/*
 * 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.web.method.annotation;

import org.springframework.core.MethodParameter;
import org.springframework.lang.Nullable;
import org.springframework.ui.ModelMap;
import org.springframework.util.Assert;
import org.springframework.util.CollectionUtils;
import org.springframework.validation.BindingResult;
import org.springframework.validation.Errors;
import org.springframework.web.bind.support.WebDataBinderFactory;
import org.springframework.web.context.request.NativeWebRequest;
import org.springframework.web.method.support.HandlerMethodArgumentResolver;
import org.springframework.web.method.support.ModelAndViewContainer;

Resolves Errors method arguments. 
An Errors method argument is expected to appear immediately after the model attribute in the method signature. It is resolved by expecting the last two attributes added to the model to be the model attribute and its BindingResult. 
Author:
Rossen Stoyanchev, Juergen Hoeller
Since:
3.1/**
 * Resolves {@link Errors} method arguments.
 *
 * <p>An {@code Errors} method argument is expected to appear immediately after
 * the model attribute in the method signature. It is resolved by expecting the
 * last two attributes added to the model to be the model attribute and its
 * {@link BindingResult}.
 *
 * @author Rossen Stoyanchev
 * @author Juergen Hoeller
 * @since 3.1
 */
public class ErrorsMethodArgumentResolver implements HandlerMethodArgumentResolver {

	@Override
	public boolean supportsParameter(MethodParameter parameter) {
		Class<?> paramType = parameter.getParameterType();
		return Errors.class.isAssignableFrom(paramType);
	}

	@Override
	@Nullable
	public Object resolveArgument(MethodParameter parameter,
			@Nullable ModelAndViewContainer mavContainer, NativeWebRequest webRequest,
			@Nullable WebDataBinderFactory binderFactory) throws Exception {

		Assert.state(mavContainer != null,
				"Errors/BindingResult argument only supported on regular handler methods");

		ModelMap model = mavContainer.getModel();
		String lastKey = CollectionUtils.lastElement(model.keySet());
		if (lastKey != null && lastKey.startsWith(BindingResult.MODEL_KEY_PREFIX)) {
			return model.get(lastKey);
		}

		throw new IllegalStateException(
				"An Errors/BindingResult argument is expected to be declared immediately after " +
				"the model attribute, the @RequestBody or the @RequestPart arguments " +
				"to which they apply: " + parameter.getMethod());
	}

}