AbstractVelocityContent.java

/*
 * Copyright (c) 2012 Data Harmonisation Panel
 * 
 * All rights reserved. This program and the accompanying materials are made
 * available under the terms of the GNU Lesser General Public License as
 * published by the Free Software Foundation, either version 3 of the License,
 * or (at your option) any later version.
 * 
 * You should have received a copy of the GNU Lesser General Public License
 * along with this distribution. If not, see <http://www.gnu.org/licenses/>.
 * 
 * Contributors:
 *     HUMBOLDT EU Integrated Project #030962
 *     Data Harmonisation Panel <http://www.dhpanel.eu>
 */

package eu.esdihumboldt.hale.doc.util.content;

import java.io.BufferedOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.FileWriter;
import java.io.InputStream;
import java.io.OutputStream;
import java.util.Locale;
import java.util.concurrent.Callable;

import org.apache.commons.io.IOUtils;
import org.apache.velocity.Template;
import org.apache.velocity.VelocityContext;
import org.apache.velocity.app.VelocityEngine;
import org.eclipse.help.IHelpContentProducer;

import com.google.common.io.Files;

/**
 * Help content based on velocity templates. Use
 * {@link #getContentFromTemplate(String, String, Callable)} in your
 * {@link #getInputStream(String, String, Locale)} implementation.
 * 
 * @author Simon Templer
 */
public abstract class AbstractVelocityContent implements IHelpContentProducer {

	/**
	 * The velocity engine used for content generation.
	 */
	private VelocityEngine ve;

	/**
	 * The temporary directory where the template is copied to, and the already
	 * generated is stored.
	 */
	private File tempDir;

	/**
	 * Generate content from the template and the given context factory. If
	 * called more than once with the same id, the previously generated content
	 * for that id is returned.
	 * 
	 * @param contentId the content id
	 * @param templateId the template id (there may be multiple templates)
	 * @param contextFactory the context factory, is called once or not at all
	 * @return the content input stream to return in
	 *         {@link #getInputStream(String, String, Locale)}
	 * @throws Exception if an error occurs creating the content
	 */
	protected InputStream getContentFromTemplate(String contentId, String templateId,
			Callable<VelocityContext> contextFactory) throws Exception {
		init(templateId);

		// creates the template file into the temporary directory
		// if it doesn't already exist
		File contentFile = new File(tempDir, templateId + "_" + contentId + ".html");
		if (!contentFile.exists()) {
			// get the template context
			VelocityContext context = contextFactory.call();

			// get the template
			Template template = ve.getTemplate(templateId + ".vm", "UTF-8");

			// write to the file
			FileWriter fw = new FileWriter(contentFile);
			template.merge(context, fw);

			fw.close();

			contentFile.deleteOnExit();
		}

		return new FileInputStream(contentFile);
	}

	/**
	 * Initialize temporary directory and template engine.
	 * 
	 * @param templateId the template id (there may be multiple templates)
	 * 
	 * @throws Exception if an error occurs during the initialization
	 */
	private void init(String templateId) throws Exception {
		synchronized (this) {
			// engine initialization
			if (ve == null) {
				ve = new VelocityEngine();
				// create a temporary directory
				tempDir = Files.createTempDir();
				tempDir.deleteOnExit();

				ve.setProperty("file.resource.loader.path", tempDir.getAbsolutePath());

				// initialize VelocityEngine
				ve.init();
			}

			File templateFile = new File(tempDir, templateId + ".vm");
			if (!templateFile.exists()) {
				OutputStream fos = new BufferedOutputStream(new FileOutputStream(templateFile));
				InputStream stream = getTemplate(templateId);

				// copy the InputStream into FileOutputStream
				IOUtils.copy(stream, fos);

				stream.close();
				fos.close();

				templateFile.deleteOnExit();
			}
		}
	}

	/**
	 * Get the template content.
	 * 
	 * @param templateId the template id (there may be multiple templates)
	 * 
	 * @return the template as input stream
	 * @throws Exception if an error occurs retrieving the template
	 */
	protected abstract InputStream getTemplate(String templateId) throws Exception;

}