I recently returned from JavaOne 2005 in San Francisco. The show was impressive for a number of reasons. The attendance seemed to be about 30% larger than last year's. The same could be said for the number of tutorials, sessions and BOFs. For example, there were enough BOFs to run until 11:00pm at night. Many of the sessions were filled to capacity, with over 600 attendees each technical presentation.

Given my strong background in C++, I am used to a more amorphous attitude toward languages. Therefore, I was surprised to see that there still is a vibrancy to Java that I do not see with C++.

Among the many interesting things that I saw, two stand out: Eclipse and the scripting language called Groovy. I already have written about Eclipse, so here I simply note that Eclipse now seems to dominate the Java IDE world. Groovy, on the other hand, is new. It prompted me to look at Python-like scripting languages that run in a Java environment.

In this article, I discuss Jython and Groovy, two scripting languages that use the Java runtime environment. By the way, JavaScript has nothing to do with the Java language, so it is not considered here.

Jython is an interpreter for the Python language. It is written in Java to run under the Java VM. To handle Python built-in entities, Jython produces Java compatible structures that it passes back and forth to the JVM. The code below uses the Jython interactive shell, in which Jython takes the Python built-in list structure and converts it to a Java list structure that is then passed to the Java println function that then passes the result to the JVM.

>>> print ['a','list','of','strings']
['a', 'list', 'of', 'strings']
>>>

Here is a simple program that uses the Java Swing library to produce a trivial GUI application:

"""\
A simple demonstration of creating a swing tree widget from a
Python dictionary.
"""

import java
import javax
from pawt import swing
from pawt import awt

sampleData = {
  'PyObject': {
        'PyInteger':None,
        'PyFloat':None,
        'PyComplex':None,
        'PySequence': {
          'PyArray':None,
          'PyList':None,
          'PyTuple':None,
          'PyString':None,
        },
        'PyClass': {
          'PyJavaClass':None,
        },
  },
  'sys':None,
  'Py':None,
  'PyException':None,
  '__builtin__':None,
  'ThreadState':None,
}

Node = swing.tree.DefaultMutableTreeNode

def addNode(tree, key, value):
  node = Node(key)
  tree.add(node)
  if value is not None:
    addLeaves(node, value.items())

def addLeaves(node, items):
  items.sort()
  for key, value in items:
    addNode(node, key, value)

def makeTree(name, data):
  tree = Node("Sample Tree")
  addLeaves(tree, data.items())
  return swing.JTree(tree)

def exit(e): 
  java.lang.System.exit(0)

if __name__ == '__main__':
  tree = makeTree('Some JPython Classes', sampleData)
  button = swing.JButton('Close Me!', actionPerformed=exit)
  f = swing.JFrame("GridBag Layout Example");
  p = swing.JFrame.getContentPane(f)
  gb = awt.GridBagLayout()
  p.setLayout(gb)
  
  c = awt.GridBagConstraints();
  c.weightx = 1.0;
  c.fill = awt.GridBagConstraints.BOTH;
  
  gb.setConstraints(tree, c);
  c.gridx = 0;
  c.gridy = awt.GridBagConstraints.RELATIVE;
  gb.setConstraints(button, c);

  p.add(tree)
  p.add(button)
  f.pack();
  f.setSize(f.getPreferredSize());
  f.show();

Jython has two main advantages. First, the language is Python and has all of its strengths, including clean and consistent syntax, introspection, dynamic creation of classes, properties and attributes. Second, it runs at the speed of the JVM. On the other hand, Jython also has two chief weakness. First is its need to convert constantly between Python and Java structures. Second, Jython has not received much work in recent years. That is about to change, however; the current maintainer, Brian Zimmer, recently received three grants to continue work on Jython.

The bottom line is if you know Python, then you know Jython. The big gain is ready access to libraries such as Swing, awt and so on.

Groovy is a new language based on features from Ruby, Python and Haskell. However, its runtime environment is any JVM. The language syntax attempts to be Java-like, while making the form of the language much simpler. Groovy is a statically typed language, so it requires a compile step before producing Java byte code. Groovy does not currently have an interpreter, although it does have a shell.

Groovy documentation offers an impressive list of features:

An interesting feature of Groovy is found its definition and use of closure:

Groovy's uses Java's standard types for things such as dictionaries and lists. This means no translation is needed between Groovy code and the JVM. This may improve execution time, depending on the application.

Groovy's documentation provides an example that illustrates the benefits of using closures in place of anonymous classes. Using an anonymous class, we can write:

Button b = new Button ("Push Me");
  b.onClick (new Action() {
    public void execute (Object target)
    {
      buttonClicked();
    }
  });

Using a closure, this becomes:

Button b = new Button ("Push Me");
  b.onClick { buttonClicked() }

Another nice feature of Groovy is its "each" operator, which is used to iterate over a collection:

SomeCollection stuff = new SomeCollection();
  stuff.each() someClosure

// for example:

def myList = [1,2,4,5,6]
myList.each { println it }  // where 'it' is current collection item

// outputs:

1
2
3
4
5
6

Groovy also has built-in structured "builders" for languages such as XML, HTML and more. Again, a simple example illustrates how natural it is to build structured text:

xml = new groovy.xml.MarkupBuilder()
def myList = ['acct1':['id':123111, 'type':'savings', 'balance':1234.56],
              'acct2':['id':221212, 'type':'checking', 'balance':2010.02]]
doc = xml.accounts() {
  for (entry in myList)
    acct(id:entry.key) {
      for (thing in entry.value)
        item(name:thing.key, type=thing.value)
    }
}

This small code fragment outputs:

<accounts>
  <acct id='acct2'>
    <item name='type'>checking</item>
    <item name='id'>221212</item>
    <item name='balance'>2010.02</item>
  </acct>
  <acct id='acct1'>
    <item name='type'>savings</item>
    <item name='id'>123111</item>
    <item name='balance'>1234.56</item>
  </acct>

Bindings

In the following example, the use of variable c is caught at compile time in a statically defined language. Only when a is greater than b is the use of the undefined variable, c, detected in an interpreted language.

a = 1
b = 2
if a > b:
  print c
print a,b

Another major difference between a compiled language and an interpreted one is when things are bound to their references. In Groovy, the following code prints "Bound to local variable":

def varA = "Bound to global variable"
def closure = { varA }
public class C {
  def varA = "Bound to local variable"
  def closure = { varA }  // bound to local varA at definition time
  public def f = closure  // f bound to local closure
};
def c = new C();          // create instance of C using new
println c.f()             // invoke f in C

in Jython, the equivalent-looking code prints "Bound to global variable":

varA = "Bound to global variable"
closure = lambda: varA
class C:
  varA = "Bound to local variable"
  closure = lambda self: varA
  f = closure
c = C()
print c.f()

For those wanting more flexibility, the Jython/Python bindings are handier. For those wanting more stability, the Groovy implementation may be more desirable.

Currying

Currying function arguments is another difference. Groovy has a special "curry" mechanism to bind arguments to a function. In the following example, "foo bar" is printed:

def c = { arg1, arg2-> println "${arg1} ${arg2}" }
def d = c.curry("foo")
d("bar")

Jython inherits Python's natural ability to curry arguments using a number of techniques. One is:

def c(arg1, arg2): print arg1,arg2
def d(arg2): c("foo",arg2)
d("bar")

Maturity

Jython has been around a long time and is based on a mature language, Python. However, its development has stalled in recent years. Groovy is a relatively new language and thus still is developing. For example, its error diagnostics leave a lot to be desired. Also, at the moment, Groovy's following is much smaller than Jython's or Python's. However, both languages are picking up development activity, so you have a chance to influence both languages if you want to become involved.