Mulththreading problems using Spock's mocks

DemoThreading.groovy

@Grab('org.spockframework:spock-core:1.1-groovy-2.4')
@Grab('cglib:cglib:2.2')
import spock.lang.Specification

import java.util.concurrent.ExecutorService
import java.util.concurrent.Executors
import java.util.concurrent.TimeUnit


class DemoThreading extends Specification {
    def "mocking with spock"() {
        given:
            Shared shared = Mock(Shared)
            def numberOfThreads = 22
            ExecutorService executor = Executors.newFixedThreadPool(numberOfThreads)
            (1..numberOfThreads).each {
                executor.execute(new Threaded(shared: shared))
            }
        when:
            executor.shutdown()
            executor.awaitTermination(20, TimeUnit.SECONDS)
        then:
            executor.terminated
            numberOfThreads * shared.share()
    }

    def "mocking with coersion"() {
        given:
            def numberOfInvocations = 0
            Shared shared = [share: {synchronized(Shared){numberOfInvocations++}}] as Shared
            def numberOfThreads = 22
            ExecutorService executor = Executors.newFixedThreadPool(numberOfThreads)
            (1..numberOfThreads).each {
                executor.execute(new Threaded(shared: shared))
            }
        when:
            executor.shutdown()
            executor.awaitTermination(20, TimeUnit.SECONDS)
        then:
            executor.terminated
            numberOfThreads * numberOfInvocations
    }
}

class Shared {
    def share() {
    }
}

class Threaded implements Runnable {
    Shared shared

    @Override
    void run() {
        shared.share()
    }
}