I’ve still been thinking about Builders so I dug into Groovy Builders and presented at the Groovy Users of Minnesota (GUM) meeting last night. Since I was inspired by Nat’s post, I used the same example domain that he used.

Builders encapsulate the logic of how an object is assembled. Groovy Builders do this using a tree based syntax. Starting with the end in mind - and after doing a little research into how Groovy Builders work (Chapter 8 of Groovy in Action is a great start) - as a basic first step, this is how I would like to describe an invoice:

def invoice = builder.invoice()
{
  recipient(name:'Sherlock Holmes')
  {
    address(street:'222b Baker Street', city:'London')
  }
  invoiceLines
  {
    invoiceLine(item:'Deerstalker Hat')
    {
      poundsShillingsPence(pounds:0, shillings:3, pence:10)
    }
    invoiceLine(item:'Tweed Cape')
    {
      poundsShillingsPence(pounds:0, shillings:4, pence:12)
    }
  }
}

Now for a quick intro into how Groovy Builders work. Creating a builder begins by extending the abstract class BuilderSupport and implementing a few createNode() methods and a setParent() method (with an option to implement nodeCompleted()). The following pseudocode illustrates how BuilderSupport uses these callback methods:

invoice = createNode('invoice')
//setParent() is not called on the root node
 
  recipient = createNode('recipient', [name:'Sherlock Holmes'])
  setParent(invoice, recipient)
 
    address = createNode('address', [street:'222b Baker', city:'London'])
    setParent(recipient, address)
    nodeCompleted(recipient, address)
 
  nodeCompleted(invoice, recipient)
  // etc ...
nodeCompleted(null, invoice)

I started down the path of creating an InvoiceBuilder as Nat had, but then I started thinking that I could do it in a more generic way that may fit what I needed to do. So rather than start in that direction, for the purposes of this example I decided to create a generic ObjectGraphBuilder that would build any object tree for me, as long as I followed some naming conventions along the way. Here’s the important bits of my builder:

class ObjectGraphBuilder extends BuilderSupport {
  def createNode(def name, Map attributes, def value) {
    if(attributes) {
      return Class.forName(capitalize(name)).newInstance(attributes)
    } else {
      return Class.forName(capitalize(name)).newInstance()
    }
  }
  def setParent(def parent, def child) {
    setObject(child, parent)
    setObject(parent, child)
  }
}

The capitalize() and setObject() methods are just helper methods I created that don’t really have any bearing on how the builder works. The first just uses WordUtils to capitalize the node name and the builder makes the assumption that it should instantiate a class by that name. The second uses reflection to fill in the association between the parent and child nodes as appropriate. The interested reader can find the code available here at the GUM Google Code repository. In addition to the builder and domain classes, there’s a test that executes the builder and asserts that the structure is set up as expected.

For building data objects for test purposes, this example doesn’t quite get to the level of simplicity that Nat’s example does. But next up, I’m hoping to work up an example using nested builders, which may do a better job for that purpose.