project review to keep compatibiliy with spring-boot-1

springdoc-openapi-common/src/main/java/org/springdoc/core/ReturnTypeParser.java

@@ -20,16 +20,99 @@
 
 import java.lang.reflect.ParameterizedType;
 import java.lang.reflect.Type;
+import java.lang.reflect.TypeVariable;
 
-import org.springframework.core.GenericTypeResolver;
 import org.springframework.core.MethodParameter;
+import org.springframework.core.ResolvableType;
+import org.springframework.lang.Nullable;
 
 public interface ReturnTypeParser {
 
 	default Type getReturnType(MethodParameter methodParameter) {
 		if (methodParameter.getGenericParameterType() instanceof ParameterizedType)
-			return GenericTypeResolver.resolveType(methodParameter.getGenericParameterType(),methodParameter.getContainingClass());
+			return ReturnTypeParser.resolveType(methodParameter.getGenericParameterType(), methodParameter.getContainingClass());
 		return methodParameter.getParameterType();
 	}
+
+	/**
+	 * This is a copy of GenericTypeResolver.resolveType which is not available on spring 4.
+	 * This also keeps compatibility with spring-boot 1 applications.
+	 * Resolve the given generic type against the given context class,
+	 * substituting type variables as far as possible.
+	 * @param genericType the (potentially) generic type
+	 * @param contextClass a context class for the target type, for example a class
+	 * in which the target type appears in a method signature (can be {@code null})
+	 * @return the resolved type (possibly the given generic type as-is)
+	 * @since 5.0
+	 */
+	static Type resolveType(Type genericType, @Nullable Class<?> contextClass) {
+		if (contextClass != null) {
+			if (genericType instanceof TypeVariable) {
+				ResolvableType resolvedTypeVariable = resolveVariable(
+						(TypeVariable<?>) genericType, ResolvableType.forClass(contextClass));
+				if (resolvedTypeVariable != ResolvableType.NONE) {
+					Class<?> resolved = resolvedTypeVariable.resolve();
+					if (resolved != null) {
+						return resolved;
+					}
+				}
+			}
+			else if (genericType instanceof ParameterizedType) {
+				ResolvableType resolvedType = ResolvableType.forType(genericType);
+				if (resolvedType.hasUnresolvableGenerics()) {
+					ParameterizedType parameterizedType = (ParameterizedType) genericType;
+					Class<?>[] generics = new Class<?>[parameterizedType.getActualTypeArguments().length];
+					Type[] typeArguments = parameterizedType.getActualTypeArguments();
+					ResolvableType contextType = ResolvableType.forClass(contextClass);
+					for (int i = 0; i < typeArguments.length; i++) {
+						Type typeArgument = typeArguments[i];
+						if (typeArgument instanceof TypeVariable) {
+							ResolvableType resolvedTypeArgument = resolveVariable(
+									(TypeVariable<?>) typeArgument, contextType);
+							if (resolvedTypeArgument != ResolvableType.NONE) {
+								generics[i] = resolvedTypeArgument.resolve();
+							}
+							else {
+								generics[i] = ResolvableType.forType(typeArgument).resolve();
+							}
+						}
+						else {
+							generics[i] = ResolvableType.forType(typeArgument).resolve();
+						}
+					}
+					Class<?> rawClass = resolvedType.getRawClass();
+					if (rawClass != null) {
+						return ResolvableType.forClassWithGenerics(rawClass, generics).getType();
+					}
+				}
+			}
+		}
+		return genericType;
+	}
+
+	static ResolvableType resolveVariable(TypeVariable<?> typeVariable, ResolvableType contextType) {
+		ResolvableType resolvedType;
+		if (contextType.hasGenerics()) {
+			resolvedType = ResolvableType.forType(typeVariable, contextType);
+			if (resolvedType.resolve() != null) {
+				return resolvedType;
+			}
+		}
+
+		ResolvableType superType = contextType.getSuperType();
+		if (superType != ResolvableType.NONE) {
+			resolvedType = resolveVariable(typeVariable, superType);
+			if (resolvedType.resolve() != null) {
+				return resolvedType;
+			}
+		}
+		for (ResolvableType ifc : contextType.getInterfaces()) {
+			resolvedType = resolveVariable(typeVariable, ifc);
+			if (resolvedType.resolve() != null) {
+				return resolvedType;
+			}
+		}
+		return ResolvableType.NONE;
+	}
 }