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Every Groovy class implements the groovy.lang.GroovyObject interface (it’s added
by the compiler) that includes these methods:
Object invokeMethod(String name, Object args)
Object getProperty(String property)
void setProperty(String property, Object newValue)
MetaClass getMetaClass()
void setMetaClass(MetaClass metaClass)
Java classes can also implement GroovyObject to add Groovy-like
behavior. The most convenient approach for this is to subclass the
groovy.lang.GroovyObjectSupport adapter class, which implements
the interface and provides sensible default implementations of the
methods that can be overridden as needed.
When you invoke a method in Groovy (including accessing a property, because that
calls the corresponding getter method), it’s actually dispatched to the object’s metaclass.
This provides an AOP-like interception layer. The calls are implemented with reflection,
which is slower than direct method invocation. But each new release of Java adds re‐
flection speed improvements, and Groovy has several optimizations to reduce the cost
of this overhead, the most significant being call site caching. Early versions of Groovy
were quite slow, but modern Groovy has seen huge performance boosts and is often
nearly as fast as Java. And because network latency and database access tend to con‐
tribute most to total web request time, the small increase in invocation time that Groovy
can add tends to be insignificant, because it’s such a small percentage of the total time.
Adding Methods
The syntax for adding a method at runtime is essentially just one that registers a closure
in the metaclass that’s associated with the specified method name and signature:
List.metaClass.removeRight = { int index ->
delegate.remove(delegate.size() - 1 - index)
}
The List interface has a remove method, but this addition removes the item considering
the position from the right instead of the left like remove:
assert 3 == [1, 2, 3].removeRight(0)
assert 2 == [1, 2, 3].removeRight(1)
assert 1 == [1, 2, 3].removeRight(2)
Recall that closures have a delegate that handles method calls invoked inside the closure.
When adding methods to the metaclass, you can access the instance in which closure
is invoked with the delegate property; in this example, it’s the list instance that remove
Right is called on.
22 | Chapter 1: Introduction to Groovy