/*
 * Copyright 2002-2016 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.view;

import java.util.Enumeration;
import java.util.LinkedHashMap;
import java.util.Map;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import org.springframework.web.servlet.support.RequestContext;

/**
 * Adapter base class for template-based view technologies such as FreeMarker,
 * with the ability to use request and session attributes in their model and
 * the option to expose helper objects for Spring's FreeMarker macro library.
 *
 * <p>JSP/JSTL and other view technologies automatically have access to the
 * HttpServletRequest object and thereby the request/session attributes
 * for the current user. Furthermore, they are able to create and cache
 * helper objects as request attributes themselves.
 *
 * @author Juergen Hoeller
 * @author Darren Davison
 * @since 1.0.2
 * @see AbstractTemplateViewResolver
 * @see org.springframework.web.servlet.view.freemarker.FreeMarkerView
 */
public abstract class AbstractTemplateView extends AbstractUrlBasedView {

        /**
         * Variable name of the RequestContext instance in the template model,
         * available to Spring's macros: e.g. for creating BindStatus objects.
         */
        public static final String SPRING_MACRO_REQUEST_CONTEXT_ATTRIBUTE = "springMacroRequestContext";


        private boolean exposeRequestAttributes = false;

        private boolean allowRequestOverride = false;

        private boolean exposeSessionAttributes = false;

        private boolean allowSessionOverride = false;

        private boolean exposeSpringMacroHelpers = true;


        /**
         * Set whether all request attributes should be added to the
         * model prior to merging with the template. Default is "false".
         */
        public void setExposeRequestAttributes(boolean exposeRequestAttributes) {
                this.exposeRequestAttributes = exposeRequestAttributes;
        }

        /**
         * Set whether HttpServletRequest attributes are allowed to override (hide)
         * controller generated model attributes of the same name. Default is "false",
         * which causes an exception to be thrown if request attributes of the same
         * name as model attributes are found.
         */
        public void setAllowRequestOverride(boolean allowRequestOverride) {
                this.allowRequestOverride = allowRequestOverride;
        }

        /**
         * Set whether all HttpSession attributes should be added to the
         * model prior to merging with the template. Default is "false".
         */
        public void setExposeSessionAttributes(boolean exposeSessionAttributes) {
                this.exposeSessionAttributes = exposeSessionAttributes;
        }

        /**
         * Set whether HttpSession attributes are allowed to override (hide)
         * controller generated model attributes of the same name. Default is "false",
         * which causes an exception to be thrown if session attributes of the same
         * name as model attributes are found.
         */
        public void setAllowSessionOverride(boolean allowSessionOverride) {
                this.allowSessionOverride = allowSessionOverride;
        }

        /**
         * Set whether to expose a RequestContext for use by Spring's macro library,
         * under the name "springMacroRequestContext". Default is "true".
         * <p>Currently needed for Spring's FreeMarker default macros.
         * Note that this is <i>not</i> required for templates that use HTML forms
         * <i>unless</i> you wish to take advantage of the Spring helper macros.
         * @see #SPRING_MACRO_REQUEST_CONTEXT_ATTRIBUTE
         */
        public void setExposeSpringMacroHelpers(boolean exposeSpringMacroHelpers) {
                this.exposeSpringMacroHelpers = exposeSpringMacroHelpers;
        }


        @Override
        protected final void renderMergedOutputModel(
                        Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws Exception {

                if (this.exposeRequestAttributes) {
                        Map<String, Object> exposed = null;
                        for (Enumeration<String> en = request.getAttributeNames(); en.hasMoreElements();) {
                                String attribute = en.nextElement();
                                if (model.containsKey(attribute) && !this.allowRequestOverride) {
                                        throw new ServletException("Cannot expose request attribute '" + attribute +
                                                "' because of an existing model object of the same name");
                                }
                                Object attributeValue = request.getAttribute(attribute);
                                if (logger.isDebugEnabled()) {
                                        exposed = exposed != null ? exposed : new LinkedHashMap<>();
                                        exposed.put(attribute, attributeValue);
                                }
                                model.put(attribute, attributeValue);
                        }
                        if (logger.isTraceEnabled() && exposed != null) {
                                logger.trace("Exposed request attributes to model: " + exposed);
                        }
                }

                if (this.exposeSessionAttributes) {
                        HttpSession session = request.getSession(false);
                        if (session != null) {
                                Map<String, Object> exposed = null;
                                for (Enumeration<String> en = session.getAttributeNames(); en.hasMoreElements();) {
                                        String attribute = en.nextElement();
                                        if (model.containsKey(attribute) && !this.allowSessionOverride) {
                                                throw new ServletException("Cannot expose session attribute '" + attribute +
                                                        "' because of an existing model object of the same name");
                                        }
                                        Object attributeValue = session.getAttribute(attribute);
                                        if (logger.isDebugEnabled()) {
                                                exposed = exposed != null ? exposed : new LinkedHashMap<>();
                                                exposed.put(attribute, attributeValue);
                                        }
                                        model.put(attribute, attributeValue);
                                }
                                if (logger.isTraceEnabled() && exposed != null) {
                                        logger.trace("Exposed session attributes to model: " + exposed);
                                }
                        }
                }

                if (this.exposeSpringMacroHelpers) {
                        if (model.containsKey(SPRING_MACRO_REQUEST_CONTEXT_ATTRIBUTE)) {
                                throw new ServletException(
                                                "Cannot expose bind macro helper '" + SPRING_MACRO_REQUEST_CONTEXT_ATTRIBUTE +
                                                "' because of an existing model object of the same name");
                        }
                        // Expose RequestContext instance for Spring macros.
                        model.put(SPRING_MACRO_REQUEST_CONTEXT_ATTRIBUTE,
                                        new RequestContext(request, response, getServletContext(), model));
                }

                applyContentType(response);

                if (logger.isDebugEnabled()) {
                        logger.debug("Rendering [" + getUrl() + "]");
                }

                renderMergedTemplateModel(model, request, response);
        }

        /**
         * Apply this view's content type as specified in the "contentType"
         * bean property to the given response.
         * <p>Only applies the view's contentType if no content type has been
         * set on the response before. This allows handlers to override the
         * default content type beforehand.
         * @param response current HTTP response
         * @see #setContentType
         */
        protected void applyContentType(HttpServletResponse response)   {
                if (response.getContentType() == null) {
                        response.setContentType(getContentType());
                }
        }

        /**
         * Subclasses must implement this method to actually render the view.
         * @param model combined output Map, with request attributes and
         * session attributes merged into it if required
         * @param request current HTTP request
         * @param response current HTTP response
         * @throws Exception if rendering failed
         */
        protected abstract void renderMergedTemplateModel(
                        Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws Exception;

}