullable() || kParameter.isOptional());
+						}
+					}
 				}
 			}
 			return false;

spring-core/src/test/kotlin/org/springframework/core/KotlinMethodParameterTests.kt

@@ -20,6 +20,7 @@ import org.assertj.core.api.Assertions.assertThat
 import org.junit.jupiter.api.Test
 import java.lang.reflect.Method
 import java.lang.reflect.TypeVariable
+import kotlin.coroutines.Continuation
 import kotlin.reflect.full.declaredFunctions
 import kotlin.reflect.jvm.javaMethod
 
@@ -101,6 +102,13 @@ class KotlinMethodParameterTests {
 		assertThat(returnGenericParameterType("suspendFun8")).isEqualTo(Object::class.java)
 	}
 
+	@Test
+	fun `Continuation parameter is optional`() {
+		val method = this::class.java.getDeclaredMethod("suspendFun", String::class.java, Continuation::class.java)
+		assertThat(MethodParameter(method, 0).isOptional).isFalse()
+		assertThat(MethodParameter(method, 1).isOptional).isTrue()
+	}
+
 	private fun returnParameterType(funName: String) = returnMethodParameter(funName).parameterType
 	private fun returnGenericParameterType(funName: String) = returnMethodParameter(funName).genericParameterType
 	private fun returnGenericParameterTypeName(funName: String) = returnGenericParameterType(funName).typeName