Builders in Groovy
Builders are, pardon the pun, a built-in language feature of Groovy. Closures and meta-programming support allow a level of flexibility in the language that would be simply impossible in Java, at present. The Groovy alternative to the Java example above, using the native builder support, looks like this:
GScreenBuilder b = new GScreenBuilder()
b.screen {
title( "My Page" )
navigation {
navItem( name: "Home", focus: true, visible: true )
navItem( name: "Admin", focus: false, visible: false )
navItem( name: "Logout", focus: false, visible: true )
}
content {
table ()
}
footer( "Our Footer Text" )
}
Screen screen = b.build()
Pretend Methods
Groovy builders allow you to define a tree-like structure in your code that represents complex object hierarchies. Each of the structure's nodes is represented by a method call that can take a closure as its last argument. All method calls within a closure represent a child node of the method that receives the closure. The first line of your tree structure definition invokes a method, called screen, on your builder. It then passes the contents of the subsequent code block as a closure:
b.screen {...}
There is no screen method on the GScreenBuilder. Instead, Groovy uses meta-programming to handle missing method invocation. Later on, you will see the rules that Groovy builders use to map the nodes to method calls—that you can override. For now, take a look at how Groovy deals with those missing methods.
The first revelation is that all Groovy classes, once compiled down to Java byte code, will extend the GroovyObject interface. This is the base Groovy interface that provides the first part of Groovy's meta-programming support. The interface defines the following methods:
public Object invokeMethod(String name, Object args);
public Object getProperty(String property);
public void setProperty(String property, Object newValue);
public MetaClass getMetaClass();
public void setMetaClass(MetaClass metaClass);
Suppose you override the invokeMethod method. You could then handle all method calls made to the object where the method does not actually exist. The following example and test shows this in action:
class PretendMethod {
def title( data ) {
println "A title is created with some data: ${data}"
return data
}
public Object invokeMethod(String name, Object args) {
return name
}
}
@Test
public void methodPretention() {
PretendMethod b = new PretendMethod()
assertEquals( "screen", b.screen() )
assertEquals( "My Title", b.title( "My Title" ) )
}
The PretendMethod class simply returns the method name of any method that is executed through the invokeMethod path—the test asserts that you can call a non-existent screen method on the class. Be aware that when you override invokeMethod, you are not able to intercept methods that have actually been implemented on the class—only handle calls on the class for methods that have not been implemented.
For example, when you call the title method on an instance of the PretendMethod class, invokeMethod will not be executed, only the title method will run because it has an implementation. Whereas any other method call, screen in the example, will be handled by invokeMethod because there is no screen method implementation. A full discussion of Groovy meta-programming is outside of the scope of this article. If you are interested in more information, I fully recommend the excellent book, Groovy In Action.
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