Question

In What JVM-based scripting language support @WebService to create services at runtime? I was suggested to use Groovy to provide web services configured in a script read in at runtime.

To make this work with our existing infrastructure I need essentially to be able to add new entries to a List<Callable<String>> which I then can ask an executor to invokeAny upon.

The basic structure will be something like:

• Groovy is embedded using GroovyScriptEngine
• Initial list passed in from Java as “l” in the Binding passed in.
• Groovy script defines and instantiates N objects, all implementing Callable<String> and add them to the list.
• Back in Java the list is then further processed and then passed to the executor.

My initial feeble steps show that I will most likely need to use def c = { ... } as Callable<String> but then I get a ClassCastException. Reading up I see that it appears that this is a bit hard and involves closures.

What is the correct way to define and instantiate an object in Groovy which implements Callable<String>?

Answer 1

In Groovy 1.8 and later, groovy.lang.Closure implements Callable by default so you don’t need any “as” magic., simply:

l << { "hello" }
l << { "world" }

For earlier versions of Groovy (1.6 and 1.7 certainly, not sure about “ancient” versions) you need to use as:

import java.util.concurrent.Callable

l << ({ "hello" } as Callable)