Programming for fun and profit

Programming tutorials, problems, solutions. Always with code.

Spock spy and partial mocking


How to create Spock Spy and do partial mocking? In this post we’re going to show practical application of partial mocking using Spy. Let’s test!

Let’s pretend that the following interface is Camel Processor ;-)

package com.farenda.spock;

public interface Processor {
    void process(String exchange);

And the code below is our ProcessorFactory that creates our complex processors and we want to unit test only its create(Strategy) method. The problem is that it calls other factory methods that do some complex setup, which we don’t want to test here.

package com.farenda.spock;

import java.util.List;

public class ProcessorFactory {

    private List<String> endpoints;

    enum Strategy {

    public ProcessorFactory(List<String> endpoints) {
        this.endpoints = endpoints;

    public Processor create(Strategy strategy) {
        switch(strategy) {
            case MULTICAST:
                return createMulticastProcessor(endpoints);
            case ROUND_ROBIN:
                return createRoundRobinProcessor(endpoints);
            case SELECTOR:
                return createSelectorProcessor(endpoints);
        throw new IllegalArgumentException("Unknown strategy: " + strategy);

    Processor createSelectorProcessor(List<String> endpoints) {
        // do complex things that we don't want to test
        return null;

    Processor createRoundRobinProcessor(List<String> endpoints) {
        // do complex things that we don't want to test
        return null;

    Processor createMulticastProcessor(List<String> endpoints) {
        // do complex things that we don't want to test
        return null;


Creating simple Mock is no go, because it mock all methods in a class/interface. What we want is to mock only part of the class and to do that we can use Spock Spy. Spy allows us to specify only the methods we want to mock and others leave from original object. In our case the test would look like this:

package com.farenda.spock

import spock.lang.Shared
import spock.lang.Specification

import static com.farenda.spock.ProcessorFactory.Strategy.*

class SpyTest extends Specification {

    @Shared def multicastProcessor = Mock(Processor)
    @Shared def roundRobinProcessor = Mock(Processor)
    @Shared def selectingProcessor = Mock(Processor)

    @Shared def endpoints = ['direct:x', 'direct:y']

    def factory = Spy(ProcessorFactory, constructorArgs: [endpoints]) {
        createMulticastProcessor(endpoints) >> multicastProcessor

    def setup() {
        factory.createRoundRobinProcessor(endpoints) >> roundRobinProcessor

    def 'should create selected processor'() {
        factory.createSelectorProcessor(endpoints) >> selectingProcessor

        def processor = factory.create(strategy)

        processor == expectedProcessor

        strategy    | expectedProcessor
        MULTICAST   | multicastProcessor
        ROUND_ROBIN | roundRobinProcessor
        SELECTOR    | selectingProcessor


The code is fairly straightforward. Spy is created with, well, Spy method, which takes implementation class as the parameter and possibly some of its arguments. Here we pass endpoints as constructor parameter. What is interesting we can specify mocked methods on Spy in different places:

  • during Spy construction (createMulticastProcessor),
  • in setup phase (createRoundRobinProcessor),
  • in test itself (createSelectorProcessor).

Spock spies should not be overused, because it usually means that the code haven’t been written correctly (for example I’ve used Spock Spy only once during 4 years of using Spock Framework). Sometimes they are useful, though.

Share with the World!