Over the last few weeks I have repeatedly linked to Ola's post about the stable layer. I didn't take the time to go into detail, and I trusted that people (if they wanted to) could follow the link and understand what Ola was talking about.
Well, that didn't work so well. Most responders clearly did not understand Ola. A few informed me that I didn't understand Ola. :-) So I am going to make a clean break, and lay out my own argument in more detail. What follows are my views about how layered architecture affects language and platform choice. First, some ideas that are hopefully uncontroversial:
- Good design is layered.
- Leakage between layers should be minimal.
- Features within a layer should be orthogonal, and should not have to be re-implemented in higher layers.
- All kinds of programs benefit from this kind of layering, including languages, libraries, frameworks, and application code.
A small leap:
- The lowest layers are the most important.
This might seem obvious. All other layers depend on the lower layers, so a problem at the bottom affects a lot of code. But if you are working several layers higher, problems at the bottom are part of the air you breathe. The air may smell terrible, but you are acclimated and don't notice.
A big but uncontroversial leap:
- Java, the VM, is a good VM for the bottom layer.
This is uncontroversial because the majority has chosen. And they are right to do so: the Java VM is well-specified, widely implemented, carefully optimized, and supported by a huge array of tools.
- Java, the language, is a good language for the bottom layer.
Noooo! Java is a high-ceremony language. At every turn, Java enforces a high busy-work/real-work ratio. Specifically:
- Java's checked exceptions bloat code, make components harder to use and maintain, and lead to tons of boilerplate code, each line of which is a bug-in-waiting.
- Java's new operator/constructors cannot pick a return type. The amount of code that exists only to work around this is staggering. Two entire cottage industries have sprung up to deal with this single issue: factory patterns and dependency injection.
- Java has no metaprogramming features to automate common tasks such as field accessors, standard constructors, and simple delegation.
- Primitives, functions, and classes are not first-class objects, leading to huge code bloat to deal with these types specially.
- Java's core reflection and interception capabilities are clunky, requiring tons of bolt-on technologies to make them workable, including AOP, annotations, and code generators.
That's a pretty big stink, but if you are used to it you probably can't smell it anymore.
The net result of these problems is that bottom layer code written in the Java language will be bloated and difficult to maintain. These problems multiply if we use the Java language for higher layers as well. What should we do?
Keeping the VM, avoiding the language
For better or worse, Java is already the bottom layer for many businesses. A complete rewrite is impossible, so we need an approach that lets us continue to use our existing Java code. There are two obvious choices:
- Use a framework to hide Java's most glaring flaws, and continue to use the Java language for development. The most popular option here is Spring. Spring provides framework-level fixes for several problems in Java: dependency injection, unchecked exception wrappers, and a powerful AOP capability, to name a few.
- Use a better VM language. There are lots of choices, including Clojure, JRuby, Rhino, and Groovy. All of them can interoperate nicely with existing Java code.
My opinion, based on extensive experience with both options, is that the "better VM language" approach is better than the "fix Java with a framework" approach.Smaller is better.
In the past few weeks, I have been approached by several organizations to advise them on platform decisions. Every organization is different, but here are some guidelines to consider.
- Your team matters far more than your language. Pick a platform at random, then take your platform analysis budget and spend it finding good team members and helping them get better.
- Your process matters far more than your language. If your team is not delivering real business value on a regular, repeating timeframe, stop worrying about your platform and start worrying about things like estimating, agility, testing, and continuous integration.
- The static/dynamic languages debate is a red herring. The Java language's problem is ceremony, not static typing. Use whatever combination of static and dynamic typing works for you. 
- There can be more than one! The Java VM simplifies the interop and deployment story. So quit trying to decide, and try a few different JVM languages.
- The hot new JVM languages have different syntaxes, but similar features. They all solve the problems with Java that I enumerated above. Throw a dart at the wall, pick one, and get started coding.
- Beware "Use the right tool for the job." This is true, but useless without context, and it is becoming the weapon of choice for pundits who write no code. Be a polyglot, but also be articulate about why tool X is the right fit for job Y.
- Stop writing plain old Java code. Groovy obsoletes plain old Java. We ought to just say "Java 7 = Groovy" and move on.
Keep an open mind. Try several approaches. Judge your choices by how easy they would be to unmake or adapt. Have fun!
 In the past I have had a lot to say about static/dynamic typing. I realize now that I was trying to talk about ceremony. I am still worried about the same problems, but I think I now know them by more accurate names.