VelocityLayoutView.java

/*
 * Copyright 2002-2009 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 java.util.Locale;
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.core.NestedIOException;

/**
 * VelocityLayoutView emulates the functionality offered by Velocity's
 * VelocityLayoutServlet to ease page composition from different templates.
 *
 * <p>The <code>url</code> property should be set to the content template
 * for the view, and the layout template location should be specified as
 * <code>layoutUrl</code> 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</code> 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" )</code>
	 * <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</code>.
	 * @param screenContentKey the name of the screen content key to use
	 */
	public void setScreenContentKey(String screenContentKey) {
		this.screenContentKey = screenContentKey;
	}


	/**
	 * Overrides <code>VelocityView.checkTemplate()</code> 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.
	 */
	@Override
	public boolean checkResource(Locale locale) throws Exception {
		if (!super.checkResource(locale)) {
			return false;
		}

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

}