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How to make a Ruby program work with a Java program?

This is not a question as such, but more like a list of things I would like to know about how to make a Ruby program work with a Java program.
Here's the thing: We have two teams. One works on Java and implements their code and module in Java. We do the same in Ruby. We are supposed to have a set of common interfaces and use some code of theirs in our Ruby program and they are to use some of our Ruby code in their Java program.
What are some of the things to keep in mind while connecting these two modules to give one common module? I guess this would be more like a guide or a set of guidelines than a straight answer. So please, have some patience and not report or close this question. Thank you!
Update:
Yes, we are to use RESTful services. I do not want any sort of coding help. All I want is your contribution which you might have from your experience with coding modules in two different languages and interconnecting them with a common interface.
That's pretty much all I would like to know. While this is indeed a class project of mine, I thought it would be better if there were guidelines somewhere so it could help other people as well.

In the situation you describe, I would be inclined to use JRuby so your Ruby code can interoperate directly with Java. You could also make the Ruby/Java parts of your system talk over a socket using JSON or something, but there is a performance cost and a complexity cost to doing that. (Whether the performance hit really makes a difference depends on how "fine-grained" the interface between the Ruby/Java parts are -- how many Ruby-to-Java or Java-to-Ruby calls are required to process a single request.)
In JRuby, you can load JAR files and use the (Java) classes defined in them as if they are Ruby classes. You can also go the other way -- you can use JRuby like a "library" from within a Java application to evaluate Ruby code.
You should probably either 1) write the "main" application in Java, and write the Ruby parts as "libraries" which the main application calls into, or 2) write the main application in Ruby, and write the Java parts as libraries. If you have circular dependencies from Java code into Ruby and back from Ruby into Java, it will make your codebase difficult to understand.

This is a huge problem, so I am not going to give specifics as to how this is done. I can give you an idea how to go about doing this. Assuming you are asking about web application ...
Java-based web application on the database side
Java-based web application doesn't really have a "view" for client. It won't generate conventional css, html view.
Instead it created JSON objects based on URL
Rails uses JSON objects as if it is data coming from database. It can create models out of them.
Basically, Rails is making an API call to Java based web app every time it needs to get to database.
Big downside of this is that you no longer can use ActiveModel, ActiveRecords etc, all the goodies that come with Rails
Upside? Potentially faster for database queries that are complicated
This is basically what companies like Twitter and Amazon do when they say they use Java on the backend while they use Rails on the frontend. But again, this is a huge endeavor, and you definitely want to think about cost and benefit of this.

An apt database (and/or) framework suggestion needed

Well, I've taken help of Google, Stackoverflow and whatever else I could find, did as much as I could, but it seems that I am unable to find out an exact answer! I have multiple queries, and I would love to have answers from the database-people as well as from the programmers and framework users.
From the programming languages, I know C/C++, Java and Python. I have undertaken a CMS project that would require frequent C's & R's of the CRUD. The project would have 50k users atleast. The head-to-toe of the project has been all figured out, and now I need to code it and make it live online.
Well, I want to use Neo4j as my database as its data representation model (nodes and relationships) is closest to the real project model. Now, neo4j has bindings for various languages, and one of them is Python (whose python bindings are very oldish, the jpype hasn't been updated since ages). I am thinking of going for some Java based framework, but then I leave this idea as I personally haven't heard much of java frameworks. But one of my partner tells me to go for Zend (PHP) as it has some kind of functionality that lets us execute Java code. Won't this slow the code? I mean executing one language's code in another language...
So, it all comes to this:
1) Database: I would want to go for Neo4j. But does it goes well off when the scalability factor kicks in? (From what I could gather from google, there are no scalability issues).
2) What framework to use in case of Neo4j? I would require a framework that is able to handle tonnes of requests and large data as the users of the project would be Creating and Reading data a lot.
P.S.: I know it is a long question, but couldn't jot it down in lesser words!

I can't speak about the scalability or suitability of Neo4J for your particular project.
However, I'd strongly advise you against trying to mix and match languages like Java and PHP. It's so much easier to stick to the best one for your particular task. I'd also strongly advise you against using JNI for anything unless you have no other option. Java is fast enough that you should almost never need JNI for performance.
That said, it's OK to run Neo4j in its "full server" mode and then have your PHP or Python application access it using some driver over the network. I just wouldn't recommend making an ugly hybrid of PHP and Java at your application layer.
Some decent Java frameworks you could check out include:
Spring
Google Guice with Sitebricks
Apache Struts 2
They're pretty standard in the industry and there are tons of good resources available on all of them.

In regards to the mini-question about language interoperability, Java provides the JNI interface, which allows the JVM and user code to make calls into other languages and vice versa. When the native code (e.g. C code called by Java, or Java called from C) runs, it is actually running in its natural environment, so there's no performance loss in terms of actual execution.

Neo4j as a standalone server has also REST API: http://docs.neo4j.org/chunked/milestone/rest-api.html, if you can embedded your requests in single REST queries, there is no need to use native embedded neo4j. If there is no need to use the embedded neo4j, you can take any language of your choice.
Regarding the scalability, recently neo4j can be used on Azure, so it must be quite easy to scale. To learn more how to scale neo4j, go to this page on neo4j.org.
UPDATE: in the newest version of Neo4j, there is added the support for a new query language - http://blog.neo4j.org/2011/06/kiruna-stol-14-milestone-4.html.

Grails or Play! for an ex-RoR developer?

I plan to start learning a Java web framework (I love the Java API) I have already used Rails and Django.
I want something close to Java but without all the complexity of J2EE.
I've found 2 frameworks that could be good for me:
Grails
Grails looks great, it uses Groovy which is better than Java for web application (I think..) but it's slower than pure-java based frameworks (Hibernate, Strut, Spring) It looks pretty simple to deploy (send .war and it's ok!), the GSP is great! It's a bit harder to debug (need to restart the server at each modification and stacktraces contain a mix of Java and Groovy traces which is not always the easiest to understand)
Play!
This framework also looks great; it's faster than Grails (It uses Java) but I don't really like how it uses Java, it modifies the source code to transform the property calls as setXXX/getXXX, I do not like that... The framework also has a caching function that Grails does not have. I don't really like the Template Engine.
It's also easer to debug (no need to restart the server, stacktraces are clearer)
What do you recommend?
I am looking for something easy to learn (I have a lot of Ruby experience, not so much Java experience but I love the Java API), fully featured (That's no a problem with all the Java Library available, but if it's bundle and integrated I prefer), has good scalability and is not too slow (faster than Ruby) Ideally I would like to use a framework with a decent community to easily find support.
PS: I am not interested in JRuby on Rails

I switched from Grails to Play and I never looked back. My biggest problem with Grails was overall robustness and developer usability. Most of the time I got bitten by the fact that Grails glues together the usual stack of Spring MVC and Hibernate while trying to hide this fact and giving you a Rails-like API (personal opinion of mine). The problem with this is, once something goes beyond the trivial samples, it easily broke and didnt work for me. Developing with it was like walking on eggs (for me). Whenever I googled for documentation of a feature I needed, I was not redirected to samples, tutorials, blogs, but to the Grails JIRA explaining me why the feature wouldnt work for my use case and that the bug was unresolved since two versions before the one I was using.
While that may not be the overall experience for every developer (I am not writing this to bash Grails, but to give my experiences with it here), I needed something that helped me and would not stand in my way or break down on me when I needed it the most. Thats when I found Play and I have quickly migrated my app to it after I found out about it (around the ~1.0 release).
So far it has been a great ride and for the first time in my web development career, I have stopped looking at other frameworks trying to find something that I would like better.
If I had to close with one thing that Play did better than Grails - at least for me - it would be the fact the Play is built from the ground up with developer usability in mind. It does not sacrifice ease of use for enterprise buzzwords. It has the guts to throw away what does not fit into this paradigm (e.g. ditchting Servlet-based runtimes during development for faster turnaround). It is willing to make compromises in order to guarantee awesomeness. And that is something I have only seen in communities like Rails or Django before I found Play.

I'd suggest Grails. It has a bigger community than the play framework does (~350 plugins covering pretty much every basic need). Also, grails is written almost completely in Java, it just lets you use Groovy for your domain specific implementation.
If you do run into a performance issue where the groovy pages that you've created are the bottleneck, you can always just switch to a Java implementation. Then you're in the same boat that you would have been with the Play framework all the time. You've optimized your development time by putting off the coding of things in Java till you know that you actually need to do it (which, in my experience is very rare).
I'm also not sure where you heard that you need to restart your server for each modification, but that's actually not true. Grails supports reloading of controllers/gsps/services/domain objects, etc without restarting your server.
The mixed stacktraces can get a little long, but tool vendors (like Intellij) have made some recent improvements that strip out all the stacktrace portions that you don't care about.
I've been using grails since the .5 days and have been very happy with the platform.

Take note that the Play! framework now supports using Scala as of 1.1

From my experience with Play it's a great framework. My favorite features are the cool controller system and the template system - both are simple but feature-rich and powerful.
However the most important benefit of Play is definitely the rapid development cycle, where virtually no reloading is needed on code changes. But if you're not careful, this greatness won't last much, and slowness will eventually creep into your code.
Why is that?
With Play there is common use of some plugins with pretty heavy initialization, notably EJB (Hibernate) and Spring. The initialization of these plugins is re-run on every code change before the new code is loaded. As a result of this, as your model and your system configuration grow, this heavy initialization starts to seriously slow down your development. In the system I used 20 seconds were a typical startup time on a virtual machine running on a kickass laptop.
What you can do to avoid this depends on your application, e.g. if you're building a NoSQL application then then EJB plugin should not give you trouble. Spring can be replaced with a custom hard-coded Java plugin, which IMHO is also easier to maintain, or run a Groovy script if scriptability is that important. In any case, watch out for these problems and kill them while the're young - and be sure not to be running your own bulky initializations on every refresh.

If you have used Ruby and Python before, you will probably enjoy Grails better than Play. It very hard to get back to Java once you are used to these dynamic languages.

There is also Lift on Scala.
Imho scala is the best static typed language and lift is a pretty nice framework (for a static typed language).

Adventures of a Web Programmer in Applicationland (or, Practical Java Help Needed)

Alright, so I am a compsci college student who, being in college, has not branched out towards a certain specialization yet. I have been programming since I was a young teenager, certainly know my stuff - well versed in about eight different languages as well as compsci theory, etc. In addition, I have about four years of web programming (PHP mainly) behind me, having started freelance work in that area since web 2.0 became hot.
My summer job now as an intern of sorts is to write an application for an industrial, not software-related startup. This application will be used to manage production lines and logistics flow. I have chosen Java for my language because I don't want to shoot myself in the foot.
I am well-versed in the syntax of Java, in its data structures, language theory, and such, but I have absolutely no idea where to start. I can picture the program perfectly in my mind, I understand the problem clearly and got the solution's theory nailed. Namely, I have no idea what libraries to use, and am scared that they won't be well documented.
Here are some general outlines of what I'm going to make:
Two applications, one server and one
client (of which there will be many
copies).
The server and clients obviously will
communicate via (I don't know).
Both the server and the client
software will have GUIs.
The server software will have to
query a MySQL database.
The client software must be 'live' in
the sense that the GUI updates when a
change is made to the database. This
is one of the reasons why it can't be
a web application.
I'm not even sure if a framework is right for me or not. I've used MVC tons of times in my web freelance work, but I dont know how that will translate for desktop applications.
In short, I'm looking for the right libraries for the job, as well as advice on whether or not I should use a framework (and if so, which). Thanks.

This is a summer intern job? To be honest, this sounds more like a major project if you ask me. You say the start-up is not software related? Who came up with this idea? Do they have any idea of the (huge) scope that something like this might actually entail?
The business of software development is something quite different to language syntax and libraries. It's about requirements gathering, defining a spec, writing code, ensuring quality of that code, having it tested and so on. These are not things an intern should reasonably be expected to pick up. For something like this you should be under more experienced supervision, someone you can learn from, someone who has done this before.
That being said, unless there's a really good reason, I would probably do such a thing as a Website rather than a desktop app. Desktop apps are a lot more complicated in many ways. You need to code both a client and a server. Communication is a bit trickier. You have to worry about the issue of maintaining state in multiple applications, how to handle updates being pushed around and so on.
In short, it's a big job. Even a Web site is a big job but a lot of these issues go away. You could do this with Java. I've certainly coded my fair share of Java Websites but PHP might be a far simpler bet.
Also, desktop development on Java is, well, torture. Swing is (imho) tried and true but also incredibly painful to develop in. Other desktop libraries (eg Netbeans RCP, Eclipse SWT) are more modern but have other idiosyncrasies.
Desktop remoting libraries include things like Spring remoting, even Web services and other things like Burlap. For the server side, I'd be using either Tomcat or an application server (Glassfish is my preferred choice), servlets and Spring. Persistence can be done via Hibernate or Ibatis (or lots of other options).
But honestly, the desktop option is so much more complex than a Web-based one. You'd probably get a lot more done faster using PHP + jQuery + MySQL.
If you are doing this keep it as absolutely simple as possible. Try to define the absolute minimum you need to initially deliver and do that. Once someone has that they'll then have a better idea of what works for them and what doesn't. Basically it's easier to refine something that already exists vs define something that doesn't.

I recommend that you only build a web application. A web application can be 'live' in the sense you are describing it by using AJAX. It would be much easier to build just one thing. If you also want to have a rich client, then you need to build the UI in a technology you are not familiar with (like Swing or SWT) and design/implement the communication mechanism.
Have a look at Hibernate (ORM tool) and Spring (IoC framework). They have a somehow steep learning curve, but they will make your life easier at the long run. For the UI part perhaps JSF is easier for a beginner.
As a last note, I think you have an over-ambitious plan. What you are describing is not an easy project and requires expertise with a lot of technologies. Do not try to do everything in one shot.

Java Desktop 6 (JRE)
JDBC (built-in in any JRE)
MySQL JDBC drivers (freely downloadable)
for communication you have several choices: RMI (built-in) however this days I recommend
learning something like Java Web Services (JAX-RS)

Libraries?
JDBC for the database. You may want to look at ORM mechanisms like Hibernate
I would recommend the Apache Commons libraries for all your utility work (handling files, IO etc.). There's a lot of stuff there to save you reinventing the wheel.
A standard logging framework like Log4j will allow you to log in lots of different ways, filter your logs and plug into monitoring solutions easily.
You don't say whether browser-based solutions are acceptable for the client/server GUI, and that decision will drive a lot of the further architecture.
If you're looking at browser-based solutions, then I would advise a grounding in servlets regardless of any framework you ultimately choose (no doubt a lot will be recommended here).
By this stage it's getting to be a major project. Perhaps you need to concentrate on getting the fundamentals (client/server functionality) working, and worry about the GUI later. Otherwise it's a huge amount of work (and GUI work can draw an enormous amount of time).

Just one nitpicking:
Both the server and the client software will have GUIs.
I advice you to have a headless (in awt parlance) server, with an administration GUI, not a GUI-server.

Well this can go as wild as you can think of or you can go and do KISS.
If you would like something that is really simple (as in not using any frameworks):
* In the server side you can use RMI. This server side will use plain JDBC to connect to your MySQL database. But some said that this is kind of old, so if you want to get funky you can try JAX-RS which can return a JSON objects/XML to your client.
* Your client can be made using Swing (assuming you are developing desktop) or Servlet + JSP (assuming you are developing webapp) and connect to your server by calling the RMI objects/JSON objects/XML that is exposed by the server.
If you would like to get nasty which will help you in terms of code maintainability you might want to plug-in Spring + Hibernate into this application.
Good luck!

Choosing Java vs Python on Google App Engine

Currently Google App Engine supports both Python & Java. Java support is less mature. However, Java seems to have a longer list of libraries and especially support for Java bytecode regardless of the languages used to write that code. Which language will give better performance and more power? Please advise. Thank you!
Edit:
http://groups.google.com/group/google-appengine-java/web/will-it-play-in-app-engine?pli=1
Edit:
By "power" I mean better expandability and inclusion of available libraries outside the framework. Python allows only pure Python libraries, though.

I'm biased (being a Python expert but pretty rusty in Java) but I think the Python runtime of GAE is currently more advanced and better developed than the Java runtime -- the former has had one extra year to develop and mature, after all.
How things will proceed going forward is of course hard to predict -- demand is probably stronger on the Java side (especially since it's not just about Java, but other languages perched on top of the JVM too, so it's THE way to run e.g. PHP or Ruby code on App Engine); the Python App Engine team however does have the advantage of having on board Guido van Rossum, the inventor of Python and an amazingly strong engineer.
In terms of flexibility, the Java engine, as already mentioned, does offer the possibility of running JVM bytecode made by different languages, not just Java -- if you're in a multi-language shop that's a pretty large positive. Vice versa, if you loathe Javascript but must execute some code in the user's browser, Java's GWT (generating the Javascript for you from your Java-level coding) is far richer and more advanced than Python-side alternatives (in practice, if you choose Python, you'll be writing some JS yourself for this purpose, while if you choose Java GWT is a usable alternative if you loathe writing JS).
In terms of libraries it's pretty much a wash -- the JVM is restricted enough (no threads, no custom class loaders, no JNI, no relational DB) to hamper the simple reuse of existing Java libraries as much, or more, than existing Python libraries are similarly hampered by the similar restrictions on the Python runtime.
In terms of performance, I think it's a wash, though you should benchmark on tasks of your own -- don't rely on the performance of highly optimized JIT-based JVM implementations discounting their large startup times and memory footprints, because the app engine environment is very different (startup costs will be paid often, as instances of your app are started, stopped, moved to different hosts, etc, all trasparently to you -- such events are typically much cheaper with Python runtime environments than with JVMs).
The XPath/XSLT situation (to be euphemistic...) is not exactly perfect on either side, sigh, though I think it may be a tad less bad in the JVM (where, apparently, substantial subsets of Saxon can be made to run, with some care). I think it's worth opening issues on the Appengine Issues page with XPath and XSLT in their titles -- right now there are only issues asking for specific libraries, and that's myopic: I don't really care HOW a good XPath/XSLT is implemented, for Python and/or for Java, as long as I get to use it. (Specific libraries may ease migration of existing code, but that's less important than being able to perform such tasks as "rapidly apply XSLT transformation" in SOME way!-). I know I'd star such an issue if well phrased (especially in a language-independent way).
Last but not least: remember that you can have different version of your app (using the same datastore) some of which are implemented with the Python runtime, some with the Java runtime, and you can access versions that differ from the "default/active" one with explicit URLs. So you could have both Python and Java code (in different versions of your app) use and modify the same data store, granting you even more flexibility (though only one will have the "nice" URL such as foobar.appspot.com -- which is probably important only for access by interactive users on browsers, I imagine;-).

Watch this app for changes in Python and Java performance:
http://gaejava.appspot.com/
(edit: apologies, link is broken now. But following para still applied when I saw it running last)
Currently, Python and using the low-level API in Java are faster than JDO on Java, for this simple test. At least if the underlying engine changes, that app should reflect performance changes.

Based on experience with running these VMs on other platforms, I'd say that you'll probably get more raw performance out of Java than Python. Don't underestimate Python's selling points, however: The Python language is much more productive in terms of lines of code - the general agreement is that Python requires a third of the code of an equivalent Java program, while remaining as or more readable. This benefit is multiplied by the ability to run code immediately without an explicit compile step.
With regards to available libraries, you'll find that much of the extensive Python runtime library works out of the box (as does Java's). The popular Django Web framework (http://www.djangoproject.com/) is also supported on AppEngine.
With regards to 'power', it's difficult to know what you mean, but Python is used in many different domains, especially the Web: YouTube is written in Python, as is Sourceforge (as of last week).

June 2013: This video is a very good answer by a google engineer:
http://www.youtube.com/watch?v=tLriM2krw2E
TLDR; is:
Pick the language that you and your team is most productive with
If you want to build something for production: Java or Python (not Go)
If you have a big team and a complex code base: Java (because of static code analysis and refactoring)
Small teams that iterate quickly: Python (although Java is also okay)

An important question to consider in deciding between Python and Java is how you will use the datastore in each language (and most other angles to the original question have already been covered quite well in this topic).
For Java, the standard method is to use JDO or JPA. These are great for portability but are not very well suited to the datastore.
A low-level API is available but this is too low level for day-to-day use - it is more suitable for building 3rd party libraries.
For Python there is an API designed specifically to provide applications with easy but powerful access to the datastore. It is great except that it is not portable so it locks you into GAE.
Fortunately, there are solutions being developed for the weaknesses listed for both languages.
For Java, the low-level API is being used to develop persistence libraries that are much better suited to the datastore then JDO/JPA (IMO). Examples include the Siena project, and Objectify.
I've recently started using Objectify and am finding it to be very easy to use and well suited to the datastore, and its growing popularity has translated into good support. For example, Objectify is officially supported by Google's new Cloud Endpoints service. On the other hand, Objectify only works with the datastore, while Siena is 'inspired' by the datastore but is designed to work with a variety of both SQL databases and NoSQL datastores.
For Python, there are efforts being made to allow the use of the Python GAE datastore API off of the GAE. One example is the SQLite backend that Google released for use with the SDK, but I doubt they intend this to grow into something production ready. The TyphoonAE project probably has more potential, but I don't think it is production ready yet either (correct me if I am wrong).
If anyone has experience with any of these alternatives or knows of others, please add them in a comment. Personally, I really like the GAE datastore - I find it to be a considerable improvement over the AWS SimpleDB - so I wish for the success of these efforts to alleviate some of the issues in using it.

I'm strongly recommending Java for GAE and here's why:
Performance: Java is potentially faster then Python.
Python development is under pressure of a lack of third-party libraries. For example, there is no XSLT for Python/GAE at all. Almost all Python libraries are C bindings (and those are unsupported by GAE).
Memcache API: Java SDK have more interesting abilities than Python SDK.
Datastore API: JDO is very slow, but native Java datastore API is very fast and easy.
I'm using Java/GAE in development right now.

As you've identified, using a JVM doesn't restrict you to using the Java language. A list of JVM languages and links can be found here. However, the Google App Engine does restrict the set of classes you can use from the normal Java SE set, and you will want to investigate if any of these implementations can be used on the app engine.
EDIT: I see you've found such a list
I can't comment on the performance of Python. However, the JVM is a very powerful platform performance-wise, given its ability to dynamically compile and optimise code during the run time.
Ultimately performance will depend on what your application does, and how you code it. In the absence of further info, I think it's not possible to give any more pointers in this area.

I've been amazed at how clean, straightforward, and problem free the Python/Django SDK is. However I started running into situations where I needed to start doing more JavaScript and thought I might want to take advantage of the GWT and other Java utilities. I've gotten just half way through the GAE Java tutorial, and have had one problem after another: Eclipse configuration issues, JRE versionitis, the mind-numbing complexity of Java, and a confusing and possibly broken tutorial. Checking out this site and others linked from here clinched it for me. I'm going back to Python, and I'll look into Pyjamas to help with my JavaScript challenges.

I'm a little late to the conversation, but here are my two cents. I really had a hard time choosing between Python and Java, since I am well versed in both languages. As we all know, there are advantages and disadvantages for both, and you have to take in account your requirements and the frameworks that work best for your project.
As I usually do in this type of dilemmas, I look for numbers to support my decision. I decided to go with Python for many reasons, but in my case, there was one plot that was the tipping point. If you search "Google App Engine" in GitHub as of September 2014, you will find the following figure:
There could be many biases in these numbers, but overall, there are three times more GAE Python repositories than GAE Java repositories. Not only that, but if you list the projects by the "number of stars" you will see that a majority of the Python projects appear at the top (you have to take in account that Python has been around longer). To me, this makes a strong case for Python because I take in account community adoption & support, documentation, and availability of open-source projects.

It's a good question, and I think many of the responses have given good view points of pros and cons on both sides of the fence. I've tried both Python and JVM-based AppEngine (in my case I was using Gaelyk which is a Groovy application framework built for AppEngine). When it comes to performance on the platform, one thing I hadn't considered until it was staring me in the face is the implication of "Loading Requests" that occur on the Java side of the fence. When using Groovy these loading requests are a killer.
I put a post together on the topic (http://distractable.net/coding/google-appengine-java-vs-python-performance-comparison/) and I'm hoping to find a way of working around the problem, but if not I think I'll be going back to a Python + Django combination until cold starting java requests has less of an impact.

Based on how much I hear Java people complain about AppEngine compared to Python users, I would say Python is much less stressful to use.

There's also project Unladen Swallow, which is apparently Google-funded if not Google-owned. They're trying to implement a LLVM-based backend for Python 2.6.1 bytecode, so they can use a JIT and various nice native code/GC/multi-core optimisations. (Nice quote: "We aspire to do no original work, instead using as much of the last 30 years of research as possible.") They're looking for a 5x speed-up to CPython.
Of course this doesn't answer your immediate question, but points towards a "closing of the gap" (if any) in the future (hopefully).

The beauty of python nowdays is how well it communicates with other languages. For instance you can have both python and java on the same table with Jython. Of course jython even though it fully supports java libraries it does not support fully python libraries. But its an ideal solution if you want to mess with Java Libraries. It even allows you to mix it with Java code with no extra coding.
But even python itself has made some steps forwared. See ctypes for example, near C speed , direct accees to C libraries all of this without leaving the comfort of python coding. Cython goes one step further , allowing to mix c code with python code with ease, or even if you dont want to mess with c or c++ , you can still code in python but use statically type variables making your python programms as fast as C apps. Cython is both used and supported by google by the way.
Yesterday I even found tools for python to inline C or even Assembly (see CorePy) , you cant get any more powerful than that.
Python is surely a very mature language, not only standing on itself , but able to coooperate with any other language with easy. I think that is what makes python an ideal solution even in a very advanced and demanding scenarios.
With python you can have acess to C/C++ ,Java , .NET and many other libraries with almost zero additional coding giving you also a language that minimises, simplifies and beautifies coding. Its a very tempting language.

Gone with Python even though GWT seems a perfect match for the kind of an app I'm developing. JPA is pretty messed up on GAE (e.g. no #Embeddable and other obscure non-documented limitations). Having spent a week, I can tell that Java just doesn't feel right on GAE at the moment.

One think to take into account are the frameworks you intend yo use. Not all frameworks on Java side are well suited for applications running on App Engine, which is somewhat different than traditional Java app servers.
One thing to consider is the application startup time. With traditional Java web apps you don't really need to think about this. The application starts and then it just runs. Doesn't really matter if the startup takes 5 seconds or couple of minutes. With App Engine you might end up in a situation where the application is only started when a request comes in. This means the user is waiting while your application boots up. New GAE features like reserved instances help here, but check first.
Another thing are the different limitations GAE psoes on Java. Not all frameworks are happy with the limitations on what classes you can use or the fact that threads are not allowed or that you can't access local filesystem. These issues are probably easy to find out by just googling about GAE compatibility.
I've also seen some people complaining about issues with session size on modern UI frameworks (Wicket, namely). In general these frameworks tend to do certain trade-offs in order to make development fun, fast and easy. Sometimes this may lead to conflicts with the App Engine limitations.
I initially started developing working on GAE with Java, but then switched to Python because of these reasons. My personal feeling is that Python is a better choice for App Engine development. I think Java is more "at home" for example on Amazon's Elastic Beanstalk.
BUT with App Engine things are changing very rapidly. GAE is changing itself and as it becomes more popular, the frameworks are also changing to work around its limitations.