FreeMarker on Google App Engine
We had several reports of issues in the past with use of FreeMarker on Google App Engine (GAE).
Analysis of these problems confirmed that runtime named "Java" in GAE is not, in fact, a Java-compliant runtime even though it's advertised as such, and looks like one at first sight. It doesn't provide access to some mandatory Java packages, and its reflection implementation has bugs; both of which issues affect FreeMarker.
We wish to stress that FreeMarker is being used in countless Java runtimes, many of them enterprise runtimes with strict security lockdowns, and it works in all of them, as long as these runtimes are conforming Java Runtime Environments. It is our belief that GAE is, unfortunately, non-conforming. Update: I have now reported the reflection bug in the GAE issue tracker.
This unfortunately doesn't help our users who would love to use FreeMarker on GAE. We can not quickly patch FreeMarker as some of the required workarounds would break backwards compatibility within the existing 2.3.x series, which is against our release policy; backwards compatible changes are only allowed when we increment first or second version numbers, that is, from current perspective, either in 2.4 or 3.0.
For this reason, I spent some of my weekend on this particular yak shaving, and have created a new version of FreeMarker's 2.3.16 JAR file that should run on GAE. I'm saying "should" because I don't use GAE myself, so it's up to you, the GAE users to verify it.
If you want to, please go to our project file releases, and grab "freemarker-gae-pre1.jar" from freemarker/2.3.16 directory. The file is labelled "pre1" as it's considered a prerelease; all you people who wish to use FreeMarker on GAE, please start using it and hammer at it, and report back any problems you can find. That said, I sincerely hope there will be none. The changes themselves are fairly light: I removed the dependency on Swing from AST classes, and worked around GAE's reflection security bugs. (If you see a different named file, it means we've updated it since, and please grab that more recent one.)
Update: There is now indeed a "freemarker-gae-pre3.jar"
This doesn't replace our longer term effort of having default FreeMarker be able to run under GAE, but is intended to help our users with an instant solution. There are no downsides to using this version instead of the official 2.3.16 release of FreeMarker, except for the backwards-incompatible change where the TemplateElement class no longer implements Swing TreeNode, but that incompatibility is probably exactly what you want under GAE. All other changes are under the hood - we had to expose few previously package-private implementation classes as being public to work around GAE's reflection security bugs. Since these classes are not documented in FreeMarker official JavaDoc anyway, they don't form the public API, so you shouldn't be using them even when you see them.