Mockito doAnswer() callbacks

MyUnitTest.java

import org.mockito.Mockito;
import org.mockito.invocation.InvocationOnMock;
import org.mockito.stubbing.Answer;
import org.testng.Assert;
import org.testng.annotations.Test;

public class MyUnitTest {
    int executions;
    class ServiceA {
        final ServiceB runner;
        ServiceA(final ServiceB runner) {
            this.runner = runner;
        }
        public void execute(Runnable runnable) {
            runner.run(runnable);
        }
    }

    interface ServiceB {
        public void run(final Runnable runnable);
    }

    @Test
    public void testCallbackInvokedOnlyOnce() {
        ServiceB singleRunService = Mockito.mock(ServiceB.class);

        // mock up the first callback to serviceB:
        Mockito.doAnswer(new Answer<Void>(){
            public Void answer(InvocationOnMock invocation) throws Throwable {
                System.out.println("Running first time");
                ((Runnable)invocation.getArguments()[0]).run();
                return null;
            }
        }).when(singleRunService).run(Mockito.isA(Runnable.class));

        // mock up the second callback - it should not run:
        Mockito.doAnswer(new Answer<Void>(){
            public Void answer(InvocationOnMock invocation) throws Throwable {
                System.out.println("cannot run second time");
                return null;
            }
        }).when(singleRunService).run(Mockito.isA(Runnable.class));

        ServiceA serviceA = new ServiceA(singleRunService);
        serviceA.execute(new Runnable(){ public void run(){ executions++; } });
        serviceA.execute(new Runnable(){ public void run(){ executions++; } });
        Mockito.verify(singleRunService);
        Assert.assertEquals(executions, 1, "The service should have run exactly once."); // this assert fails - executions = 0
    }
}