/*
 * Copyright 2002-2012 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
 *
 *      http://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 java.util.ArrayList;

import org.springframework.core.MethodParameter;
import org.springframework.ui.ModelMap;
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 {@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
 * @since 3.1
 */
public class ErrorsMethodArgumentResolver implements HandlerMethodArgumentResolver {

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

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

		ModelMap model = mavContainer.getModel();
		if (model.size() > 0) {
			int lastIndex = model.size()-1;
			String lastKey = new ArrayList<String>(model.keySet()).get(lastIndex);
			if (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());
	}

}
