Note: the following examples are snippets, not ready-to-run examples.

Groovy has native support for various markup languages from XML, HTML, SAX, W3C DOM, Ant tasks, Swing user interfaces and so forth.
This is all accomplished via the following syntax...

def someBuilder = new NodeBuilder()

someBuilder.people(kind:'folks', groovy:true) {
  person(x:123,  name:'James', cheese:'edam') {
    project(name:'groovy')
    project(name:'geronimo')
  }
  person(x:234,  name:'bob', cheese:'cheddar') {
    project(name:'groovy')
    project(name:'drools')
  }
}

Whichever kind of builder object is used, the syntax is the same. What the above means is that the someBuilder object has a method called 'people' invoked with 2 parameters...

So we can easily represent any arbitrary nested markup with ease using a simple concise syntax. No pointy brackets! (smile)

What's more is this is native Groovy syntax; so you can mix and match this markup syntax with any other Groovy features (iteration, branching, method calls, variables, expressions etc). e.g.

// lets create a form with a label & text field for each property of a bean
def swing = new SwingBuilder()
def widget = swing.frame(title:'My Frame', defaultCloseOperation:javax.swing.WindowConstants.EXIT_ON_CLOSE) {
    panel() {
       for (entry in someBean) {
          label(text:entry.key)
          textField(text:entry.value)
       }
       button(text:'OK', actionPerformed:{ println("I've been clicked with event ${it}") })
    }
}
widget.show()

Trees, DOMs, beans and event processing

The really neat thing about GroovyMarkup is that its just a syntax which maps down to method calls. So it can easily support the building of any arbitrary object structure - so it can build any DOMish model, a bean structure, JMX MBeans, PicoComponents, Swing front ends, Ant tasks etc. What's more since its just normal method invocations it can naturally map to SAX event processing too.

Out of the box Groovy comes with a few different markup builders you can use :

Examples

Here's a simple example which shows how you could iterate through some SQL result set and output a dynamic XML document containing the results in a custom format using GroovyMarkup

// lets output some XML builder (could be SAX / DOM / TrAX / text)
def xml = new NodeBuilder()
xml.customers() {
    loc = 'London'
    sql.eachRow("select * from customer where location = ${loc}) {

        // lets process each row by emitting some markup
        xml.customer(id:it.id, type:'Customer', foo:someVariable)) {
            role(it.person_role)
            name(it.customer_name)
            location(id:it.location_id, name:it.location_name)
        }
    }
}

The interesting thing about the above is that the XML technology used at the other end could be push-event based (SAX) or pull-event based (StAX) or a DOM-ish API (W3C, dom4j, JDOM, EXML, XOM) or some JAXB-ish thing (XMLBeans, Castor) or just beans or just good old text files.
e.g. a pull parser could literally pull the data out of the database - or the data could be pushed into data some structure or piped straight to a file using IO or async NIO.

The use of GroovyMarkup means developers can hide the XML plumbing and focus on tackling the real problems we're trying to solve.

To see more examples of using GroovyMarkup try looking at our unit test cases

There is more detail on markup here Make a builder.