/*
 * Copyright 2002-2012 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.ui.velocity;

import java.io.File;
import java.io.IOException;
import java.util.HashMap;
import java.util.Map;
import java.util.Properties;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.apache.velocity.app.VelocityEngine;
import org.apache.velocity.exception.VelocityException;
import org.apache.velocity.runtime.RuntimeConstants;

import org.springframework.core.io.DefaultResourceLoader;
import org.springframework.core.io.Resource;
import org.springframework.core.io.ResourceLoader;
import org.springframework.core.io.support.PropertiesLoaderUtils;
import org.springframework.util.CollectionUtils;
import org.springframework.util.StringUtils;

/**
 * Factory that configures a VelocityEngine. Can be used standalone,
 * but typically you will either use {@link VelocityEngineFactoryBean}
 * for preparing a VelocityEngine as bean reference, or
 * {@link org.springframework.web.servlet.view.velocity.VelocityConfigurer}
 * for web views.
 *
 * <p>The optional "configLocation" property sets the location of the Velocity
 * properties file, within the current application. Velocity properties can be
 * overridden via "velocityProperties", or even completely specified locally,
 * avoiding the need for an external properties file.
 *
 * <p>The "resourceLoaderPath" property can be used to specify the Velocity
 * resource loader path via Spring's Resource abstraction, possibly relative
 * to the Spring application context.
 *
 * <p>If "overrideLogging" is true (the default), the VelocityEngine will be
 * configured to log via Commons Logging, that is, using the Spring-provided
 * {@link CommonsLoggingLogSystem} as log system.
 *
 * <p>The simplest way to use this class is to specify a
 * {@link #setResourceLoaderPath(String) "resourceLoaderPath"}; the
 * VelocityEngine typically then does not need any further configuration.
 *
 * @author Juergen Hoeller
 * @see #setConfigLocation
 * @see #setVelocityProperties
 * @see #setResourceLoaderPath
 * @see #setOverrideLogging
 * @see #createVelocityEngine
 * @see CommonsLoggingLogSystem
 * @see VelocityEngineFactoryBean
 * @see org.springframework.web.servlet.view.velocity.VelocityConfigurer
 * @see org.apache.velocity.app.VelocityEngine
 */
public class VelocityEngineFactory {

	protected final Log logger = LogFactory.getLog(getClass());

	private Resource configLocation;

	private final Map<String, Object> velocityProperties = new HashMap<String, Object>();

	private String resourceLoaderPath;

	private ResourceLoader resourceLoader = new DefaultResourceLoader();

	private boolean preferFileSystemAccess = true;

	private boolean overrideLogging = true;


	/**
	 * Set the location of the Velocity config file.
	 * Alternatively, you can specify all properties locally.
	 * @see #setVelocityProperties
	 * @see #setResourceLoaderPath
	 */
	public void setConfigLocation(Resource configLocation) {
		this.configLocation = configLocation;
	}

	/**
	 * Set Velocity properties, like "file.resource.loader.path".
	 * Can be used to override values in a Velocity config file,
	 * or to specify all necessary properties locally.
	 * <p>Note that the Velocity resource loader path also be set to any
	 * Spring resource location via the "resourceLoaderPath" property.
	 * Setting it here is just necessary when using a non-file-based
	 * resource loader.
	 * @see #setVelocityPropertiesMap
	 * @see #setConfigLocation
	 * @see #setResourceLoaderPath
	 */
	public void setVelocityProperties(Properties velocityProperties) {
		CollectionUtils.mergePropertiesIntoMap(velocityProperties, this.velocityProperties);
	}

	/**
	 * Set Velocity properties as Map, to allow for non-String values
	 * like "ds.resource.loader.instance".
	 * @see #setVelocityProperties
	 */
	public void setVelocityPropertiesMap(Map<String, Object> velocityPropertiesMap) {
		if (velocityPropertiesMap != null) {
			this.velocityProperties.putAll(velocityPropertiesMap);
		}
	}

	/**
	 * Set the Velocity resource loader path via a Spring resource location.
	 * Accepts multiple locations in Velocity's comma-separated path style.
	 * <p>When populated via a String, standard URLs like "file:" and "classpath:"
	 * pseudo URLs are supported, as understood by ResourceLoader. Allows for
	 * relative paths when running in an ApplicationContext.
	 * <p>Will define a path for the default Velocity resource loader with the name
	 * "file". If the specified resource cannot be resolved to a {@code java.io.File},
	 * a generic SpringResourceLoader will be used under the name "spring", without
	 * modification detection.
	 * <p>Note that resource caching will be enabled in any case. With the file
	 * resource loader, the last-modified timestamp will be checked on access to
	 * detect changes. With SpringResourceLoader, the resource will be cached
	 * forever (for example for class path resources).
	 * <p>To specify a modification check interval for files, use Velocity's
	 * standard "file.resource.loader.modificationCheckInterval" property. By default,
	 * the file timestamp is checked on every access (which is surprisingly fast).
	 * Of course, this just applies when loading resources from the file system.
	 * <p>To enforce the use of SpringResourceLoader, i.e. to not resolve a path
	 * as file system resource in any case, turn off the "preferFileSystemAccess"
	 * flag. See the latter's javadoc for details.
	 * @see #setResourceLoader
	 * @see #setVelocityProperties
	 * @see #setPreferFileSystemAccess
	 * @see SpringResourceLoader
	 * @see org.apache.velocity.runtime.resource.loader.FileResourceLoader
	 */
	public void setResourceLoaderPath(String resourceLoaderPath) {
		this.resourceLoaderPath = resourceLoaderPath;
	}

	/**
	 * Set the Spring ResourceLoader to use for loading Velocity template files.
	 * The default is DefaultResourceLoader. Will get overridden by the
	 * ApplicationContext if running in a context.
	 * @see org.springframework.core.io.DefaultResourceLoader
	 * @see org.springframework.context.ApplicationContext
	 */
	public void setResourceLoader(ResourceLoader resourceLoader) {
		this.resourceLoader = resourceLoader;
	}

	/**
	 * Return the Spring ResourceLoader to use for loading Velocity template files.
	 */
	protected ResourceLoader getResourceLoader() {
		return this.resourceLoader;
	}

	/**
	 * Set whether to prefer file system access for template loading.
	 * File system access enables hot detection of template changes.
	 * <p>If this is enabled, VelocityEngineFactory will try to resolve the
	 * specified "resourceLoaderPath" as file system resource (which will work
	 * for expanded class path resources and ServletContext resources too).
	 * <p>Default is "true". Turn this off to always load via SpringResourceLoader
	 * (i.e. as stream, without hot detection of template changes), which might
	 * be necessary if some of your templates reside in an expanded classes
	 * directory while others reside in jar files.
	 * @see #setResourceLoaderPath
	 */
	public void setPreferFileSystemAccess(boolean preferFileSystemAccess) {
		this.preferFileSystemAccess = preferFileSystemAccess;
	}

	/**
	 * Return whether to prefer file system access for template loading.
	 */
	protected boolean isPreferFileSystemAccess() {
		return this.preferFileSystemAccess;
	}

	/**
	 * Set whether Velocity should log via Commons Logging, i.e. whether Velocity's
	 * log system should be set to CommonsLoggingLogSystem. Default value is true.
	 * @see CommonsLoggingLogSystem
	 */
	public void setOverrideLogging(boolean overrideLogging) {
		this.overrideLogging = overrideLogging;
	}


	/**
	 * Prepare the VelocityEngine instance and return it.
	 * @return the VelocityEngine instance
	 * @throws IOException if the config file wasn't found
	 * @throws VelocityException on Velocity initialization failure
	 */
	public VelocityEngine createVelocityEngine() throws IOException, VelocityException {
		VelocityEngine velocityEngine = newVelocityEngine();
		Map<String, Object> props = new HashMap<String, Object>();

		// Load config file if set.
		if (this.configLocation != null) {
			if (logger.isInfoEnabled()) {
				logger.info("Loading Velocity config from [" + this.configLocation + "]");
			}
			CollectionUtils.mergePropertiesIntoMap(PropertiesLoaderUtils.loadProperties(this.configLocation), props);
		}

		// Merge local properties if set.
		if (!this.velocityProperties.isEmpty()) {
			props.putAll(this.velocityProperties);
		}

		// Set a resource loader path, if required.
		if (this.resourceLoaderPath != null) {
			initVelocityResourceLoader(velocityEngine, this.resourceLoaderPath);
		}

		// Log via Commons Logging?
		if (this.overrideLogging) {
			velocityEngine.setProperty(RuntimeConstants.RUNTIME_LOG_LOGSYSTEM, new CommonsLoggingLogSystem());
		}

		// Apply properties to VelocityEngine.
		for (Map.Entry<String, Object> entry : props.entrySet()) {
			velocityEngine.setProperty(entry.getKey(), entry.getValue());
		}

		postProcessVelocityEngine(velocityEngine);

		try {
			// Perform actual initialization.
			velocityEngine.init();
		}
		catch (IOException ex) {
			throw ex;
		}
		catch (VelocityException ex) {
			throw ex;
		}
		catch (RuntimeException ex) {
			throw ex;
		}
		catch (Exception ex) {
			logger.error("Why does VelocityEngine throw a generic checked exception, after all?", ex);
			throw new VelocityException(ex.toString());
		}

		return velocityEngine;
	}

	/**
	 * Return a new VelocityEngine. Subclasses can override this for
	 * custom initialization, or for using a mock object for testing.
	 * <p>Called by {@code createVelocityEngine()}.
	 * @return the VelocityEngine instance
	 * @throws IOException if a config file wasn't found
	 * @throws VelocityException on Velocity initialization failure
	 * @see #createVelocityEngine()
	 */
	protected VelocityEngine newVelocityEngine() throws IOException, VelocityException {
		return new VelocityEngine();
	}

	/**
	 * Initialize a Velocity resource loader for the given VelocityEngine:
	 * either a standard Velocity FileResourceLoader or a SpringResourceLoader.
	 * <p>Called by {@code createVelocityEngine()}.
	 * @param velocityEngine the VelocityEngine to configure
	 * @param resourceLoaderPath the path to load Velocity resources from
	 * @see org.apache.velocity.runtime.resource.loader.FileResourceLoader
	 * @see SpringResourceLoader
	 * @see #initSpringResourceLoader
	 * @see #createVelocityEngine()
	 */
	protected void initVelocityResourceLoader(VelocityEngine velocityEngine, String resourceLoaderPath) {
		if (isPreferFileSystemAccess()) {
			// Try to load via the file system, fall back to SpringResourceLoader
			// (for hot detection of template changes, if possible).
			try {
				StringBuilder resolvedPath = new StringBuilder();
				String[] paths = StringUtils.commaDelimitedListToStringArray(resourceLoaderPath);
				for (int i = 0; i < paths.length; i++) {
					String path = paths[i];
					Resource resource = getResourceLoader().getResource(path);
					File file = resource.getFile();  // will fail if not resolvable in the file system
					if (logger.isDebugEnabled()) {
						logger.debug("Resource loader path [" + path + "] resolved to file [" + file.getAbsolutePath() + "]");
					}
					resolvedPath.append(file.getAbsolutePath());
					if (i < paths.length - 1) {
						resolvedPath.append(',');
					}
				}
				velocityEngine.setProperty(RuntimeConstants.RESOURCE_LOADER, "file");
				velocityEngine.setProperty(RuntimeConstants.FILE_RESOURCE_LOADER_CACHE, "true");
				velocityEngine.setProperty(RuntimeConstants.FILE_RESOURCE_LOADER_PATH, resolvedPath.toString());
			}
			catch (IOException ex) {
				if (logger.isDebugEnabled()) {
					logger.debug("Cannot resolve resource loader path [" + resourceLoaderPath +
							"] to [java.io.File]: using SpringResourceLoader", ex);
				}
				initSpringResourceLoader(velocityEngine, resourceLoaderPath);
			}
		}
		else {
			// Always load via SpringResourceLoader
			// (without hot detection of template changes).
			if (logger.isDebugEnabled()) {
				logger.debug("File system access not preferred: using SpringResourceLoader");
			}
			initSpringResourceLoader(velocityEngine, resourceLoaderPath);
		}
	}

	/**
	 * Initialize a SpringResourceLoader for the given VelocityEngine.
	 * <p>Called by {@code initVelocityResourceLoader}.
	 * @param velocityEngine the VelocityEngine to configure
	 * @param resourceLoaderPath the path to load Velocity resources from
	 * @see SpringResourceLoader
	 * @see #initVelocityResourceLoader
	 */
	protected void initSpringResourceLoader(VelocityEngine velocityEngine, String resourceLoaderPath) {
		velocityEngine.setProperty(
				RuntimeConstants.RESOURCE_LOADER, SpringResourceLoader.NAME);
		velocityEngine.setProperty(
				SpringResourceLoader.SPRING_RESOURCE_LOADER_CLASS, SpringResourceLoader.class.getName());
		velocityEngine.setProperty(
				SpringResourceLoader.SPRING_RESOURCE_LOADER_CACHE, "true");
		velocityEngine.setApplicationAttribute(
				SpringResourceLoader.SPRING_RESOURCE_LOADER, getResourceLoader());
		velocityEngine.setApplicationAttribute(
				SpringResourceLoader.SPRING_RESOURCE_LOADER_PATH, resourceLoaderPath);
	}

	/**
	 * To be implemented by subclasses that want to to perform custom
	 * post-processing of the VelocityEngine after this FactoryBean
	 * performed its default configuration (but before VelocityEngine.init).
	 * <p>Called by {@code createVelocityEngine()}.
	 * @param velocityEngine the current VelocityEngine
	 * @throws IOException if a config file wasn't found
	 * @throws VelocityException on Velocity initialization failure
	 * @see #createVelocityEngine()
	 * @see org.apache.velocity.app.VelocityEngine#init
	 */
	protected void postProcessVelocityEngine(VelocityEngine velocityEngine)
			throws IOException, VelocityException {
	}

}
