RickJWagner writes "This is a book targeted primarily towards the Java programmer who doesn't yet know Groovy. Groovy is a dynamic scripting language that runs on the JVM and offers easy integration with the Java language. Groovy also offers dynamic language features that lend themselves to the creation of DSLs (Domain-Specific Languages). This book attempts to simultaneously teach the reader how to use Groovy and how to build a DSL." Keep reading for the rest of RickJWagner's review.DSLs are something of a hot topic these days. A DSL is a 'Little Language' or a language written just for one business domain. As programmers, we're familiar with DSLs whether we realize it or not-- each time you use 'Make' to build a C project, or use 'Ant' to make a Java project, you're using a DSL. A DSL is a purpose-built language, with it's own verbs, rules, syntax, etc. It's made for one purpose only, and is meant to make things as easy as possible for the intended user. If the user has a need outside what the DSL can provide-- they're just tough out of luck! General purpose languages are for doing whatever the user needs-- DSLs are for doing only one task, but making that one task exceedingly easy for the user.
The book does a good job of making Groovy understandable for a Java coder. Programmers of other stripes would be able to use the book, but the author aimed squarely for the Java sweet spot and I think did a good job of hitting it. Where language integration was to be shown, the JDK was the reciprocal part in the examples, and the classes singled out for interfacing were ones that would be well-known to a Java coder.
Writing DSLs has been done for many years, but was largely an undocumented process until just recently. I think the rise of dynamic languages has something to do with that. Dynamic languages offer some key features that make DSL creation a little easier. A few of these features the book covers are closures and meta-programming. Closures are anonymous methods, packaged in such a way that they can be passed as parameters to other methods. Meta-programming is using code to write other code, and can be used to perform some neat tricks like allowing the user to call methods that don't have a previously written definition. (If that sounds really strange to you, I agree. But it turns out this is something very handy if you're writing a DSL!)
The book is written in a way that thoroughly the breadth of features offered by the Groovy language, but not a lot of depth in all of them. For this reason, I found myself sometimes referring to external sources when I was encountering some language construct that seemed a little less familiar than most. It wasn't a problem-- the book did a fine job of letting me know of the existence of some language feature and how it might be used in writing a DSL. If I didn't immediately grasp the use of the feature after the typical single example of seeing it used in sample code, I'd just pop out to the internet and get a few more examples. The book is nearly 300 pages as it is, and very little of that is wasted. I give the book good grades for technical content.
Style-wise, I found the book reasonable to read but not great. As well as the author does spelling out the nuances of the Groovy language, the DSL parts are interspersed among the chapters and sometimes could be hard to find unless you read a lot of surrounding technical content. Maybe this is the only way to cover two meaty topics at once, I don't know. But I found myself doing quite a bit of paging when I wanted to refer back to DSL-specific advice, sometimes falling back to the chapter-by-chapter summary in the front of the book to get myself in the right neighborhood.
The book has plenty of sample code, which I found well constructed and easy to read. The author adapted the practice of writing small snippets of code, then used assertions to demonstrate the behavior the reader would expect if the code were to be executed. I really liked that. The code itself was always cleanly formatted and minimal in length, which made it very easy to read.
So, did the book hit it's target? I believe it did. The strengths of using Groovy for Domain Specific Languages are drawn out, and the Groovy language is given a thorough overview. Java coders will find this book a reference that will let them start using dynamic features while still staying on the JVM. (By the way, Groovy compiles to Java, so all your existing Java code is usable from Groovy. So you might write an application that's 95% Java and 5% Groovy, and that's just fine with Groovy.) I think this book covered a lot of ground and did it well.
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