Book excerpt: Your way to Groovy

All the power of the Java platform can be harnessed with Groovy

The Groovy Website gives one of the best definitions of Groovy: "Groovy is an agile dynamic language for the Java Platform with many features that are inspired by languages like Python, Ruby and Smalltalk, making them available to Java developers using a Java-like syntax."

Groovy is often referred to as a scripting language—and it works very well for scripting. It's a mistake to label Groovy purely in those terms, though. It can be precompiled into Java bytecode, be integrated into Java applications, power Web applications, add an extra degree of control within build files, and be the basis of whole applications on its own—Groovy is too flexible to be pigeon-holed.

What we can say about Groovy is that it is closely tied to the Java platform. This is true in terms of both implementation (many parts of Groovy are written in Java, with the rest being written in Groovy itself) and interaction. When you program in Groovy, in many ways, you're writing a special kind of Java. All the power of the Java platform—including the massive set of available libraries—is there to be harnessed.

Does this make Groovy just a layer of syntactic sugar? Not at all. Although everything you do in Groovy could be done in Java, it would be madness to write the Java code required to work Groovy's magic. Groovy performs a lot of work behind the scenes to achieve its agility and dynamic nature. As you read this book, try to think every so often about what would be required to mimic the effects of Groovy using Java. Many of the Groovy features that seem extraordinary at first—encapsulating logic in objects in a natural way, building hierarchies with barely any code other than what is absolutely required to compute the data, expressing database queries in the normal application language before they are translated into SQL, manipulating the runtime behavior of individual objects after they have been created—all of these are tasks that Java cannot perform. You might like to think of Groovy as being a "full color" language compared with the monochrome nature of Java—the miracle being that the color pictures are created out of lots of carefully engineered black and white dots.

Let's take a closer look at what makes Groovy so appealing, starting with how Groovy and Java work hand-in-hand.

Note: This article is an excerpt from Groovy in Action, by Dierk Koenig, with Andrew Glover, Paul King, Guillaume Laforge, and Jon Skeet (Manning Publications, January 2007; ISBN: 1932394842).

Playing nicely with Java: seamless integration

Being Java friendly means two things: seamless integration with the Java Runtime Environment and having a syntax that is aligned with Java.

Seamless integration

Figure 1 shows the integration aspect of Groovy: It runs inside the Java Virtual Machine and makes use of Java's libraries (together called the Java Runtime Environment, or JRE). Groovy is only a new way of creating ordinary Java classes—from a runtime perspective, Groovy is Java with an additional jar file as a dependency.

Figure 1. Groovy and Java join together in a tongue-and-groove fashion

Consequently, calling Java from Groovy is a nonissue. When developing in Groovy, you end up doing this all the time without noticing. Every Groovy type is a subtype of java.lang.Object. Every Groovy object is an instance of a type in the normal way. A Groovy date is a java.util.Date, and so on. Integration in the opposite direction is just as easy. Suppose a Groovy class MyGroovyClass is compiled into a *.class file and put on the classpath. You can use this Groovy class from within a Java class by typing: new MyGroovyClass(); // create from Java.

In other words, instantiating a Groovy class is identical to instantiating a Java class. After all, a Groovy class is a Java class. You can then call methods on the instance, pass the reference as an argument to methods, and so forth. The JVM is blissfully unaware that the code was written in Groovy.

Syntax alignment

The second dimension of Groovy's friendliness is its syntax alignment. Let's compare the different mechanisms to obtain today's date in Java, Groovy, and Ruby in order to demonstrate what alignment should mean:

import java.util.*; // Java
Date today = new Date(); // Java

today = new Date() // a Groovy Script

require 'date' # Ruby
today = Date.new # Ruby

The Groovy solution is short, precise, and more compact than normal Java. Groovy does not need to import the java.util package or specify the Date type; moreover, Groovy doesn't require semicolons when it can understand the code without them. Despite being more compact, Groovy is fully comprehensible to a Java programmer.

The Ruby solution is listed to illustrate what Groovy avoids: a different packaging concept (require), a different comment syntax, and a different object-creation syntax. Although the Ruby way makes sense in itself (and may even be more consistent than Java), it does not align as nicely with the Java syntax and architecture as Groovy does.

Now you have an idea what Java friendliness means in terms of integration and syntax alignment. But how about feature richness?

Power in your code: a feature-rich language

Giving a list of Groovy features is a bit like giving a list of moves a dancer can perform. Although each feature is important in itself, it's how well they work together that makes Groovy shine. Groovy has three main types of features over and above those of Java: language features, libraries specific to Groovy, and additions to the existing Java standard classes (GDK). Figure 2 shows some of these features and how they fit together. The shaded circles indicate the way that the features use each other. For instance, many of the library features rely heavily on language features. Idiomatic Groovy code rarely uses one feature in isolation—instead, it usually uses several of them together, like notes in a chord.

Figure 2. Many of the additional libraries and JDK enhancements in Groovy build on the new language features. The combination of the three forms a "sweet spot" for clear and powerful code. Click on thumbnail to view full-sized image.

Unfortunately, many of the features can't be understood in just a few words. Closures, for example, are an invaluable language concept in Groovy, but the word on its own doesn't tell you anything. We won't go into all the details now, but here are a few examples to whet your appetite.

Listing a file: closures and I/O additions

Closures are blocks of code that can be treated as first-class objects: passed around as references, stored, executed at arbitrary times, and so on. Java's anonymous inner classes are often used this way, particularly with adapter classes, but the syntax of inner classes is ugly, and they're limited in terms of the data they can access and change.

File handling in Groovy is made significantly easier with the addition of various methods to classes in the java.io package. A great example is the File.eachLine method. How often have you needed to read a file, a line at a time, and perform the same action on each line, closing the file at the end? This is such a common task, it shouldn't be difficult—so in Groovy, it isn't.

Let's put the two features together and create a complete program that lists a file with line numbers:

 def number=0
new File ('test.groovy').eachLine { line ->
   number++
   println "$number: $line"
}

The closure in curly braces gets executed for each line, and File's new eachLine method makes this happen.

Printing a list: Collection literals and simplified property access

java.util.List and java.util.Map are probably the most widely used interfaces in Java, but there is little language support for them. Groovy adds the ability to declare list and map literals just as easily as you would a string or numeric literal, and it adds many methods to the collection classes.

Similarly, the JavaBean conventions for properties are almost ubiquitous in Java, but the language makes no use of them. Groovy simplifies property access, allowing for far more readable code.

Here's an example using these two features to print the package for each of a list of classes. Note that the word package needs to be quoted because it's a keyword, but it can still be used for the property name. Although Java would allow a similar first line to declare an array, we're using a real list here—elements could be added or removed with no extra work:

 def classes = [String, List, File]
for (clazz in classes)
{
   println clazz.'package'.name
}

In Groovy, you can even avoid such commonplace for loops by applying property access to a list—the result is a list of the properties. Using this feature, an equivalent solution to the previous code is println( [String, List, File].'package'.name ) to produce the output ["java.lang", "java.util", "java.io"]. Pretty cool, eh?

XML handling the Groovy way: GPath with dynamic properties

Whether you're reading it or writing it, working with XML in Java requires a considerable amount of work. Alternatives to the W3C DOM make life easier, but Java itself doesn't help you in language terms—it's unable to adapt to your needs. Groovy allows classes to act as if they have properties at runtime even if the names of those properties aren't known when the class is compiled. GPath was built on this feature, and it allows seamless XPath-like navigation of XML documents.

Suppose you have a file called customers.xml such as this:

 <?xml version="1.0" ?>
<customers>
   <corporate>
      <customer name="Bill Gates" company="Microsoft" />
      <customer name="Steve Jobs" company="Apple" />
      <customer name="Jonathan Schwartz" company="Sun" />
   </corporate>

   <consumer>
      <customer name="John Doe" />
      <customer name="Jane Doe" />
   </consumer>
</customers>

You can print out all the corporate customers with their names and companies using just the following code. (Generating the file in the first place with Groovy using a Builder would be considerably easier than in Java, too.)

 def customers = new XmlSlurper().parse(new File('customers.xml'))
for (customer in customers.corporate.customer)
{
   println "${customer.@name} works for ${customer.@company}"
}

Even trying to demonstrate just a few features of Groovy, you've seen other features in the preceding examples—string interpolation with GString, simpler for loops, optional typing, and optional statement terminators and parentheses, just for starters. The features work so well with each other and become second nature so quickly, you hardly notice you're using them.

Although being Java friendly and feature-rich are the main driving forces for Groovy, there are more aspects worth considering. So far, we have focused on the hard technical facts about Groovy, but a language needs more than that to be successful. It needs to attract people. In the world of computer languages, building a better mousetrap doesn't guarantee that the world will beat a path to your door. It has to appeal to both developers and their managers, in different ways.

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