tests double-checked locking approach

DoubleCheckedLocking.groovy

import java.util.concurrent.*

class Factory {
    def rand = new Random()
    def instance
    def getInstance() {
        if (instance == null) {
            synchronized(this) {
                if (instance == null) {
                    instance = createInstance()
                }
            }
        }
        instance
    }  
    def createInstance() {
        Thread.sleep(rand.nextInt(1000)) // for simulation purposes
        new Object()
    }
}

def factory = new Factory()

def createdInstances = [] as Set
int nThreads = 10
int nTasks = 100
ExecutorService executor = Executors.newFixedThreadPool(nThreads);
def tasks = []
(0..nTasks).each {
    tasks << { createdInstances << factory.getInstance() } as Callable<Boolean>
}
executor.invokeAll(tasks)
executor.shutdown()
executor.awaitTermination(1, TimeUnit.MINUTES)
// this "Set" approach assumes the generated instances are distinct
assert createdInstances.size() == 1