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Best dynamic language to pair with Java on a Java project

What is the best dynamic language to pair with Java on a large Java project?
We are considering using a dynamic language for tests, controllers, services. Some options are Groovy, JRuby or Jython. What are the pros and cons of each for this? Ideally we'd be able to call Java from the dynamic language as well as call the dynamic language from Java.
EDIT: If it helps, we're using Hibernate with PicoContainer and Webwork.
Thanks,
Alex

There are really three dynamic languages that offer a very seamless interop with Java - scala, groovy and clojure. From there, I'd ask your team which language they would rather work in or have them try a prototype in each language and see what they think.
If the team efficiency isn't important in the beginning, look to what problem each language attempts to solve:
Groovy is going to be very loose but natural to experienced Java developers and allows fast prototype development due to it's duck typing.
Scala is going to enable you to write DSLs making it a good for frameworks and tools where you want to solve the problem in a language more akin to how you would describe the problem.
Clojure is going to impose lisp's functional programming and immutable state concepts and could be a very natural fit for problems in AI, natural language processing, etc.
Finally, I've gone down the path of looking for the perfect language to base projects on and have found there is no perfect language. All of the languages I've mentioned above compile to native JVM byte code and are quite solid. Sometimes you just need to pick a language that might not be as cool as the others but gets you on the way to solving your problem.

I recommend Groovy, principally because it interoperates seamlessly with Java, and is almost a superset of Java, so very easy for a Java developer to learn. I have absolutely no evidence to support this, but based on hearsay, guesswork, and personal experience, I suspect the Groovy community is much larger than that of either JRuby or Jython.
Incidentally, Groovy++ is way too immature to consider for production use, in my opinion.

The answer is, of course, going to depend somewhat on matters of taste and flexibility. If there are folk who don't have experience with Ruby or Python then Groovy is going to have a syntax much closer to Java (in fact it is a superset of Java), and consequently be a much easier sell.
I can't really speak to JRuby as I haven't used it.
Groovy gives you probably the easiest interop with Java of the three you listed. It also has a very nice BDD library in EasyB which I like a lot. On the negative side I don't think the features or syntax of Groovy really hang together very well. It can kind of feel like a whole bunch of separate extensions to Java.
Jython is of course Python so the syntax is different, but also has all the consistency of Python. Interop is very good at the script level but at least used to be a bit awkward if you wanted to write pre-compiled classes in Jython that you call from Java. The other main pro over Groovy for me is that is that you have a real REPL to interact with the Java project.
I would also mention Clojure, the syntax is even more different but the Java interop is excellent, probably the best of all, and again you have a REPL. On the down-side if folk have trouble adjusting to Ruby or Python syntax then a Lisp is probably right out.

Clojure is probably the best dynamic language for controllers and services. (depending on what you mean with "services".
Scala and Groovy++ has the best java interop, but those are not dynamic (well in Groovy++ you decide for yourself what is typed.). Scala has the look and feel of a dynamic language. Scala has good testing frameworks http://www.scalatest.org/ and Specs and AKKA is very mature for services and also has Java APIs

I'd suggest going with Jython. The syntax is clean, and you get whatever additional power/conveniences that Python gives you.
For example, if you were to go with Groovy, you are basically limited to only what Java will give you. Jython would add the powers of Python to that as well.
If it helps any, I've used Jython with Hibernate, SOAP, Corba, and EJBs and it is much easier than doing the same with just plain Java.

Knowing the fundamentals of Java what is the right approach to learn Groovy?

As my question already implies I want to learn a new language and have read several articles about groovy and its more pragmatic syntax. SO I have choosen Groovy as the language of my choice.
What is a good way to learn a new language like Groovy when I already know the fundamentals of Java. When I understand correctly Groovy will be running in the Java Virtual Machine and allows me to always rely on what I know from Java when I don't know how to solve the problem in Groovy.
I am looking for hints on how to organize a learning track to learn this language. I found that page explaining the differences: http://groovy.codehaus.org/Differences+from+Java But what I am looking for is more a tutorial where I can get through and get introduced to the differences.

There are a couple of articles which have been written with that specific goal in mind: "From Java to Groovy in a few easy steps", part 1 and part 2. They were written by Gillaume Laforge, one of the masterminds behind Groovy. The articles are well written, easy to follow and provide a nice introduction for Java developers. Also, there is the Beginners tutorial in the official Groovy documentation.
Once comfortable with the basics, it's easy to move on to the on-line Groovy task-specific documentation.

Personally I find that referencing an old language that I know to learn a new language is not a good idea. The reason I say this is that in a new language you want to learn to express and use data structures, idioms, style, etc in THAT language. You want to Groovy developers to say that the code is written by a Groovy developer rather than a Java developer.
An example, using Groovy and Java, is that Java developers tend to use a loop for collections, but in Groovy tend to use closure. They may look the same, but they are not. In Java we are applying to data to the code, whereas in Groovy we are applying the code to data. Learning this subtle difference is important to understanding the rest of Groovy API and will help you to grasp that concept.
Also I think that this approach of learning a new language referencing and old might not work that well with other styles of programming like functional.

You might also take a look at this article by James Strachan, the author of Groovy:
http://macstrac.blogspot.com/2009/04/scala-as-long-term-replacement-for.html

Which is more similar to AS3, Java or C++?

I am ActionScript 3/Flex programmer, it is the first language I learned.
I want to learn either Java or C++.
Would one of these be easier to learn based on my current knowledge?

It really depends what you want to do. C++ is more powerful and fast. But Java has a smaller learning curve.
I'd say learn C++, only because it will require you to gain a better understanding of how computers work under the hood. It will also help position you to learn Java, C#, or any other language down the road.

Java seems more likely to be directly relatable to your work in AS3; C++ is better for giving you a grounding in a different technology (pointer-style OO rather than object-style. C++ may feel eerily similar yet different).
If you are doing C-ish C++, the pointer language learning process can be very informative as to how OS-level calls are written...
The places where you will reach for one tool or the other are very different, and the use you have for the tool may be more important in deciding than their relative merits as languages - employability IS a use for a tool, after all.

Well, AS3 more closely resembles JavaScript - they both follow the ECMAScript specification.
But to answer your question, I would say learning Java will be more beneficial and easier for you. Java supports Interfaces, and single inheritance, like Flex, whereas C++ supports multiple inheritance and lacks a formal notion of Interfaces. Java and Flex both manage memory for you, whereas C++ forces you to manage your own memory. Both Java and Flex have large helper libraries out of the box, and both have native String types, whereas C++ forces you to find a library to use and doesn't support native String types.
This may be my own bias, but it also seems more likely that you'll find a job that is looking for a Flex/Java developer, rather than a Flex/C++ developer. Java and Flex seem to work well together, with Flex as a front end and Java as a middle and back end.

I would say Java is more similar to AS3 than C++. You will find lots of familiar metaphors and mechanisms.
C++ on the other hand, will require more effort to learn. It is closer to the machine and demands an undertanding of lower level mechanisms. For instance, there is no garbage collection so you have to manage your memory resources yourself.
Which is better to know depends on what you plan to do with the language. C++ is good at performance critical applications (games, all kinds of real time simulations etc.). Java makes it easier and safer to build things, but at the cost of performance.
hope this helps
regards

C++ is, generally, harder to learn than Java. You will find this true pretty much no matter what your previous experience. Therefore, if you want to learn the easier, learn Java.
(This is partly a matter of design philosophy. C++ was designed to be mostly upward compatible from C, at least the C at the time C++ was being designed. It was also designed
to be useful and permissive, with ease of learning being secondary. Java was designed partly as a reaction to C++, as a generally safer and easier language. C++ is more expressive than Java, but this comes at a price: it's harder to learn C++, and easier to screw up with it.)
That being said, you never said why you wanted to learn another language. You might well be better off learning C++, even though it is more difficult.

Whatever you'll finally decide to learn, do yourself a favor and read either Thinking in Java or Thinking in C++.
Both books are available - for free! - at Bruce Eckels website. They are very, very good.
C++ is more complex than Java so I'd suggest to take a look at Java, first.
However, this isn't meant as C++ bashing. Both are great language, it just depends on what you'd like to do in the end.

I would go with Java. It will be easier to learn and, given your background, I'm guessing you do more web/internet work in which case you'll probably find more ways to apply the Java knowledge then you would C++.

I think you would find Java more similar.
Learning C++ will likely require you to learn concepts that you could avoid with other high level languages (such Java and AS3) e.g. manual memory management, pointers, non forward looking compiler, multiple inheritance, etc.

How is Java an 'imperative' programming language and not a 'declarative' one?

Specially in comparison to C/C++ (which are declarative), how is Java imperative?

C/C++ is imperative too.
Edit: Imperative is do this, then do that, then do the next thing and so on. Declarative is, this are the rules, now what's the answer to this question. Google, you'll find plenty of info.

You are confusing the concepts, C and C++ are not declarative languages. Refer to Declarative Programming and Imperative Programming. Basically, with declarative languages (e.g. Prolog), you specifiy what you want to accomplish, without specifying how to accomplish it, which contrasts with imperative languages.

For future readers, although both Java and C/C++ are usually written imperatively,
Both languages support writing in a more declarative way for example by applying functional principles.
In the past years it has been more and more prominent and is encouraged by many influential software engineers such as Uncle Bob to go to a more functional (and thus declarative) approach.
In Java this was made much more easy with Java 8 that introduced Lambda, Streams etc.
I read this book about functional programming in Java and found it useful:
https://pragprog.com/book/vsjava8/functional-programming-in-java

Best approach for learning Java after C++?

I've been using C++ for about 6 or 7 years now, and I consider myself fluent in it. I've never bothered with Java until now, but I find myself out of the job (company went under) and I need to expand my skill set. Someone recommended Java, so I am wondering if there is any advice for where somebody like me might start. I am also interested to know what the key aspects of Java are that are most likely to come up in an interview.

There are some popular areas that I think of when we talk about Java
Concepts of OOP: I'm sure this will not be much different from C++:
Class, Abstract Class, Interface,
Polymorphism, Overriding,
Overloading, Inheritance, Static
member, ... Interface will likely be
area that you might want to focus.
Since this is not directly available
in C++, I don't know.
Core Java: Such as the basic syntax and the common classes such
as Math, String, System.out,
Scanner, Basic file I/O, Stream.
Know the concept of Garbage
Collection, Reference Type (since
pointers are not available here.)
Know the Java platform/technologies,
J2SE, J2EE. Basic GUI with Swing and
its layout managers. Web
Applications with Servlet/JSP.
Popular tools, frameworks, and libraries: This may not as
important as above bullets. But
maybe you should know what Eclipse,
Netbeans, Spring, Struts, Hibernate,
EJB, Ant, JUnit, JavaDoc, are for.
You can always search for "C++ to Java" in Google. I'm sure there will plenty of good start points. Don't forget not to put * in front of var name next time you code Java :)

I recommend you read the book Effective Java cover to cover. Not only will you learn lots of good programming practices, but you will also learn more about Java than you otherwise would. Highly recommended. :-)
As a simple example, in Java, rather than rolling bitfields by hand (which is rather painful to do in Java, and for good reason), you use an EnumSet instead. This is documented in Item 32.
BTW, if you don't mind reading a PDF (no DRM) version, it's cheaper than hardcopy too: http://www.informit.com/title/0132345285

There are a number of great "in depth" Java books that will teach you not only Java, but also best programming practices.
If you're just interested in syntax and an introduction to the Class Hierarchy, Java in 24 hours is pretty good. From there, you will know enough to go and explore more on your own.
I highly recommend thinking of a small project to help you learn. In fact, think of a teeny tiny project -- it will turn out much bigger than you think! ;) Maybe a simple shoot-em-up or a Daleks- or Asteroids-alike, or some such.
The main thing being: for an expert-level programmer, don't try to learn Java from a book. You can get introduced to Java via a book, but then you have to go learn on your own (or from another.) It's like learning to drive a car -- you can get the basics from a book, but you need to explore & try stuff, and someone who knows how can be very helpful.
Some things that will help:
Download and bookmark the HTML Java docs. You will reference them often.
Have a small project, a simple game or utility app that you estimate will take a couple of days to write. Spend a couple of weeks writing it ;)
Note that "Java" is 3 things:
Java the programming language. Like C++ or Lisp or Perl.
Java the Class Hierarchy. Like STL or Cocoa.
Java the run-time virtual machine, the JVM.
People use the word "Java" to mean all 3 things, and slip between them without much notice, so pay attention to that, when you start talking Java to people.
Good luck!
(And let us know when your game is available :)

The biggest issue with the C++ to Java conversion is that they're so similar -- you'll find yourself doing C++-isms for a while. But it's no big deal: as suggested, read Josh Bloch's book, and run through the tutorials on java.sun.com and you'll do fine.

In my opinion, Java sets itself apart from C++ in three ways:
Automated memory management
Simplified syntax (and no preprocessor)
The Java standard libraries
As a former C++ guy, the first two shouldn't be a problem. (In fact, I would wager they will strike you as rather refreshing.) To get used to the libraries, I recommend Bruce Eckel's Thinking in Java - the 3rd edition is free to download. Though the beginning chapters will probably be a little too basic, the sections about GUI programming, concurrency, and the various container classes will be very helpful.
Beyond that, the best way to learn any language is to read good code.

I started by reading the Java Language Specification, version 2 at the time (2001).
In my view it is quite readable as specifications go.
Effective Java is good advice.
Java Puzzlers is good to learn corner cases and possible pitfalls.

The Peter Norton's Guide to Java Programming, "Making the Transition from C/C++ to Java".