Mocking same method multiple times using Mockito? Can't be easier than that!

This is fairly short blog post. I recently had the case I needed to mock the same method twice during my Junit test case, with different behavior.

To shorten your path how that is being made, take a quick look at :

package com.mydomain;

import org.junit.Test;
import org.mockito.Mockito;

import java.util.Optional;

import static org.assertj.core.api.Assertions.assertThat;
import static org.mockito.Mockito.*;

public class RepeatedMocker {
    @Test
    public void getsRightVersionWithNoRepeat() throws Exception {
        VersionManager versionManager = Mockito.mock(VersionManager.class);
        doReturn(1).when(versionManager).getVersion();
        SystemUnderTest systemUnderTest = new SystemUnderTest(versionManager);
        Optional<Integer> version = systemUnderTest.getVersion();
        assertThat(version.get()).isEqualTo(1);
        verify(versionManager).getVersion();
    }

    @Test
    public void getsRightVersionWithRepeat() throws Exception {
        VersionManager versionManager = Mockito.mock(VersionManager.class);
        doReturn(-1).doReturn(1).when(versionManager).getVersion();
        SystemUnderTest systemUnderTest = new SystemUnderTest(versionManager);
        Optional<Integer> version = systemUnderTest.getVersion();
        assertThat(version.get()).isEqualTo(1);
        verify(versionManager, times(2)).getVersion();
    }

    @Test
    public void failsGettingRightVersion() throws Exception {
        VersionManager versionManager = Mockito.mock(VersionManager.class);
        doReturn(-1).doReturn(-1).when(versionManager).getVersion();
        SystemUnderTest systemUnderTest = new SystemUnderTest(versionManager);
        Optional<Integer> version = systemUnderTest.getVersion();
        assertThat(version.isPresent()).isFalse();
        verify(versionManager, times(2)).getVersion();
    }

    private static class SystemUnderTest {
        private VersionManager versionManager;

        SystemUnderTest(VersionManager versionManager) {
            this.versionManager = versionManager;
        }

        Optional<Integer> getVersion() {
            int version = versionManager.getVersion();
            if (version == -1) {
                System.out.println("Trying once more, version can't be 1 ....");
                version = versionManager.getVersion();
                if (version == -1) {
                    System.out.println("Error: version can't be 1!");
                    return Optional.empty();
                }
            }
            return Optional.of(version);
        }
    }

    private static class VersionManager {
        int getVersion() {
            return Math.random() < 0.5 ? 1 : -1;
        }
    }
}

To shorten solution, I've implement class to be tested in the Junit test class itself. The class to test is SystemUnderTest. It has only one public method, getVersion, which basically just delegates that task to VersionManager dependent class. Here, we have some dummy implementation in VersionManager, where our method randomly returns 1 or -1. Our SystemUnderTest will retry retrieving correct version once more, if VersionManager returns -1 in the first attempt. It will eventually, if it again receives -1 from VersionManager, return an empty Optional.

Long story short - when we need to mock the same method differently more than once, we can just chain it as seen in above snippet.

doReturn(-1).doReturn(1).when(versionManager).getVersion();

Given that, if our dependency class's method is called more than once during our system under test method call, it would use that different behavior you specify in this doReturn()....doReturn() methods chain.

Fairly simple!

You can find source code @ Github

That was all for today! Hope you liked it!