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I figure it is going to take me at least four iterations to get this right. The first one will be building a GWT application with a simple UI that has no server logic behind it (just to learn how layout in GWT works). Step two will be adding a fake servlet backend (not app engine, just in memory). While not exactly App Engine yet, I should have a completely specified client-server API by the end of this process that I can subsequently implement on App Engine (iteration 3). Iteration four will handle deployment, CSS and whatever I may screw up in iterations one and two. I will log my notes of things I run into while I code.

GaeVFS ships with a servlet that handles file uploads and abstracts how it writes them to its virtual file system. 

GaeVFS is built on top of Apache Commons VFS, and you use their FileObject instead of the standard File class.  There are some examples on the GaeVFS wiki, but I had to play around for a bit before I figured out some of the differences I needed to know. 

The easiest way to get setup rapidly is to download the template project from the download section. It provides a ready-to-go project with the right configuration files pre-filled and an appropriate directory layout: web.xml preconfigured with the Gaelyk servlets appengine-web.xml with the right settings predefined (static file directive) a sample Groovlet and template the needed JARs (Groovy, Gaelyk and Google App Engine SDK)

Running your application locally Google App Engine provides a local servlet container, powered by Jetty, which lets you run your applications locally. If you're using the Gaelyk template, when you're at the root of your project — and we assume you have installed the App Engine SDK on your machine — you can run your application with the following command-line: dev_appserver.sh war

Deploying your application in the cloud Once you're at the root of your application, simply run the usual deployment command: appcfg.sh update war

you can use the sample project that the google plugin for eclipse create for you. Then you can easily replace the generated java code by groovy / gaelyk code. Note that you need to add the jars in the war/WEB-INF/lib of your project, and add them in the project build path options.

to write a string to a twitter account. Here’s my original java code:

The Blobstore API provides two types of functions:   An ability to upload and save the blob automaticallyThe BlobstoreService which is provided by the com.google.appengine.api.blobstore.BlobstoreService allows us to specify a URL where users can upload their large files. You can think of this url as the action element in the HTML form. The implementation at this URL is internal to the BlobstoreService. But what it does is significant. It will extract out the file contents that you uploaded and store it as a Blob in the database. Each blob that is stored in the database is associated with the a Blob Key. This Blob key is then provided to your url and you can then use the Blob Key to do anything within your application. In our case, we form a url that is tweeted to the users who can then view the picture that we uploaded. An ability to serve or retrieve the blob.The BlobstoreService also provides an ability to serve or retrieve the blob that was saved successfully. It will provide the blob as a response that could then use as a source in an <img> element for example. All you need to do is provide it the Blob Key and the response stream and in return, it will provide the content properly encoded as per its type that you could then use.