Another important feature in Groovy is Builder. Groovy Builders allow you to create complex tree-like hierarchical object structures. For example, SwingUI or XML documents can be created very easily using the DSL or Closure-like features in Groovy, with the support of the BuilderSupport class and its subclasses, MarkupBuilder and SwingBuilder.

Let's try to understand with an example. We created the Order class earlier in this chapter. Assume we have a list of orders and we want to store the details in a file called orders.xml. So every Order object in our list should be saved as a node in the XML file. Each of these Order nodes, again should contain child nodes, grand children nodes, and so on. Creating this tree-like structure can be complex if we try to implement a DOM-like parser in Java:

<orders>
  <order>
    <no>1</no>
    <description>Ordered by customer 1</description>
    <customer>
      <name firstname='Customer1' />
      <email>[email protected]</email>
    </customer>
  </order>
  <order>
    <no>2</no>
    <description>Ordered by customer 2</description>
    <customer>
      <name firstname='Customer2' />
      <email>[email protected]</email>
    </customer>
  </order>
  ….
</orders>

But in Groovy, this is just few lines of code with some method calls combined with Closure and named parameters. In the following example, we have created a builder object from the MarkupBuilder class to create the XML document. Then we have defined orders as the root of the document. However, builder has no method defined as orders. So then, how does this work?

As mentioned earlier, the MarkupBuilder class is a subclass of the BuilderSupport class. BuilderSupport has methods such as createNode, invokeMethod, nodeCompleted, setCurrent, setParent, and a few more. In runtime, an object is created by calling the createNode method on the builder with the name orders. In a similar fashion, for each order object, no, description, and customer nodes are created. Finally, each order node is attached to the parent orders node by calling the setParent method of the builder object:

def builder = new groovy.xml.MarkupBuilder(new FileWriter("orders.xml"))

  builder.orders{
    for(i in orderlist){
      order{
        no(i.orderNo)
        description(i.description)
        customer{
          name(firstname : i.orderedBy.name)
          email(i.orderedBy.email)
        }
      }
    }
  }