Thanks Bryan that looks very useful. I'll let you know how I get on.
> -----Original Message-----
> From: Bryan Lewis [mailto:brya..aine.rr.com]
> Sent: 06 January 2006 17:57
> To: cayenne-use..bjectstyle.org
> Subject: Re: Note to Cayenne/Tapestry users ...
>
> Philip Miller wrote:
>
> >Unfortunately I can only find documentation for v3.0. If anyone has
> >successfully implemented squeezers in Tapestry 4 and can
> explain to a
> >newbie, I'd be delighted to hear.
> >
> I've been upgrading our apps from Tap3 to 4, including a
> DataSqueezer.
> I think I fgiured it out from the source code for the
> built-in DataSqueezers.
>
> I created a new class ModelObjectAdaptor, so named because
> all my DataObjects derive from a ModelObject "wedge" class.
> Could've called it CayenneDataObjectAdaptor, I guess.
>
>
> import org.apache.tapestry.services.DataSqueezer;
> import org.apache.tapestry.util.io.SqueezeAdaptor;
>
> import model.Model;
> import model.ModelObject;
> import model.UserContext;
>
> /**
> * Squeezes a ModelObject, our version of a CayenneDataObject.
> * Allows our page code to deal only with objects, not oids.
> * It uses the prefix 'k', resulting in a squeezed parameter like
> * "kCompany-5885".
> */
> public class ModelObjectAdaptor implements SqueezeAdaptor {
> public String getPrefix()
> {
> return Model.DATASQUEEZER_PREFIX;
> }
>
> public Class getDataClass()
> {
> return ModelObject.class;
> }
>
> public String squeeze(DataSqueezer squeezer, Object object)
> {
> return Model.getDataSqueezerKeyFromObject(object);
> }
>
> public Object unsqueeze(DataSqueezer squeezer, String key)
> {
> // Magically get the current request's DataContext.
> return
> Model.getObjectFromDataSqueezerKey(UserContext.getDataContext(), key);
> }
> }
>
> I have some static methods in a Model utility class for
> historical reasosn. Come to think of it, I could've included
> these methods in the ModelObjectAdaptor code... they're not
> needed anywhere else. They are:
>
>
> public static final String DATASQUEEZER_PREFIX = "k";
> private static final String DATASQUEEZER_SEPARATOR = "-";
>
> /** Returns the key String needed by our custom
> ModelObject DataSqueezer;
> * see Engine.createDataSqueezer(). The string will
> start with a "k"
> * prefix, then the entityName, a hyphen and the oid, e.g.,
> * "kCompany-5885".
> */
> public static String getDataSqueezerKeyFromObject(Object object)
> {
> ModelObject modelObject = (ModelObject) object;
>
> // This works only if the object has a single-column
> primary key. It
> // could probably be generalized to any ObjectId -- see
> // compoundPKForObject() -- but this should satisfy our needs.
> Object pkObject = null;
> try {
> pkObject = DataObjectUtils.pkForObject(modelObject);
> }
> catch (Exception ex) {
> log.error("getDataSqueezerKeyFromObject(): object
> that doesn't have a single-column pk?! " + ex);
> return null;
> }
>
> // There might be a simpler way to do this. Get the
> class name and
> // strip off the "model." prefix.
> String className = modelObject.getClass().getName();
> int index = className.lastIndexOf('.');
> String entityName = className.substring(index + 1);
>
> StringBuffer sb = new StringBuffer();
>
> sb.append(DATASQUEEZER_PREFIX).append(entityName);
> sb.append(DATASQUEEZER_SEPARATOR).append(pkObject.toString());
> return sb.toString();
> }
>
> /** Returns a ModelObject corresponding to the given key. */
> public static ModelObject getObjectFromDataSqueezerKey(DataContext
> dc, String key)
> {
> // A minimum key would be 4 characters long, "kX-1".
> assert key != null && key.length() >= 4;
> assert dc != null;
>
> int index = key.indexOf(DATASQUEEZER_SEPARATOR);
> assert index > 1;
> // We drop the initial 'k' prefix too.
> String entityName = key.substring(1, index);
>
> // So far all our oids are Integers. Some day we
> might need to deal
> // with another type, either by creating another
> DataSqueezer or by
> // asking Cayenne for the entity's pk type and doing
> the right thing.
> Integer oid = new Integer(key.substring(index + 1));
>
> // Call the nice utility method that gets the object
> from the cache.
> return (ModelObject) DataObjectUtils.objectForPK(dc,
> entityName, oid);
> }
>
> The UserContext class is the same trick from Tapestry 3, a
> ThreadLocal to hold the DataContext where anyone can get it.
>
> public final class UserContext
> {
> private static final ThreadLocal dataContextLocal = new
> ThreadLocal();
>
> // Disable construction.
> private UserContext() {}
>
> /** Engine.requestDestroyed() calls this. It prevents
> lingering object
> * references by nullifying all threadLocals.
> */
> public static void reset()
> {
> setDataContext(null);
> setAppName(null);
> }
>
> public static DataContext getDataContext()
> {
> return (DataContext) dataContextLocal.get();
> }
> public static void setDataContext(DataContext dc)
> {
> dataContextLocal.set(dc);
> }
> }
>
> I tell hivemind to add my DataSqueezer to the list with this
> snippet of hivemodule.xml in the same package as the
> ModelObjectAdapter code:.
>
> <?xml version="1.0"?>
> <module id="cview" version="1.0.0">
> <contribution configuration-id="tapestry.data.SqueezeAdaptors">
> <adaptor object="instance:cview.base.ModelObjectAdaptor"/>
> </contribution>
> </module>
>
> The final trick came from Cayenne 1.2's WebApplicationContextProvider.
> You can use that directly if you don't have a custom
> configuration. I have this code in my Engine class:
>
> public static final String DATA_CONTEXT_KEY =
> "cayenne.datacontext";
>
> /** This method is called by the servlet container when a
> session is
> * created. It creates a DataContext and stores it in
> the session for
> * later retrieval by requestInitialized(). This is the
> same approach
> * used by Cayenne 1.2's WebApplicationContextProvider,
> except calling
> * our custom getConfiguration().
> */
> public void sessionCreated(HttpSessionEvent se)
> {
> log.info("sessionCreated, no login yet; creating
> DataContext");
>
> Configuration config = Model.getConfiguration();
> DataContext dataContext =
> config.getDomain().createDataContext();
>
> // Store the DC in the session.
> se.getSession().setAttribute(DATA_CONTEXT_KEY, dataContext);
> }
>
> /** This is called at the beginning of each request; we
> use it as we did
> * the old Tapestry 3 setupForRequest(). Primarily it
> retrieves the
> * DataContext stored earlier in the HttpSession and
> binds it to the
> * current thread.
> */
> public void requestInitialized(ServletRequestEvent sre)
> {
> // Get the DataContext from the session and bind to
> the thread.
> DataContext dc = null;
> ServletRequest req = sre.getServletRequest();
> HttpSession session = null;
> if (req instanceof HttpServletRequest) {
> session = ((HttpServletRequest) req).getSession();
> dc = (DataContext) session.getAttribute(DATA_CONTEXT_KEY);
> }
> UserContext.setDataContext(dc);
> }
>
> In order to get notified about the session and request
> initialization, the Engine implements HttpSessionListener and
> ServletRequestListener.
> Each web-app's web.xml specifies Engine as a listener:
>
> <listener>
> <listener-class>cview.base.Engine</listener-class>
> </listener>
>
> I think that's it. It seems like a bit of plumbing, eh? I
> wouldn't be surprised (or offended :-) if someone pointed out
> how it could be more elegant. It works.
>
>
>
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