[Tapestry Wiki] Update of "SessionPagePersistence" by MarkLehmacher

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The following page has been changed by MarkLehmacher:
http://wiki.apache.org/tapestry/SessionPagePersistence

The comment on the change is:
added a different way of implementing session:page strategy

------------------------------------------------------------------------------
  Tapestry's session persistence is great, but some times you just want a value to work like a "session page instance variable" - only around while the user is working within a page (calling listener methods on that page, but not rendering any other page).  This strategy avoids all the problems of using real instance variables in a page object.
  
- Values are discarded if the last page rendered was not the same as the current page being rendered.
+ Values are discarded if the last page rendered was not the same as the current page being rendered (see below for another implementation).
  
  You'll need a [http://tapestry.apache.org/tapestry4/tapestry/apidocs/org/apache/tapestry/record/PropertyPersistenceStrategy.html PropertyPersistenceStrategy], some hivemodule magic, and a [http://tapestry.apache.org/tapestry4/tapestry/apidocs/org/apache/tapestry/event/PageEndRenderListener.html PageEndRenderListener].  Once it's done, just use [http://tapestry.apache.org/tapestry4/tapestry-annotations/apidocs/org/apache/tapestry/annotations/Persist.html @Persist]("session:page") to take advantage of it.
  
@@ -33, +33 @@

  	 * unique persistence name
  	 */
  	public static final String	STRATEGY_ID	= "session:page";
- 	
+ 
  	/**
  	 * Session key for the last rendered page name
  	 */
@@ -63, +63 @@

  		WebSession session = _request.getSession(false);
  
  		if (session == null) return Collections.EMPTY_LIST;
- 		
+ 
  		/*
  		 * Single largest difference between this and Session persistence:
- 		 * If the last rendered page is not this page (or is null) then don't return any 
+ 		 * If the last rendered page is not this page (or is null) then don't return any
  		 * property values, and set any values found to null.
  		 */
  		String lastPage = (String) session.getAttribute(getLastPageKey(_applicationId));
@@ -75, +75 @@

  			discardStoredChanges(pageName);
  			return Collections.EMPTY_LIST;
  		}
- 		
+ 
  		final Collection result = new ArrayList();
  
  		WebSessionAttributeCallback callback = new WebSessionAttributeCallback() {
@@ -117, +117 @@

  	public void setRequest(WebRequest request) {
  		_request = request;
  	}
- 	
+ 
  	/**
- 	 * @param appId 
+ 	 * @param appId
  	 * @return application specific session key name for the last rendered page
  	 * static so the PageEndRenderListener can also use it - there may be a better way for this
  	 */
@@ -140, +140 @@

  				<set-object property="applicationId" value="infrastructure:applicationId"/>
  			</construct>
  		</invoke-factory>
-     </service-point> 
+     </service-point>
  
  	<contribution configuration-id="tapestry.persist.PersistenceStrategy">
  		<strategy name="session:page" object="service:SessionPagePropertyPersistenceStrategy"/>
@@ -176, +176 @@

  	}
  }}}
  
+ A completely different implementation for the same purpose (of implementing a session:page strategy) is presented below. The advantage to the solution above is, that it is stand alone and does not rely on the PageEndRenderListener impl. However, there are some disadvantages to it as well, as it breaks encapsulation of some Tapestry internals (RecordUtils in particular) and might incur performance-penalties when you have lots and lots of properties in your session (because it iterates over *all* session properties for each request).
+ 
+ The strategy:
+ {{{
+ package de.rwth.rz.tapestry.record;
+ 
+ 
+ import java.util.ArrayList;
+ import java.util.Collection;
+ import java.util.Collections;
+ import java.util.Iterator;
+ 
+ import org.apache.hivemind.util.Defense;
+ import org.apache.log4j.Logger;
+ import org.apache.tapestry.IRequestCycle;
+ import org.apache.tapestry.engine.ServiceEncoding;
+ import org.apache.tapestry.record.PropertyChange;
+ import org.apache.tapestry.record.PropertyPersistenceStrategy;
+ import org.apache.tapestry.record.RecordUtils;
+ import org.apache.tapestry.record.SessionPropertyPersistenceStrategy;
+ import org.apache.tapestry.record.WebSessionAttributeCallback;
+ import org.apache.tapestry.web.WebRequest;
+ import org.apache.tapestry.web.WebSession;
+ 
+ 
+ /**
+  * <p>
+  * Strategy similar to client:page but for server properties.
+  * </p>
+  *
+  * <h4> Implementation notes: </h4>
+  *
+  * <p>
+  * Can't extend {@link SessionPropertyPersistenceStrategy} because all methods use STRATEGY_ID which
+  * is final in <code>SessionPropertyPersistenceStrategy</code>. So basically this is lot's of
+  * duplicated code from said class, with changed strategy id and a one-line change in
+  * {@link #getStoredChanges(String)}.
+  * </p>
+  *
+  * <p>
+  * Moreover implementation of {@link #evictStaleProperties(String)} is based upon internal knowledge
+  * of {@link RecordUtils} workings (yeah, sucks). It works by iterating over all session properties
+  * and discarding those properties which have been recorded by this strategy (together with the page
+  * name they originated from) but don't belong to the current page.</p >
+  *
+  * <p>
+  * To top it all off, I don't even know whether or not it's correct to hook into
+  * <code>getStoredChanges</code> for evicting properties. I don't really know what that method is
+  * doing in the first place but I guess it returns those changed properties which should be handled
+  * by the page recorder for distribution over a clustered session or something (but that's just a
+  * wild guess).
+  * </p>
+  *
+  * <p>
+  * The strategy seems to work though.
+  * </p>
+  *
+  * <p>
+  * When using this strategy be aware what it means when the user uses the browser back button in
+  * order to return from another page to the page before: The page before will have lost all it's
+  * page persistent properties, so keep that in mind.
+  * </p>
+  *
+  * @author lehmacher
+  */
+ public class PageSessionPropertyPersistenceStrategy implements PropertyPersistenceStrategy {
+ 
+ 	private static final Logger logger = Logger.getLogger(PageSessionPropertyPersistenceStrategy.class);
+ 
+ 	/**
+ 	 * For page names starting with that prefix no properties are evicted. Basing this on the name
+ 	 * is brittle at best, but when I tried retrieving the current page from the request cycle in
+ 	 * getStoredChanges method (in order to base that change on an implemented IntermediatePage
+ 	 * interface), it did not work (forgot the exact reason why it did not work).
+ 	 */
+ 	public static final String SKIP_PREFIX = "intermediate";
+ 	public static final String STRATEGY_ID = "session-page";
+ 
+ 	private String _applicationId;
+ 	private WebRequest _request;
+ 	private IRequestCycle _cycle;
+ 
+     public void store(String pageName, String idPath, String propertyName, Object newValue) {
+ 		Defense.notNull(pageName, "pageName");
+ 		Defense.notNull(propertyName, "propertyName");
+ 
+ 		WebSession session = _request.getSession(true);
+ 
+ 		String attributeName = RecordUtils.buildChangeKey(STRATEGY_ID, _applicationId, pageName,
+ 			idPath, propertyName);
+ 
+ 		session.setAttribute(attributeName, newValue);
+ 	}
+ 
+ 	public Collection getStoredChanges(String pageName) {
+ 		Defense.notNull(pageName, "pageName");
+ 
+ 		evictStaleProperties(pageName);
+ 
+ 		WebSession session = _request.getSession(false);
+ 
+ 		if (session == null)
+ 			return Collections.EMPTY_LIST;
+ 
+ 		final Collection result = new ArrayList();
+ 
+ 		WebSessionAttributeCallback callback = new WebSessionAttributeCallback() {
+ 			public void handleAttribute(WebSession session, String name) {
+ 				PropertyChange change = RecordUtils.buildChange(name, session.getAttribute(name));
+ 
+ 				result.add(change);
+ 			}
+ 		};
+ 
+ 		RecordUtils.iterateOverMatchingAttributes(STRATEGY_ID, _applicationId, pageName, session,
+ 			callback);
+ 
+ 		return result;
+ 	}
+ 
+ 	public void addParametersForPersistentProperties(ServiceEncoding encoding, boolean post) {
+ 	}
+ 
+ 
+ 	public void discardStoredChanges(String pageName) {
+ 		WebSession session = _request.getSession(false);
+ 
+ 		if (session == null)
+ 			return;
+ 
+ 		WebSessionAttributeCallback callback = new WebSessionAttributeCallback() {
+ 			public void handleAttribute(WebSession session, String name) {
+ 				session.setAttribute(name, null);
+ 			}
+ 		};
+ 
+ 		RecordUtils.iterateOverMatchingAttributes(STRATEGY_ID, _applicationId, pageName, session,
+ 			callback);
+ 	}
+ 
+ 	/**
+ 	 * Iterates over all persistent properties and removes those which are page persistent but where
+ 	 * current page does not match recorded page.<p/>
+ 	 *
+ 	 * Implementation breaks encapsulation of RecordUtils.
+ 	 */
+ 	protected void evictStaleProperties(String currentPageName) {
+ 
+ 		// no session? do nothing
+ 		WebSession session = _request.getSession(false);
+ 		if (session == null)
+ 			return;
+ 
+ 		// this is for assuring that properties don't get evicted when a page is retrieved from the
+ 		// request cycle from another page. For example CurrentPage is the current page and it looks
+ 		// up AnotherPage in one of it's listener methods. Persistent properties will be restored
+ 		// for AnotherPage but at the same time we don't want the CurrentPage properties to get
+ 		// evicted just yet. The checks works on the assumption that when a new page is being surfed
+ 		// to, the request cycle will still return null when the persistence strategy is used to
+ 		// get stored changes.
+ 		if (_cycle.getPage() != null)
+ 			return;
+ 
+ 		if (currentPageName.startsWith(SKIP_PREFIX) || currentPageName.indexOf("/" + SKIP_PREFIX) != -1)
+ 			return;
+ 
+ 		// that's how RecordUtils build the prefix string:
+ 		// String prefix = strategyId + "," + applicationId + "," + pageName + ",";
+ 
+ 		// this breaks encapsulation for building the attributeName keys in RecordUtils
+ 		String prefix = STRATEGY_ID + "," + _applicationId + ",";
+ 
+ 		Iterator names = session.getAttributeNames().iterator();
+ 		while (names.hasNext()) {
+ 
+ 			String name = (String)names.next();
+ 
+ 			// if property is not of this strategy or application we skip it
+ 			if (!name.startsWith(prefix))
+ 				continue;
+ 
+ 			// means strategy and application match, but page doesn't
+ 			if (!name.startsWith(prefix + currentPageName)) {
+ 				session.setAttribute(name, null);
+ 				logger.debug("evicting from session " + name);
+ 			}
+ 
+ 		}
+ 	}
+ 
+ 	public void setApplicationId(String applicationName) {
+ 		_applicationId = applicationName;
+ 	}
+ 
+ 	public void setRequest(WebRequest request) {
+ 		_request = request;
+ 	}
+ 
+ 	public void setRequestCycle(IRequestCycle cycle) {
+ 		_cycle = cycle;
+ 	}
+ }
+ }}}
+ 
+ The hivemodule config:
+ {{{
+ 	<service-point id="PageSessionPropertyPersistenceStrategy"
+ 		interface="org.apache.tapestry.record.PropertyPersistenceStrategy">
+ 
+ 		Property persistence Strategy for session properties with page scope, similar to "client:page".
+ 
+ 		<invoke-factory>
+ 			<construct class="de.rwth.rz.tapestry.record.PageSessionPropertyPersistenceStrategy">
+ 				<set-object property="applicationId" value="infrastructure:applicationId"/>
+ 				<set-object property="request" value="infrastructure:request"/>
+ 				<set-object property="requestCycle" value="infrastructure:requestCycle"/>
+ 			</construct>
+ 		</invoke-factory>
+ 	</service-point>
+ 
+ 	<contribution configuration-id="tapestry.persist.PersistenceStrategy">
+ 		<strategy name="session:page" object="service:PageSessionPropertyPersistenceStrategy"/>
+ 	</contribution>
+ }}}
+ 

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