/*
* Copyright 2002-2011 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;
import java.util.HashMap;
import java.util.Locale;
import java.util.Map;
import org.springframework.web.context.support.WebApplicationObjectSupport;
import org.springframework.web.servlet.View;
import org.springframework.web.servlet.ViewResolver;
/**
* Convenient base class for {@link org.springframework.web.servlet.ViewResolver}
* implementations. Caches {@link org.springframework.web.servlet.View} objects
* once resolved: This means that view resolution won't be a performance problem,
* no matter how costly initial view retrieval is.
*
* <p>Subclasses need to implement the {@link #loadView} template method,
* building the View object for a specific view name and locale.
*
* @author Rod Johnson
* @author Juergen Hoeller
* @see #loadView
*/
public abstract class AbstractCachingViewResolver extends WebApplicationObjectSupport implements ViewResolver {
/** Whether we should cache views, once resolved */
private boolean cache = true;
/** Whether we should refrain from resolving views again if unresolved once */
private boolean cacheUnresolved = true;
/** Map from view key to View instance */
private final Map<Object, View> viewCache = new HashMap<Object, View>();
/**
* Enable or disable caching.
* <p>Default is "true": caching is enabled.
* Disable this only for debugging and development.
* <p><b>Warning: Disabling caching can severely impact performance.</b>
*/
public void setCache(boolean cache) {
this.cache = cache;
}
/**
* Return if caching is enabled.
*/
public boolean isCache() {
return this.cache;
}
/**
* Whether a view name once resolved to {@code null} should be cached and
* automatically resolved to {@code null} subsequently.
* <p>Default is "true": unresolved view names are being cached, as of Spring 3.1.
* Note that this flag only applies if the general {@link #setCache "cache"}
* flag is kept at its default of "true" as well.
* <p>Of specific interest is the ability for some AbstractUrlBasedView
* implementations (FreeMarker, Velocity, Tiles) to check if an underlying
* resource exists via {@link AbstractUrlBasedView#checkResource(Locale)}.
* With this flag set to "false", an underlying resource that re-appears
* is noticed and used. With the flag set to "true", one check is made only.
*/
public void setCacheUnresolved(boolean cacheUnresolved) {
this.cacheUnresolved = cacheUnresolved;
}
/**
* Return if caching of unresolved views is enabled.
*/
public boolean isCacheUnresolved() {
return this.cacheUnresolved;
}
public View resolveViewName(String viewName, Locale locale) throws Exception {
if (!isCache()) {
return createView(viewName, locale);
}
else {
Object cacheKey = getCacheKey(viewName, locale);
synchronized (this.viewCache) {
View view = this.viewCache.get(cacheKey);
if (view == null && (!this.cacheUnresolved || !this.viewCache.containsKey(cacheKey))) {
// Ask the subclass to create the View object.
view = createView(viewName, locale);
if (view != null || this.cacheUnresolved) {
this.viewCache.put(cacheKey, view);
if (logger.isTraceEnabled()) {
logger.trace("Cached view [" + cacheKey + "]");
}
}
}
return view;
}
}
}
/**
* Return the cache key for the given view name and the given locale.
* <p>Default is a String consisting of view name and locale suffix.
* Can be overridden in subclasses.
* <p>Needs to respect the locale in general, as a different locale can
* lead to a different view resource.
*/
protected Object getCacheKey(String viewName, Locale locale) {
return viewName + "_" + locale;
}
/**
* Provides functionality to clear the cache for a certain view.
* <p>This can be handy in case developer are able to modify views
* (e.g. Velocity templates) at runtime after which you'd need to
* clear the cache for the specified view.
* @param viewName the view name for which the cached view object
* (if any) needs to be removed
* @param locale the locale for which the view object should be removed
*/
public void removeFromCache(String viewName, Locale locale) {
if (!this.cache) {
logger.warn("View caching is SWITCHED OFF -- removal not necessary");
}
else {
Object cacheKey = getCacheKey(viewName, locale);
Object cachedView;
synchronized (this.viewCache) {
cachedView = this.viewCache.remove(cacheKey);
}
if (cachedView == null) {
// Some debug output might be useful...
if (logger.isDebugEnabled()) {
logger.debug("No cached instance for view '" + cacheKey + "' was found");
}
}
else {
if (logger.isDebugEnabled()) {
logger.debug("Cache for view " + cacheKey + " has been cleared");
}
}
}
}
/**
* Clear the entire view cache, removing all cached view objects.
* Subsequent resolve calls will lead to recreation of demanded view objects.
*/
public void clearCache() {
logger.debug("Clearing entire view cache");
synchronized (this.viewCache) {
this.viewCache.clear();
}
}
/**
* Create the actual View object.
* <p>The default implementation delegates to {@link #loadView}.
* This can be overridden to resolve certain view names in a special fashion,
* before delegating to the actual <code>loadView</code> implementation
* provided by the subclass.
* @param viewName the name of the view to retrieve
* @param locale the Locale to retrieve the view for
* @return the View instance, or <code>null</code> if not found
* (optional, to allow for ViewResolver chaining)
* @throws Exception if the view couldn't be resolved
* @see #loadView
*/
protected View createView(String viewName, Locale locale) throws Exception {
return loadView(viewName, locale);
}
/**
* Subclasses must implement this method, building a View object
* for the specified view. The returned View objects will be
* cached by this ViewResolver base class.
* <p>Subclasses are not forced to support internationalization:
* A subclass that does not may simply ignore the locale parameter.
* @param viewName the name of the view to retrieve
* @param locale the Locale to retrieve the view for
* @return the View instance, or <code>null</code> if not found
* (optional, to allow for ViewResolver chaining)
* @throws Exception if the view couldn't be resolved
* @see #resolveViewName
*/
protected abstract View loadView(String viewName, Locale locale) throws Exception;
}