/*
 * Copyright 2002-2018 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.core;

import java.lang.annotation.Annotation;
import java.lang.reflect.AnnotatedElement;
import java.lang.reflect.Constructor;
import java.lang.reflect.Executable;
import java.lang.reflect.Member;
import java.lang.reflect.Method;
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;
import kotlin.reflect.jvm.ReflectJvmMapping;

import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;

/**
 * Helper class that encapsulates the specification of a method parameter, i.e. a {@link Method}
 * or {@link Constructor} plus a parameter index and a nested type index for a declared generic
 * type. Useful as a specification object to pass along.
 *
 * <p>As of 4.2, there is a {@link org.springframework.core.annotation.SynthesizingMethodParameter}
 * subclass available which synthesizes annotations with attribute aliases. That subclass is used
 * for web and message endpoint processing, in particular.
 *
 * @author Juergen Hoeller
 * @author Rob Harrop
 * @author Andy Clement
 * @author Sam Brannen
 * @author Sebastien Deleuze
 * @since 2.0
 * @see org.springframework.core.annotation.SynthesizingMethodParameter
 */
public class MethodParameter {

	private static final Annotation[] EMPTY_ANNOTATION_ARRAY = new Annotation[0];

	private final Executable executable;

	private final int parameterIndex;

	@Nullable
	private volatile Parameter parameter;

	private int nestingLevel = 1;

	/** Map from Integer level to Integer type index. */
	@Nullable
	Map<Integer, Integer> typeIndexesPerLevel;

	@Nullable
	private volatile Class<?> containingClass;

	@Nullable
	private volatile Class<?> parameterType;

	@Nullable
	private volatile Type genericParameterType;

	@Nullable
	private volatile Annotation[] parameterAnnotations;

	@Nullable
	private volatile ParameterNameDiscoverer parameterNameDiscoverer;

	@Nullable
	private volatile String parameterName;

	@Nullable
	private volatile MethodParameter nestedMethodParameter;


	/**
	 * Create a new {@code MethodParameter} for the given method, with nesting level 1.
	 * @param method the Method to specify a parameter for
	 * @param parameterIndex the index of the parameter: -1 for the method
	 * return type; 0 for the first method parameter; 1 for the second method
	 * parameter, etc.
	 */
	public MethodParameter(Method method, int parameterIndex) {
		this(method, parameterIndex, 1);
	}

	/**
	 * Create a new {@code MethodParameter} for the given method.
	 * @param method the Method to specify a parameter for
	 * @param parameterIndex the index of the parameter: -1 for the method
	 * return type; 0 for the first method parameter; 1 for the second method
	 * parameter, etc.
	 * @param nestingLevel the nesting level of the target type
	 * (typically 1; e.g. in case of a List of Lists, 1 would indicate the
	 * nested List, whereas 2 would indicate the element of the nested List)
	 */
	public MethodParameter(Method method, int parameterIndex, int nestingLevel) {
		Assert.notNull(method, "Method must not be null");
		this.executable = method;
		this.parameterIndex = validateIndex(method, parameterIndex);
		this.nestingLevel = nestingLevel;
	}

	/**
	 * Create a new MethodParameter for the given constructor, with nesting level 1.
	 * @param constructor the Constructor to specify a parameter for
	 * @param parameterIndex the index of the parameter
	 */
	public MethodParameter(Constructor<?> constructor, int parameterIndex) {
		this(constructor, parameterIndex, 1);
	}

	/**
	 * Create a new MethodParameter for the given constructor.
	 * @param constructor the Constructor to specify a parameter for
	 * @param parameterIndex the index of the parameter
	 * @param nestingLevel the nesting level of the target type
	 * (typically 1; e.g. in case of a List of Lists, 1 would indicate the
	 * nested List, whereas 2 would indicate the element of the nested List)
	 */
	public MethodParameter(Constructor<?> constructor, int parameterIndex, int nestingLevel) {
		Assert.notNull(constructor, "Constructor must not be null");
		this.executable = constructor;
		this.parameterIndex = validateIndex(constructor, parameterIndex);
		this.nestingLevel = nestingLevel;
	}

	/**
	 * Copy constructor, resulting in an independent MethodParameter object
	 * based on the same metadata and cache state that the original object was in.
	 * @param original the original MethodParameter object to copy from
	 */
	public MethodParameter(MethodParameter original) {
		Assert.notNull(original, "Original must not be null");
		this.executable = original.executable;
		this.parameterIndex = original.parameterIndex;
		this.parameter = original.parameter;
		this.nestingLevel = original.nestingLevel;
		this.typeIndexesPerLevel = original.typeIndexesPerLevel;
		this.containingClass = original.containingClass;
		this.parameterType = original.parameterType;
		this.genericParameterType = original.genericParameterType;
		this.parameterAnnotations = original.parameterAnnotations;
		this.parameterNameDiscoverer = original.parameterNameDiscoverer;
		this.parameterName = original.parameterName;
	}


	/**
	 * Return the wrapped Method, if any.
	 * <p>Note: Either Method or Constructor is available.
	 * @return the Method, or {@code null} if none
	 */
	@Nullable
	public Method getMethod() {
		return (this.executable instanceof Method ? (Method) this.executable : null);
	}

	/**
	 * Return the wrapped Constructor, if any.
	 * <p>Note: Either Method or Constructor is available.
	 * @return the Constructor, or {@code null} if none
	 */
	@Nullable
	public Constructor<?> getConstructor() {
		return (this.executable instanceof Constructor ? (Constructor<?>) this.executable : null);
	}

	/**
	 * Return the class that declares the underlying Method or Constructor.
	 */
	public Class<?> getDeclaringClass() {
		return this.executable.getDeclaringClass();
	}

	/**
	 * Return the wrapped member.
	 * @return the Method or Constructor as Member
	 */
	public Member getMember() {
		return this.executable;
	}

	/**
	 * Return the wrapped annotated element.
	 * <p>Note: This method exposes the annotations declared on the method/constructor
	 * itself (i.e. at the method/constructor level, not at the parameter level).
	 * @return the Method or Constructor as AnnotatedElement
	 */
	public AnnotatedElement getAnnotatedElement() {
		return this.executable;
	}

	/**
	 * Return the wrapped executable.
	 * @return the Method or Constructor as Executable
	 * @since 5.0
	 */
	public Executable getExecutable() {
		return this.executable;
	}

	/**
	 * Return the {@link Parameter} descriptor for method/constructor parameter.
	 * @since 5.0
	 */
	public Parameter getParameter() {
		if (this.parameterIndex < 0) {
			throw new IllegalStateException("Cannot retrieve Parameter descriptor for method return type");
		}
		Parameter parameter = this.parameter;
		if (parameter == null) {
			parameter = getExecutable().getParameters()[this.parameterIndex];
			this.parameter = parameter;
		}
		return parameter;
	}

	/**
	 * Return the index of the method/constructor parameter.
	 * @return the parameter index (-1 in case of the return type)
	 */
	public int getParameterIndex() {
		return this.parameterIndex;
	}

	/**
	 * Increase this parameter's nesting level.
	 * @see #getNestingLevel()
	 */
	public void increaseNestingLevel() {
		this.nestingLevel++;
	}

	/**
	 * Decrease this parameter's nesting level.
	 * @see #getNestingLevel()
	 */
	public void decreaseNestingLevel() {
		getTypeIndexesPerLevel().remove(this.nestingLevel);
		this.nestingLevel--;
	}

	/**
	 * Return the nesting level of the target type
	 * (typically 1; e.g. in case of a List of Lists, 1 would indicate the
	 * nested List, whereas 2 would indicate the element of the nested List).
	 */
	public int getNestingLevel() {
		return this.nestingLevel;
	}

	/**
	 * Set the type index for the current nesting level.
	 * @param typeIndex the corresponding type index
	 * (or {@code null} for the default type index)
	 * @see #getNestingLevel()
	 */
	public void setTypeIndexForCurrentLevel(int typeIndex) {
		getTypeIndexesPerLevel().put(this.nestingLevel, typeIndex);
	}

	/**
	 * Return the type index for the current nesting level.
	 * @return the corresponding type index, or {@code null}
	 * if none specified (indicating the default type index)
	 * @see #getNestingLevel()
	 */
	@Nullable
	public Integer getTypeIndexForCurrentLevel() {
		return getTypeIndexForLevel(this.nestingLevel);
	}

	/**
	 * Return the type index for the specified nesting level.
	 * @param nestingLevel the nesting level to check
	 * @return the corresponding type index, or {@code null}
	 * if none specified (indicating the default type index)
	 */
	@Nullable
	public Integer getTypeIndexForLevel(int nestingLevel) {
		return getTypeIndexesPerLevel().get(nestingLevel);
	}

	/**
	 * Obtain the (lazily constructed) type-indexes-per-level Map.
	 */
	private Map<Integer, Integer> getTypeIndexesPerLevel() {
		if (this.typeIndexesPerLevel == null) {
			this.typeIndexesPerLevel = new HashMap<>(4);
		}
		return this.typeIndexesPerLevel;
	}

	/**
	 * Return a variant of this {@code MethodParameter} which points to the
	 * same parameter but one nesting level deeper. This is effectively the
	 * same as {@link #increaseNestingLevel()}, just with an independent
	 * {@code MethodParameter} object (e.g. in case of the original being cached).
	 * @since 4.3
	 */
	public MethodParameter nested() {
		MethodParameter nestedParam = this.nestedMethodParameter;
		if (nestedParam != null) {
			return nestedParam;
		}
		nestedParam = clone();
		nestedParam.nestingLevel = this.nestingLevel + 1;
		this.nestedMethodParameter = nestedParam;
		return nestedParam;
	}

	/**
	 * Return whether this method indicates a parameter which is not required:
	 * 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.
	 * @since 4.3
	 */
	public boolean isOptional() {
		return (getParameterType() == Optional.class || hasNullableAnnotation() ||
				(KotlinDetector.isKotlinReflectPresent() &&
						KotlinDetector.isKotlinType(getContainingClass()) &&
						KotlinDelegate.isOptional(this)));
	}

	/**
	 * Check whether this method parameter is annotated with any variant of a
	 * {@code Nullable} annotation, e.g. {@code javax.annotation.Nullable} or
	 * {@code edu.umd.cs.findbugs.annotations.Nullable}.
	 */
	private boolean hasNullableAnnotation() {
		for (Annotation ann : getParameterAnnotations()) {
			if ("Nullable".equals(ann.annotationType().getSimpleName())) {
				return true;
			}
		}
		return false;
	}

	/**
	 * Return a variant of this {@code MethodParameter} which points to
	 * the same parameter but one nesting level deeper in case of a
	 * {@link java.util.Optional} declaration.
	 * @since 4.3
	 * @see #isOptional()
	 * @see #nested()
	 */
	public MethodParameter nestedIfOptional() {
		return (getParameterType() == Optional.class ? nested() : this);
	}

	/**
	 * Set a containing class to resolve the parameter type against.
	 */
	void setContainingClass(Class<?> containingClass) {
		this.containingClass = containingClass;
	}

	public Class<?> getContainingClass() {
		Class<?> containingClass = this.containingClass;
		return (containingClass != null ? containingClass : getDeclaringClass());
	}

	/**
	 * Set a resolved (generic) parameter type.
	 */
	void setParameterType(@Nullable Class<?> parameterType) {
		this.parameterType = parameterType;
	}

	/**
	 * Return the type of the method/constructor parameter.
	 * @return the parameter type (never {@code null})
	 */
	public Class<?> getParameterType() {
		Class<?> paramType = this.parameterType;
		if (paramType == null) {
			if (this.parameterIndex < 0) {
				Method method = getMethod();
				paramType = (method != null ?
						(KotlinDetector.isKotlinReflectPresent() && KotlinDetector.isKotlinType(getContainingClass()) ?
						KotlinDelegate.getReturnType(method) : method.getReturnType()) : void.class);
			}
			else {
				paramType = this.executable.getParameterTypes()[this.parameterIndex];
			}
			this.parameterType = paramType;
		}
		return paramType;
	}

	/**
	 * Return the generic type of the method/constructor parameter.
	 * @return the parameter type (never {@code null})
	 * @since 3.0
	 */
	public Type getGenericParameterType() {
		Type paramType = this.genericParameterType;
		if (paramType == null) {
			if (this.parameterIndex < 0) {
				Method method = getMethod();
				paramType = (method != null ?
						(KotlinDetector.isKotlinReflectPresent() && KotlinDetector.isKotlinType(getContainingClass()) ?
						KotlinDelegate.getGenericReturnType(method) : method.getGenericReturnType()) : void.class);
			}
			else {
				Type[] genericParameterTypes = this.executable.getGenericParameterTypes();
				int index = this.parameterIndex;
				if (this.executable instanceof Constructor &&
						ClassUtils.isInnerClass(this.executable.getDeclaringClass()) &&
						genericParameterTypes.length == this.executable.getParameterCount() - 1) {
					// Bug in javac: type array excludes enclosing instance parameter
					// for inner classes with at least one generic constructor parameter,
					// so access it with the actual parameter index lowered by 1
					index = this.parameterIndex - 1;
				}
				paramType = (index >= 0 && index < genericParameterTypes.length ?
						genericParameterTypes[index] : getParameterType());
			}
			this.genericParameterType = paramType;
		}
		return paramType;
	}

	/**
	 * Return the nested type of the method/constructor parameter.
	 * @return the parameter type (never {@code null})
	 * @since 3.1
	 * @see #getNestingLevel()
	 */
	public Class<?> getNestedParameterType() {
		if (this.nestingLevel > 1) {
			Type type = getGenericParameterType();
			for (int i = 2; i <= this.nestingLevel; i++) {
				if (type instanceof ParameterizedType) {
					Type[] args = ((ParameterizedType) type).getActualTypeArguments();
					Integer index = getTypeIndexForLevel(i);
					type = args[index != null ? index : args.length - 1];
				}
				// TODO: Object.class if unresolvable
			}
			if (type instanceof Class) {
				return (Class<?>) type;
			}
			else if (type instanceof ParameterizedType) {
				Type arg = ((ParameterizedType) type).getRawType();
				if (arg instanceof Class) {
					return (Class<?>) arg;
				}
			}
			return Object.class;
		}
		else {
			return getParameterType();
		}
	}

	/**
	 * Return the nested generic type of the method/constructor parameter.
	 * @return the parameter type (never {@code null})
	 * @since 4.2
	 * @see #getNestingLevel()
	 */
	public Type getNestedGenericParameterType() {
		if (this.nestingLevel > 1) {
			Type type = getGenericParameterType();
			for (int i = 2; i <= this.nestingLevel; i++) {
				if (type instanceof ParameterizedType) {
					Type[] args = ((ParameterizedType) type).getActualTypeArguments();
					Integer index = getTypeIndexForLevel(i);
					type = args[index != null ? index : args.length - 1];
				}
			}
			return type;
		}
		else {
			return getGenericParameterType();
		}
	}

	/**
	 * Return the annotations associated with the target method/constructor itself.
	 */
	public Annotation[] getMethodAnnotations() {
		return adaptAnnotationArray(getAnnotatedElement().getAnnotations());
	}

	/**
	 * Return the method/constructor annotation of the given type, if available.
	 * @param annotationType the annotation type to look for
	 * @return the annotation object, or {@code null} if not found
	 */
	@Nullable
	public <A extends Annotation> A getMethodAnnotation(Class<A> annotationType) {
		A annotation = getAnnotatedElement().getAnnotation(annotationType);
		return (annotation != null ? adaptAnnotation(annotation) : null);
	}

	/**
	 * Return whether the method/constructor is annotated with the given type.
	 * @param annotationType the annotation type to look for
	 * @since 4.3
	 * @see #getMethodAnnotation(Class)
	 */
	public <A extends Annotation> boolean hasMethodAnnotation(Class<A> annotationType) {
		return getAnnotatedElement().isAnnotationPresent(annotationType);
	}

	/**
	 * Return the annotations associated with the specific method/constructor parameter.
	 */
	public Annotation[] getParameterAnnotations() {
		Annotation[] paramAnns = this.parameterAnnotations;
		if (paramAnns == null) {
			Annotation[][] annotationArray = this.executable.getParameterAnnotations();
			int index = this.parameterIndex;
			if (this.executable instanceof Constructor &&
					ClassUtils.isInnerClass(this.executable.getDeclaringClass()) &&
					annotationArray.length == this.executable.getParameterCount() - 1) {
				// Bug in javac in JDK <9: annotation array excludes enclosing instance parameter
				// for inner classes, so access it with the actual parameter index lowered by 1
				index = this.parameterIndex - 1;
			}
			paramAnns = (index >= 0 && index < annotationArray.length ?
					adaptAnnotationArray(annotationArray[index]) : EMPTY_ANNOTATION_ARRAY);
			this.parameterAnnotations = paramAnns;
		}
		return paramAnns;
	}

	/**
	 * Return {@code true} if the parameter has at least one annotation,
	 * {@code false} if it has none.
	 * @see #getParameterAnnotations()
	 */
	public boolean hasParameterAnnotations() {
		return (getParameterAnnotations().length != 0);
	}

	/**
	 * Return the parameter annotation of the given type, if available.
	 * @param annotationType the annotation type to look for
	 * @return the annotation object, or {@code null} if not found
	 */
	@SuppressWarnings("unchecked")
	@Nullable
	public <A extends Annotation> A getParameterAnnotation(Class<A> annotationType) {
		Annotation[] anns = getParameterAnnotations();
		for (Annotation ann : anns) {
			if (annotationType.isInstance(ann)) {
				return (A) ann;
			}
		}
		return null;
	}

	/**
	 * Return whether the parameter is declared with the given annotation type.
	 * @param annotationType the annotation type to look for
	 * @see #getParameterAnnotation(Class)
	 */
	public <A extends Annotation> boolean hasParameterAnnotation(Class<A> annotationType) {
		return (getParameterAnnotation(annotationType) != null);
	}

	/**
	 * Initialize parameter name discovery for this method parameter.
	 * <p>This method does not actually try to retrieve the parameter name at
	 * this point; it just allows discovery to happen when the application calls
	 * {@link #getParameterName()} (if ever).
	 */
	public void initParameterNameDiscovery(@Nullable ParameterNameDiscoverer parameterNameDiscoverer) {
		this.parameterNameDiscoverer = parameterNameDiscoverer;
	}

	/**
	 * Return the name of the method/constructor parameter.
	 * @return the parameter name (may be {@code null} if no
	 * parameter name metadata is contained in the class file or no
	 * {@link #initParameterNameDiscovery ParameterNameDiscoverer}
	 * has been set to begin with)
	 */
	@Nullable
	public String getParameterName() {
		if (this.parameterIndex < 0) {
			return null;
		}
		ParameterNameDiscoverer discoverer = this.parameterNameDiscoverer;
		if (discoverer != null) {
			String[] parameterNames = null;
			if (this.executable instanceof Method) {
				parameterNames = discoverer.getParameterNames((Method) this.executable);
			}
			else if (this.executable instanceof Constructor) {
				parameterNames = discoverer.getParameterNames((Constructor<?>) this.executable);
			}
			if (parameterNames != null) {
				this.parameterName = parameterNames[this.parameterIndex];
			}
			this.parameterNameDiscoverer = null;
		}
		return this.parameterName;
	}


	/**
	 * A template method to post-process a given annotation instance before
	 * returning it to the caller.
	 * <p>The default implementation simply returns the given annotation as-is.
	 * @param annotation the annotation about to be returned
	 * @return the post-processed annotation (or simply the original one)
	 * @since 4.2
	 */
	protected <A extends Annotation> A adaptAnnotation(A annotation) {
		return annotation;
	}

	/**
	 * A template method to post-process a given annotation array before
	 * returning it to the caller.
	 * <p>The default implementation simply returns the given annotation array as-is.
	 * @param annotations the annotation array about to be returned
	 * @return the post-processed annotation array (or simply the original one)
	 * @since 4.2
	 */
	protected Annotation[] adaptAnnotationArray(Annotation[] annotations) {
		return annotations;
	}


	@Override
	public boolean equals(Object other) {
		if (this == other) {
			return true;
		}
		if (!(other instanceof MethodParameter)) {
			return false;
		}
		MethodParameter otherParam = (MethodParameter) other;
		return (this.parameterIndex == otherParam.parameterIndex && getExecutable().equals(otherParam.getExecutable()));
	}

	@Override
	public int hashCode() {
		return (getExecutable().hashCode() * 31 + this.parameterIndex);
	}

	@Override
	public String toString() {
		Method method = getMethod();
		return (method != null ? "method '" + method.getName() + "'" : "constructor") +
				" parameter " + this.parameterIndex;
	}

	@Override
	public MethodParameter clone() {
		return new MethodParameter(this);
	}


	/**
	 * Create a new MethodParameter for the given method or constructor.
	 * <p>This is a convenience factory method for scenarios where a
	 * Method or Constructor reference is treated in a generic fashion.
	 * @param methodOrConstructor the Method or Constructor to specify a parameter for
	 * @param parameterIndex the index of the parameter
	 * @return the corresponding MethodParameter instance
	 * @deprecated as of 5.0, in favor of {@link #forExecutable}
	 */
	@Deprecated
	public static MethodParameter forMethodOrConstructor(Object methodOrConstructor, int parameterIndex) {
		if (!(methodOrConstructor instanceof Executable)) {
			throw new IllegalArgumentException(
					"Given object [" + methodOrConstructor + "] is neither a Method nor a Constructor");
		}
		return forExecutable((Executable) methodOrConstructor, parameterIndex);
	}

	/**
	 * Create a new MethodParameter for the given method or constructor.
	 * <p>This is a convenience factory method for scenarios where a
	 * Method or Constructor reference is treated in a generic fashion.
	 * @param executable the Method or Constructor to specify a parameter for
	 * @param parameterIndex the index of the parameter
	 * @return the corresponding MethodParameter instance
	 * @since 5.0
	 */
	public static MethodParameter forExecutable(Executable executable, int parameterIndex) {
		if (executable instanceof Method) {
			return new MethodParameter((Method) executable, parameterIndex);
		}
		else if (executable instanceof Constructor) {
			return new MethodParameter((Constructor<?>) executable, parameterIndex);
		}
		else {
			throw new IllegalArgumentException("Not a Method/Constructor: " + executable);
		}
	}

	/**
	 * Create a new MethodParameter for the given parameter descriptor.
	 * <p>This is a convenience factory method for scenarios where a
	 * Java 8 {@link Parameter} descriptor is already available.
	 * @param parameter the parameter descriptor
	 * @return the corresponding MethodParameter instance
	 * @since 5.0
	 */
	public static MethodParameter forParameter(Parameter parameter) {
		return forExecutable(parameter.getDeclaringExecutable(), findParameterIndex(parameter));
	}

	protected static int findParameterIndex(Parameter parameter) {
		Executable executable = parameter.getDeclaringExecutable();
		Parameter[] allParams = executable.getParameters();
		// Try first with identity checks for greater performance.
		for (int i = 0; i < allParams.length; i++) {
			if (parameter == allParams[i]) {
				return i;
			}
		}
		// Potentially try again with object equality checks in order to avoid race
		// conditions while invoking java.lang.reflect.Executable.getParameters().
		for (int i = 0; i < allParams.length; i++) {
			if (parameter.equals(allParams[i])) {
				return i;
			}
		}
		throw new IllegalArgumentException("Given parameter [" + parameter +
				"] does not match any parameter in the declaring executable");
	}

	private static int validateIndex(Executable executable, int parameterIndex) {
		int count = executable.getParameterCount();
		Assert.isTrue(parameterIndex >= -1 && parameterIndex < count,
				() -> "Parameter index needs to be between -1 and " + (count - 1));
		return parameterIndex;
	}


	/**
	 * Inner class to avoid a hard dependency on Kotlin at runtime.
	 */
	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).
		 */
		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());
				}
				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());
				}
			}
			return false;
		}

		/**
		 * Return the generic return type of the method, with support of suspending
		 * functions via Kotlin reflection.
		 */
		static private Type getGenericReturnType(Method method) {
			KFunction<?> function = ReflectJvmMapping.getKotlinFunction(method);
			if (function != null && function.isSuspend()) {
				return ReflectJvmMapping.getJavaType(function.getReturnType());
			}
			return method.getGenericReturnType();
		}

		/**
		 * Return the return type of the method, with support of suspending
		 * functions via Kotlin reflection.
		 */
		static private Class<?> getReturnType(Method method) {
			KFunction<?> function = ReflectJvmMapping.getKotlinFunction(method);
			if (function != null && function.isSuspend()) {
				Type paramType = ReflectJvmMapping.getJavaType(function.getReturnType());
				Class<?> paramClass = ResolvableType.forType(paramType).resolve();
				Assert.notNull(paramClass, "Type " + paramType + "can't be resolved to a class");
				return paramClass;
			}
			return method.getReturnType();
		}
	}
}
