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Spring Framework example source code file (VelocityLayoutView.java)

This example Spring Framework source code file (VelocityLayoutView.java) is included in the DevDaily.com "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

Java - Spring Framework tags/keywords

applicationcontextexception, applicationcontextexception, default_layout_key, default_layout_url, default_screen_content_key, exception, exception, http, io, request, response, servlet, string, string, stringwriter, stringwriter, template, url, velocity

The Spring Framework VelocityLayoutView.java source code

/*
 * Copyright 2002-2006 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.servlet.view.velocity;

import java.io.StringWriter;

import javax.servlet.http.HttpServletResponse;

import org.apache.velocity.Template;
import org.apache.velocity.context.Context;
import org.apache.velocity.exception.ResourceNotFoundException;

import org.springframework.context.ApplicationContextException;

/**
 * VelocityLayoutView emulates the functionality offered by Velocity's
 * VelocityLayoutServlet to ease page composition from different templates.
 *
 * <p>The url property should be set to the content template
 * for the view, and the layout template location should be specified as
 * <code>layoutUrl property. A view can override the configured
 * layout template location by setting the appropriate key (the default
 * is "layout") in the content template.
 *
 * <p>When the view is rendered, the VelocityContext is first merged with
 * the content template (specified by the <code>url property) and
 * then merged with the layout template to produce the final output.
 *
 * <p>The layout template can include the screen content through a
 * VelocityContext variable (the default is "screen_content").
 * At runtime, this variable will contain the rendered content template.
 *
 * @author Darren Davison
 * @author Juergen Hoeller
 * @since 1.2
 * @see #setLayoutUrl
 * @see #setLayoutKey
 * @see #setScreenContentKey
 */
public class VelocityLayoutView extends VelocityToolboxView {

	/**
	 * The default {@link #setLayoutUrl(String) layout url}.
	 */
	public static final String DEFAULT_LAYOUT_URL = "layout.vm";

	/**
	 * The default {@link #setLayoutKey(String) layout key}.
	 */
	public static final String DEFAULT_LAYOUT_KEY = "layout";

	/**
	 * The default {@link #setScreenContentKey(String) screen content key}.
	 */
	public static final String DEFAULT_SCREEN_CONTENT_KEY = "screen_content";


	private String layoutUrl = DEFAULT_LAYOUT_URL;

	private String layoutKey = DEFAULT_LAYOUT_KEY;

	private String screenContentKey = DEFAULT_SCREEN_CONTENT_KEY;


	/**
	 * Set the layout template to use. Default is {@link #DEFAULT_LAYOUT_URL "layout.vm"}.
	 * @param layoutUrl the template location (relative to the template
	 * root directory)
	 */
	public void setLayoutUrl(String layoutUrl) {
		this.layoutUrl = layoutUrl;
	}

	/**
	 * Set the context key used to specify an alternate layout to be used instead
	 * of the default layout. Screen content templates can override the layout
	 * template that they wish to be wrapped with by setting this value in the
	 * template, for example:<br>
	 * <code>#set( $layout = "MyLayout.vm" )
	 * <p>Default key is {@link #DEFAULT_LAYOUT_KEY "layout"}, as illustrated above.
	 * @param layoutKey the name of the key you wish to use in your
	 * screen content templates to override the layout template
	 */
	public void setLayoutKey(String layoutKey) {
		this.layoutKey = layoutKey;
	}

	/**
	 * Set the name of the context key that will hold the content of
	 * the screen within the layout template. This key must be present
	 * in the layout template for the current screen to be rendered.
	 * <p>Default is {@link #DEFAULT_SCREEN_CONTENT_KEY "screen_content"}:
	 * accessed in VTL as <code>$screen_content.
	 * @param screenContentKey the name of the screen content key to use
	 */
	public void setScreenContentKey(String screenContentKey) {
		this.screenContentKey = screenContentKey;
	}


	/**
	 * Overrides <code>VelocityView.checkTemplate() to additionally check
	 * that both the layout template and the screen content template can be loaded.
	 * Note that during rendering of the screen content, the layout template
	 * can be changed which may invalidate any early checking done here.
	 */
	protected void checkTemplate() throws ApplicationContextException {
		super.checkTemplate();

		try {
			// Check that we can get the template, even if we might subsequently get it again.
			getTemplate(this.layoutUrl);
		}
		catch (ResourceNotFoundException ex) {
			throw new ApplicationContextException("Cannot find Velocity template for URL [" + this.layoutUrl +
					"]: Did you specify the correct resource loader path?", ex);
		}
		catch (Exception ex) {
			throw new ApplicationContextException(
					"Could not load Velocity template for URL [" + this.layoutUrl + "]", ex);
		}
	}

	/**
	 * Overrides the normal rendering process in order to pre-process the Context,
	 * merging it with the screen template into a single value (identified by the
	 * value of screenContentKey). The layout template is then merged with the
	 * modified Context in the super class.
	 */
	protected void doRender(Context context, HttpServletResponse response) throws Exception {
		renderScreenContent(context);

		// Velocity context now includes any mappings that were defined
		// (via #set) in screen content template.
		// The screen template can overrule the layout by doing
		// #set( $layout = "MyLayout.vm" )
		String layoutUrlToUse = (String) context.get(this.layoutKey);
		if (layoutUrlToUse != null) {
			if (logger.isDebugEnabled()) {
				logger.debug("Screen content template has requested layout [" + layoutUrlToUse + "]");
			}
		}
		else {
			// No explicit layout URL given -> use default layout of this view.
			layoutUrlToUse = this.layoutUrl;
		}

		mergeTemplate(getTemplate(layoutUrlToUse), context, response);
	}

	/**
	 * The resulting context contains any mappings from render, plus screen content.
	 */
	private void renderScreenContent(Context velocityContext) throws Exception {
		if (logger.isDebugEnabled()) {
			logger.debug("Rendering screen content template [" + getUrl() + "]");
		}

		StringWriter sw = new StringWriter();
		Template screenContentTemplate = getTemplate(getUrl());
		screenContentTemplate.merge(velocityContext, sw);

		// Put rendered content into Velocity context.
		velocityContext.put(this.screenContentKey, sw.toString());
	}

}

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