/*
 * 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.servlet.view.freemarker;

import java.io.FileNotFoundException;
import java.io.IOException;
import java.util.Collections;
import java.util.Enumeration;
import java.util.Locale;
import java.util.Map;

import javax.servlet.GenericServlet;
import javax.servlet.ServletConfig;
import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import freemarker.core.ParseException;
import freemarker.ext.jsp.TaglibFactory;
import freemarker.ext.servlet.AllHttpScopesHashModel;
import freemarker.ext.servlet.FreemarkerServlet;
import freemarker.ext.servlet.HttpRequestHashModel;
import freemarker.ext.servlet.HttpRequestParametersHashModel;
import freemarker.ext.servlet.HttpSessionHashModel;
import freemarker.ext.servlet.ServletContextHashModel;
import freemarker.template.Configuration;
import freemarker.template.DefaultObjectWrapperBuilder;
import freemarker.template.ObjectWrapper;
import freemarker.template.SimpleHash;
import freemarker.template.Template;
import freemarker.template.TemplateException;

import org.springframework.beans.BeansException;
import org.springframework.beans.factory.BeanFactoryUtils;
import org.springframework.beans.factory.BeanInitializationException;
import org.springframework.beans.factory.NoSuchBeanDefinitionException;
import org.springframework.context.ApplicationContextException;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.web.servlet.support.RequestContextUtils;
import org.springframework.web.servlet.view.AbstractTemplateView;

/**
 * View using the FreeMarker template engine.
 *
 * <p>Exposes the following JavaBean properties:
 * <ul>
 * <li><b>url</b>: the location of the FreeMarker template to be wrapped,
 * relative to the FreeMarker template context (directory).
 * <li><b>encoding</b> (optional, default is determined by FreeMarker configuration):
 * the encoding of the FreeMarker template file
 * </ul>
 *
 * <p>Depends on a single {@link FreeMarkerConfig} object such as {@link FreeMarkerConfigurer}
 * being accessible in the current web application context, with any bean name.
 * Alternatively, you can set the FreeMarker {@link Configuration} object as a
 * bean property. See {@link #setConfiguration} for more details on the impacts
 * of this approach.
 *
 * <p>Note: Spring's FreeMarker support requires FreeMarker 2.3 or higher.
 *
 * @author Darren Davison
 * @author Juergen Hoeller
 * @since 03.03.2004
 * @see #setUrl
 * @see #setExposeSpringMacroHelpers
 * @see #setEncoding
 * @see #setConfiguration
 * @see FreeMarkerConfig
 * @see FreeMarkerConfigurer
 */
public class FreeMarkerView extends AbstractTemplateView {

        @Nullable
        private String encoding;

        @Nullable
        private Configuration configuration;

        @Nullable
        private TaglibFactory taglibFactory;

        @Nullable
        private ServletContextHashModel servletContextHashModel;


        /**
         * Set the encoding of the FreeMarker template file. Default is determined
         * by the FreeMarker Configuration: "ISO-8859-1" if not specified otherwise.
         * <p>Specify the encoding in the FreeMarker Configuration rather than per
         * template if all your templates share a common encoding.
         */
        public void setEncoding(@Nullable String encoding) {
                this.encoding = encoding;
        }

        /**
         * Return the encoding for the FreeMarker template.
         */
        @Nullable
        protected String getEncoding() {
                return this.encoding;
        }

        /**
         * Set the FreeMarker Configuration to be used by this view.
         * <p>If this is not set, the default lookup will occur: a single {@link FreeMarkerConfig}
         * is expected in the current web application context, with any bean name.
         * <strong>Note:</strong> using this method will cause a new instance of {@link TaglibFactory}
         * to created for every single {@link FreeMarkerView} instance. This can be quite expensive
         * in terms of memory and initial CPU usage. In production it is recommended that you use
         * a {@link FreeMarkerConfig} which exposes a single shared {@link TaglibFactory}.
         */
        public void setConfiguration(@Nullable Configuration configuration) {
                this.configuration = configuration;
        }

        /**
         * Return the FreeMarker configuration used by this view.
         */
        @Nullable
        protected Configuration getConfiguration() {
                return this.configuration;
        }

        /**
         * Obtain the FreeMarker configuration for actual use.
         * @return the FreeMarker configuration (never {@code null})
         * @throws IllegalStateException in case of no Configuration object set
         * @since 5.0
         */
        protected Configuration obtainConfiguration() {
                Configuration configuration = getConfiguration();
                Assert.state(configuration != null, "No Configuration set");
                return configuration;
        }


        /**
         * Invoked on startup. Looks for a single FreeMarkerConfig bean to
         * find the relevant Configuration for this factory.
         * <p>Checks that the template for the default Locale can be found:
         * FreeMarker will check non-Locale-specific templates if a
         * locale-specific one is not found.
         * @see freemarker.cache.TemplateCache#getTemplate
         */
        @Override
        protected void initServletContext(ServletContext servletContext) throws BeansException {
                if (getConfiguration() != null) {
                        this.taglibFactory = new TaglibFactory(servletContext);
                }
                else {
                        FreeMarkerConfig config = autodetectConfiguration();
                        setConfiguration(config.getConfiguration());
                        this.taglibFactory = config.getTaglibFactory();
                }

                GenericServlet servlet = new GenericServletAdapter();
                try {
                        servlet.init(new DelegatingServletConfig());
                }
                catch (ServletException ex) {
                        throw new BeanInitializationException("Initialization of GenericServlet adapter failed", ex);
                }
                this.servletContextHashModel = new ServletContextHashModel(servlet, getObjectWrapper());
        }

        /**
         * Autodetect a {@link FreeMarkerConfig} object via the ApplicationContext.
         * @return the Configuration instance to use for FreeMarkerViews
         * @throws BeansException if no Configuration instance could be found
         * @see #getApplicationContext
         * @see #setConfiguration
         */
        protected FreeMarkerConfig autodetectConfiguration() throws BeansException {
                try {
                        return BeanFactoryUtils.beanOfTypeIncludingAncestors(
                                        obtainApplicationContext(), FreeMarkerConfig.class, true, false);
                }
                catch (NoSuchBeanDefinitionException ex) {
                        throw new ApplicationContextException(
                                        "Must define a single FreeMarkerConfig bean in this web application context " +
                                        "(may be inherited): FreeMarkerConfigurer is the usual implementation. " +
                                        "This bean may be given any name.", ex);
                }
        }

        /**
         * Return the configured FreeMarker {@link ObjectWrapper}, or the
         * {@link ObjectWrapper#DEFAULT_WRAPPER default wrapper} if none specified.
         * @see freemarker.template.Configuration#getObjectWrapper()
         */
        protected ObjectWrapper getObjectWrapper() {
                ObjectWrapper ow = obtainConfiguration().getObjectWrapper();
                return (ow != null ? ow :
                                new DefaultObjectWrapperBuilder(Configuration.DEFAULT_INCOMPATIBLE_IMPROVEMENTS).build());
        }

        /**
         * Check that the FreeMarker template used for this view exists and is valid.
         * <p>Can be overridden to customize the behavior, for example in case of
         * multiple templates to be rendered into a single view.
         */
        @Override
        public boolean checkResource(Locale locale) throws Exception {
                String url = getUrl();
                Assert.state(url != null, "'url' not set");

                try {
                        // Check that we can get the template, even if we might subsequently get it again.
                        getTemplate(url, locale);
                        return true;
                }
                catch (FileNotFoundException ex) {
                        // Allow for ViewResolver chaining...
                        return false;
                }
                catch (ParseException ex) {
                        throw new ApplicationContextException("Failed to parse [" + url + "]", ex);
                }
                catch (IOException ex) {
                        throw new ApplicationContextException("Failed to load [" + url + "]", ex);
                }
        }


        /**
         * Process the model map by merging it with the FreeMarker template.
         * Output is directed to the servlet response.
         * <p>This method can be overridden if custom behavior is needed.
         */
        @Override
        protected void renderMergedTemplateModel(
                        Map<String, Object> model, HttpServletRequest request, HttpServletResponse response) throws Exception {

                exposeHelpers(model, request);
                doRender(model, request, response);
        }

        /**
         * Expose helpers unique to each rendering operation. This is necessary so that
         * different rendering operations can't overwrite each other's formats etc.
         * <p>Called by {@code renderMergedTemplateModel}. The default implementation
         * is empty. This method can be overridden to add custom helpers to the model.
         * @param model the model that will be passed to the template at merge time
         * @param request current HTTP request
         * @throws Exception if there's a fatal error while we're adding information to the context
         * @see #renderMergedTemplateModel
         */
        protected void exposeHelpers(Map<String, Object> model, HttpServletRequest request) throws Exception {
        }

        /**
         * Render the FreeMarker view to the given response, using the given model
         * map which contains the complete template model to use.
         * <p>The default implementation renders the template specified by the "url"
         * bean property, retrieved via {@code getTemplate}. It delegates to the
         * {@code processTemplate} method to merge the template instance with
         * the given template model.
         * <p>Adds the standard Freemarker hash models to the model: request parameters,
         * request, session and application (ServletContext), as well as the JSP tag
         * library hash model.
         * <p>Can be overridden to customize the behavior, for example to render
         * multiple templates into a single view.
         * @param model the model to use for rendering
         * @param request current HTTP request
         * @param response current servlet response
         * @throws IOException if the template file could not be retrieved
         * @throws Exception if rendering failed
         * @see #setUrl
         * @see org.springframework.web.servlet.support.RequestContextUtils#getLocale
         * @see #getTemplate(java.util.Locale)
         * @see #processTemplate
         * @see freemarker.ext.servlet.FreemarkerServlet
         */
        protected void doRender(Map<String, Object> model, HttpServletRequest request,
                        HttpServletResponse response) throws Exception {

                // Expose model to JSP tags (as request attributes).
                exposeModelAsRequestAttributes(model, request);
                // Expose all standard FreeMarker hash models.
                SimpleHash fmModel = buildTemplateModel(model, request, response);

                // Grab the locale-specific version of the template.
                Locale locale = RequestContextUtils.getLocale(request);
                processTemplate(getTemplate(locale), fmModel, response);
        }

        /**
         * Build a FreeMarker template model for the given model Map.
         * <p>The default implementation builds a {@link AllHttpScopesHashModel}.
         * @param model the model to use for rendering
         * @param request current HTTP request
         * @param response current servlet response
         * @return the FreeMarker template model, as a {@link SimpleHash} or subclass thereof
         */
        protected SimpleHash buildTemplateModel(Map<String, Object> model, HttpServletRequest request,
                        HttpServletResponse response) {

                AllHttpScopesHashModel fmModel = new AllHttpScopesHashModel(getObjectWrapper(), getServletContext(), request);
                fmModel.put(FreemarkerServlet.KEY_JSP_TAGLIBS, this.taglibFactory);
                fmModel.put(FreemarkerServlet.KEY_APPLICATION, this.servletContextHashModel);
                fmModel.put(FreemarkerServlet.KEY_SESSION, buildSessionModel(request, response));
                fmModel.put(FreemarkerServlet.KEY_REQUEST, new HttpRequestHashModel(request, response, getObjectWrapper()));
                fmModel.put(FreemarkerServlet.KEY_REQUEST_PARAMETERS, new HttpRequestParametersHashModel(request));
                fmModel.putAll(model);
                return fmModel;
        }

        /**
         * Build a FreeMarker {@link HttpSessionHashModel} for the given request,
         * detecting whether a session already exists and reacting accordingly.
         * @param request current HTTP request
         * @param response current servlet response
         * @return the FreeMarker HttpSessionHashModel
         */
        private HttpSessionHashModel buildSessionModel(HttpServletRequest request, HttpServletResponse response) {
                HttpSession session = request.getSession(false);
                if (session != null) {
                        return new HttpSessionHashModel(session, getObjectWrapper());
                }
                else {
                        return new HttpSessionHashModel(null, request, response, getObjectWrapper());
                }
        }

        /**
         * Retrieve the FreeMarker template for the given locale,
         * to be rendering by this view.
         * <p>By default, the template specified by the "url" bean property
         * will be retrieved.
         * @param locale the current locale
         * @return the FreeMarker template to render
         * @throws IOException if the template file could not be retrieved
         * @see #setUrl
         * @see #getTemplate(String, java.util.Locale)
         */
        protected Template getTemplate(Locale locale) throws IOException {
                String url = getUrl();
                Assert.state(url != null, "'url' not set");
                return getTemplate(url, locale);
        }

        /**
         * Retrieve the FreeMarker template specified by the given name,
         * using the encoding specified by the "encoding" bean property.
         * <p>Can be called by subclasses to retrieve a specific template,
         * for example to render multiple templates into a single view.
         * @param name the file name of the desired template
         * @param locale the current locale
         * @return the FreeMarker template
         * @throws IOException if the template file could not be retrieved
         */
        protected Template getTemplate(String name, Locale locale) throws IOException {
                return (getEncoding() != null ?
                                obtainConfiguration().getTemplate(name, locale, getEncoding()) :
                                obtainConfiguration().getTemplate(name, locale));
        }

        /**
         * Process the FreeMarker template to the servlet response.
         * <p>Can be overridden to customize the behavior.
         * @param template the template to process
         * @param model the model for the template
         * @param response servlet response (use this to get the OutputStream or Writer)
         * @throws IOException if the template file could not be retrieved
         * @throws TemplateException if thrown by FreeMarker
         * @see freemarker.template.Template#process(Object, java.io.Writer)
         */
        protected void processTemplate(Template template, SimpleHash model, HttpServletResponse response)
                        throws IOException, TemplateException {

                template.process(model, response.getWriter());
        }


        /**
         * Simple adapter class that extends {@link GenericServlet}.
         * Needed for JSP access in FreeMarker.
         */
        @SuppressWarnings("serial")
        private static class GenericServletAdapter extends GenericServlet {

                @Override
                public void service(ServletRequest servletRequest, ServletResponse servletResponse) {
                        // no-op
                }
        }


        /**
         * Internal implementation of the {@link ServletConfig} interface,
         * to be passed to the servlet adapter.
         */
        private class DelegatingServletConfig implements ServletConfig {

                @Override
                @Nullable
                public String getServletName() {
                        return FreeMarkerView.this.getBeanName();
                }

                @Override
                @Nullable
                public ServletContext getServletContext() {
                        return FreeMarkerView.this.getServletContext();
                }

                @Override
                @Nullable
                public String getInitParameter(String paramName) {
                        return null;
                }

                @Override
                public Enumeration<String> getInitParameterNames() {
                        return Collections.enumeration(Collections.emptySet());
                }
        }

}