/*
 * Copyright 2002-2020 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.servlet.mvc.method.annotation;

import java.lang.reflect.Method;
import java.util.List;

import org.springframework.core.MethodParameter;
import org.springframework.lang.Nullable;
import org.springframework.ui.ExtendedModelMap;
import org.springframework.util.Assert;
import org.springframework.web.context.request.NativeWebRequest;
import org.springframework.web.method.annotation.ModelAttributeMethodProcessor;
import org.springframework.web.method.support.HandlerMethodReturnValueHandler;
import org.springframework.web.method.support.ModelAndViewContainer;
import org.springframework.web.servlet.ModelAndView;
import org.springframework.web.servlet.mvc.annotation.ModelAndViewResolver;

This return value handler is intended to be ordered after all others as it attempts to handle _any_ return value type (i.e. returns true for all return types). 
The return value is handled either with a ModelAndViewResolver or otherwise by regarding it as a model attribute if it is a non-simple type. If neither of these succeeds (essentially simple type other than String), UnsupportedOperationException is raised. 
Note: This class is primarily needed to support ModelAndViewResolver, which unfortunately cannot be properly adapted to the HandlerMethodReturnValueHandler contract since the HandlerMethodReturnValueHandler.supportsReturnType method cannot be implemented. Hence ModelAndViewResolvers are limited to always being invoked at the end after all other return value handlers have been given a chance. It is recommended to re-implement a ModelAndViewResolver as HandlerMethodReturnValueHandler, which also provides better access to the return type and method information. 
Author:
Rossen Stoyanchev
Since:
3.1/**
 * This return value handler is intended to be ordered after all others as it
 * attempts to handle _any_ return value type (i.e. returns {@code true} for
 * all return types).
 *
 * <p>The return value is handled either with a {@link ModelAndViewResolver}
 * or otherwise by regarding it as a model attribute if it is a non-simple
 * type. If neither of these succeeds (essentially simple type other than
 * String), {@link UnsupportedOperationException} is raised.
 *
 * <p><strong>Note:</strong> This class is primarily needed to support
 * {@link ModelAndViewResolver}, which unfortunately cannot be properly
 * adapted to the {@link HandlerMethodReturnValueHandler} contract since the
 * {@link HandlerMethodReturnValueHandler#supportsReturnType} method
 * cannot be implemented. Hence {@code ModelAndViewResolver}s are limited
 * to always being invoked at the end after all other return value
 * handlers have been given a chance. It is recommended to re-implement
 * a {@code ModelAndViewResolver} as {@code HandlerMethodReturnValueHandler},
 * which also provides better access to the return type and method information.
 *
 * @author Rossen Stoyanchev
 * @since 3.1
 */
public class ModelAndViewResolverMethodReturnValueHandler implements HandlerMethodReturnValueHandler {

	@Nullable
	private final List<ModelAndViewResolver> mavResolvers;

	private final ModelAttributeMethodProcessor modelAttributeProcessor =
			new ServletModelAttributeMethodProcessor(true);


	
Create a new instance.
/**
	 * Create a new instance.
	 */
	public ModelAndViewResolverMethodReturnValueHandler(@Nullable List<ModelAndViewResolver> mavResolvers) {
		this.mavResolvers = mavResolvers;
	}


	
Always returns true. See class-level note. /**
	 * Always returns {@code true}. See class-level note.
	 */
	@Override
	public boolean supportsReturnType(MethodParameter returnType) {
		return true;
	}

	@Override
	public void handleReturnValue(@Nullable Object returnValue, MethodParameter returnType,
			ModelAndViewContainer mavContainer, NativeWebRequest webRequest) throws Exception {

		if (this.mavResolvers != null) {
			for (ModelAndViewResolver mavResolver : this.mavResolvers) {
				Class<?> handlerType = returnType.getContainingClass();
				Method method = returnType.getMethod();
				Assert.state(method != null, "No handler method");
				ExtendedModelMap model = (ExtendedModelMap) mavContainer.getModel();
				ModelAndView mav = mavResolver.resolveModelAndView(method, handlerType, returnValue, model, webRequest);
				if (mav != ModelAndViewResolver.UNRESOLVED) {
					mavContainer.addAllAttributes(mav.getModel());
					mavContainer.setViewName(mav.getViewName());
					if (!mav.isReference()) {
						mavContainer.setView(mav.getView());
					}
					return;
				}
			}
		}

		// No suitable ModelAndViewResolver...
		if (this.modelAttributeProcessor.supportsReturnType(returnType)) {
			this.modelAttributeProcessor.handleReturnValue(returnValue, returnType, mavContainer, webRequest);
		}
		else {
			throw new UnsupportedOperationException("Unexpected return type: " +
					returnType.getParameterType().getName() + " in method: " + returnType.getMethod());
		}
	}

}