I've used the python app engine before for a few small personal projects, and it rocks. Not having to worry, at all, about physical machines, or deployment is a huge win. And, on top of that, they provide some really nice services right out of the box (authentication, memcache, a data store, etc.). So, when they announced the availability of app engine for java, we were all really excited.
Of course, there are some limitations. No Threads, no Sockets (this really sucks), and not all JRE classes.... BUT, it's probably enough for most applications...
And, it just so happens I'm working on a project where all this sounds okay.
They provide a really nice local development environment. Of course, there's not 100% correlation between things working locally and things working remotely. It's to be expected, but can be a pain.
Some things to watch out for:
- crypto problems with the local Mac SDK -- luckily you can work around this by disabling all permission checking, but unfortunately this means that all permission checking will be disabled locally :).
- You can't seem to use PUT or POST locally with the java.net connection services.
- XPathFactory.newInstance() fails.
Connecting to S3
We normally use jets3t for all of our s3 access. It's a great library. However, it's not a great app engine choice because it uses Threads, and Sockets... It seemed like a big task to modify it to work for app engine... I thought using a simpler library as a base would be better.
The author of jets3t has some s3 sample code he published as part of his AWS book. After making some small changes to get over the XPath problem, I just couldn't get it to work. The code worked fine locally, and it would work when I first deployed to app engine, but after that it would fail with IOException: Unknown... Everything looked pretty straightforward... I even tried setting up a fake s3 server to see if there was some weird issues with headers, or what... nothing...
So, I decided to try out some even simpler approaches. After all, it's just a simple REST service, right? That lead me to two different paths that (sort-of) worked.
J2ME and J2SE Toolkit for Amazon S3 -- this is a small SOAP library designed for use in a J2ME environment. This works in app engine! At least for GETs (all I tested). It is very basic and only has support for 1MB objects.
S3Shell in Java -- This is a small REST library designed for use as a shell. There's a small bug (mentioned in the comments), and you need to remove some references to com.sun classes (for Base64 encoding), but otherwise it seems to work pretty well! You will have problems using PUTs locally, but it works fine in production.
Putting it all together
I decided to go with the S3Shell code. I had to make a few changes (as mentioned above), but so far so good. I put up a modified copy of the code on github, or you can download the jar directly. This code should work fine as-is on google app engine. As mentioned, there's an issue with local PUTs (Please vote for it!).
The functionality is pretty basic. If you add any other app-engine supported features (would definitely like to get some meta-data support), definitely let me know.