By Marcin Zajaczkowski

A unit test is a test related to a single responsibility of a single class, often referred to as the System Under Test (SUT). The purpose of unit tests is to verify that the code in an SUT works. A tested object usually talks to other objects known as collaborators. These collaborators need to be created so the tested object can be assigned to them in the test. To make unit testing simpler and allow control of all aspects of the execution context, it is useful to replace the real cooperating objects with their fake replacements called test doubles. They look like the originals, but do not have any dependencies to other objects. Test doubles can also be easily programmed with specific expectations, such as recording any interactions they've had.

To make it clearer, try to imagine code for a typical en- terprise system. Now here's a service with some logic that needs two classes to fulfill its responsibility. Both classes then require a few other classes. One of these other classes could be a DAO, which needs access to a database, while yet another requires a message queue. It would be quite an effort to create that hierarchy and provide required resources. There could also be problems while running that kind of test, e.g., long startup times or the inability to test multiple developer stations simultaneously. Using Mocks, though, the same test could be much cleaner and faster.

Test doubles can be divided into a few groups1 :

• Dummy - an empty object passed in an invocation (usually only to satisfy a compiler when a method ar- gument is required)
• Fake - an object having a functional implementation, but usually in a simplified form, just to satisfy the test (e.g., an in-memory database)
• Stub - an object with hardcoded behavior suitable for a given test (or a group of tests)
• Mock - an object with the ability to a) have a programmed expected behavior, and b) verify the interactions occurring in its lifetime (this object is usually created with the help of mocking framework)
• Spy - a mock created as a proxy to an existing real object; some methods can be stubbed, while the un- stubbed ones are forwarded to the covered object

Mockito is a mocking framework helpful in creating mocks and spies in a simple and intuitive way, while at the same time providing great control of the whole process.

Mockito artifacts are available in the Maven Central Repository (MCR). The easiest way to make MCR available in your project is to put the following configuration in your dependency manager:

Maven:

<dependency>
    <groupId>org.mockito</groupId>
    <artifactId>mockito‚àícore</artifactId>
    <version>1.9.0</version>
    <scope>test</scope>
    </dependency>

Gradle:

testCompile “org.mockito:mockito−core:1.9.0“

Ivy:

<dependency org=”org.mockito” name=”mockito-core” rev=”1.9.0” conf=”test->default”/>

It will add JAR with Mockito classes as well as all re- quired dependencies. Change 1.9.0 with the latest released version of Mockito.

This Refcard is based on the latest stable version 1.9.0. Some things are about to change in the further Mockito versions.

A mock can be created with the help of a static method mock():

Flower flowerMock=Mockito.mock(Flower.class);

But there's another option: use of @Mock annotation:

@Mock
    private Flower flowerMock;

Warning: If you want to use @Mock or any other Mockito annotations, it is required to call MockitoAnnotations.initMocks( testClass ) or use MockitoJUnit4Runner as a JUnit runner (see the annotation section below for more information).

One of the basic functions of mocking frameworks is an ability to return a given value when a specific method is called. It can be done using Mockito.when() in conjunction with thenReturn () . This process of defining how a given mock method should behave is called stubbing.

Warning: Note that the examples in this Refcardwere created to demonstrate behaviors of Mockito in a specific context. Of course, when writing the test for your codebase, there is no need to ensure that mocks are stubbed correctly.

package info.solidsoft.blog.refcard.mockito;

    import org.testng.annotations.Test;

    import static org.mockito.Mockito.mock;
    import static org.mockito.Mockito.when;
    import static org.testng.Assert.assertEquals;

    public class SimpleStubbingTest {
    public static final int TEST_NUMBER_OF_LEAFS = 5;

    @Test
    public void shouldReturnGivenValue() {
    Flower flowerMock = mock(Flower.class);
    when(flowerMock.getNumberOfLeafs()).thenReturn(TEST_NUMBER_OF_LEAFS);

    int numberOfLeafs = flowerMock.getNumberOfLeafs();

    assertEquals(numberOfLeafs, TEST_NUMBER_OF_LEAFS);
    }
    }

Mockito provides a family of functions for requesting specific behaviors ring.

Following an arrange-act-assert pattern (similar to given- when-then, from Behavior Driven Development) a test should be split into three parts (blocks), each with a specified responsibility.

This way, what is being tested, is clearly separated from the setup and verification parts. To integrate cleanly with Behavior Driven Development semantics Mockito contains BDDMockito class which introduces an alias given() which can be used instead of when() method while stubbing. Here's the previous example, but now using the BDD semantics:

import static org.mockito.BDDMockito.given;
    import static org.mockito.Mockito.mock;
    import static org.testng.Assert.assertEquals;

    @Test
    public void shouldReturnGivenValueUsingBDDSemantics() {
    //given
    Flower flowerMock = mock(Flower.class);
    given(flowerMock.getNumberOfLeafs()).willReturn(TEST_NUMBER_OF_LEAFS);

    //when
    int numberOfLeafs = flowerMock.getNumberOfLeafs();

    //then
    assertEquals(numberOfLeafs, TEST_NUMBER_OF_LEAFS);
    }

given-when-then comments make intentions of tests clearer.

Mockito, by default, compares arguments using equals () methods. Sometimes it's convenient to know exactly what parameter the method will be called with.

@Test
    public void shouldMatchSimpleArgument() {
    WateringScheduler schedulerMock = mock(WateringScheduler.class);
    given(schedulerMock.getNumberOfPlantsScheduledOnDate(WANTED_DATE)).willReturn(VALUE_FOR_WANTED_ARGUMENT);

    int numberForWantedArgument = schedulerMock.getNumberOfPlantsScheduledOnDate(WANTED_DATE);
    int numberForAnyOtherArgument = schedulerMock.getNumberOfPlantsScheduledOnDate(ANY_OTHER_DATE);

    assertEquals(numberForWantedArgument, VALUE_FOR_WANTED_ARGUMENT);
    assertEquals(numberForAnyOtherArgument, 0);  //default value for int
    }

Very often we needed to define a wider matching range. Mockito provides a set of build-in matchers defined in Matchers and AdditionalMatchers classes (see corresponding table).

given(plantSearcherMock.smellyMethod(anyInt(), contains("asparag"), eq("red"))).willReturn(true);
    //given(plantSearcherMock.smellyMethod(anyInt(), contains("asparag"), "red")).willReturn(true);
    //incorrect - would throw an exception

Luckily, in the last case, Mockito will protest by throwing a meaningful exception:

org.mockito.exceptions.misusing.InvalidUseOfMatchersException:
    Invalid use of argument matchers!
    3 matchers expected, 2 recorded.
    This exception may occur if matchers are combined with raw values:
    //incorrect:
    someMethod(anyObject(), "raw String");
    When using matchers, all arguments have to be provided by matchers.
    For example:
    //correct:
    someMethod(anyObject(), eq("String by matcher"));

    For more info see javadoc for Matchers class.

It is also possible to create a custom matcher by extending the ArgumentMatcher class and using it together with argThat ()

given(schedulerMock.getNumberOfPlantsScheduledOnDate(
    argThat(haveHourFieldEqualTo(7)))).willReturn(1);

    //with the util method to create a matcher
    private ArgumentMatcher

    haveHourFieldEqualTo(final int hour) {
        return new ArgumentMatcher

    () {
            @Override
            public boolean matches(Object argument) {
            return ((Date) argument).getHours() == hour;
            }
            };
            }

Table 1: Selected Mockito matchers

Sometimes you want to return different values for subsequent calls of the same method. Returned values can be mixed with exceptions. The last value/behavior is used for all following calls.

@Test
    public void shouldReturnLastDefinedValueConsistently() {
    WaterSource waterSource = mock(WaterSource.class);
    given(waterSource.getWaterPressure()).willReturn(3, 5);

    assertEquals(waterSource.getWaterPressure(), 3);
    assertEquals(waterSource.getWaterPressure(), 5);
    assertEquals(waterSource.getWaterPressure(), 5);
    }

As we've seen before, the stubbed method is passed as a parameter to a given / when method. This obviously means that you cannot use this construct for void methods. Instead, you should use willXXX..given or doXXX..when. See here:

@Test(expectedExceptions = WaterException.class)
    public void shouldStubVoidMethod() {
    WaterSource waterSourceMock = mock(WaterSource.class);
    doThrow(WaterException.class).when(waterSourceMock).doSelfCheck();
    //the same with BDD semantics
    //willThrow(WaterException.class).given(waterSourceMock).doSelfCheck();

    waterSourceMock.doSelfCheck();
    //exception expected
    }

do/willXXX methods family:

will/doXXX methods are also handy when working with spy objects, as will be seen in the following section.

A given / when construct allows Mockito to internally use the type returned by a stubbed method to provide a typed argument in the will/ thenReturn methods. Void is not a valid type of it causes a compilation error. Will/doReturn does not use that trick. Will/doReturn can be also used for stubbing non-void methods, though it is not recommended because it cannot detect wrong return method types at a compilation time (only an exception at runtime will be thrown).

//compilation error - int expected, not boolean
    //given(flowerMock.getNumberOfLeafs()).willReturn(true);

    //only runtime exception
    willReturn(true).given(flowerMock).getNumberOfLeafs();

In some rare cases it can be useful to implement a custom logic, later used on a stubbed method invocation. Mockito contains a generic Answer interface allowing the implementation of a callback method and providing access to invocation parameters (used arguments, a called method, and a mock instance).

@Test
    public void shouldReturnTheSameValue() {
    FlowerFilter filterMock = mock(FlowerFilter.class);
    given(filterMock.filterNoOfFlowers(anyInt())).will(returnFirstArgument());

    int filteredNoOfFlowers = filterMock.filterNoOfFlowers(TEST_NUMBER_OF_FLOWERS);

    assertEquals(filteredNoOfFlowers, TEST_NUMBER_OF_FLOWERS);
    }
    //reusable answer class
    public class ReturnFirstArgument implements Answer<Object> {
    @Override
    public Object answer(InvocationOnMock invocation) throws Throwable {
    Object[] arguments = invocation.getArguments();
    if (arguments.length == 0) {
    throw new MockitoException("...");
    }
    return arguments[0];
    }
    public static ReturnFirstArgument returnFirstArgument() {
    return new ReturnFirstArgument();
    }
    }

Warning: The need to use a custom answer may indicate that tested code is too complicated and should be re-factored.

Once created, a mock remembers all operations performed on it. Important from the SUT perspective, these operations can easily be verified. In the basic form, use Mockito. verify (T mock) on the mocked method.

WaterSourcewaterSourceMock=mock(WaterSource.class);

    waterSourceMock.doSelfCheck();

    verify(waterSourceMock).doSelfCheck();

By default, Mockito checks if a given method (with given arguments) was called once and only once. This can be modified using a VerificationMode. Mockito provides the number of very meaningful verification modes. It is also possible to create a custom verification mode.

verify(waterSourceMock,never()).doSelfCheck();
    verify(waterSourceMock,times(2)).getWaterPressure();
    verify(waterSourceMock,atLeast(1)).getWaterTemperature();

As an alternative to never (), which works only for the specified call, verifyZeroInteractions ( Object ... mocks) method can be used to verify no interaction with any method of the given mock(s). Additionally, there is one more method available, called verifyNoMoreInteractions ( Object ... mocks), which allows to ensure that no more interaction (except the already verified ones) was performed with the mock(s).verifyNoMoreInteractions can be useful in some cases, but shouldn't be overused by using on all mocks in every test. Unlike other mocking frameworks, Mockito does not automatically verify all stubbed calls. It is possible to do it manually, but usually it is just redundant. The tested code should mainly care about values returned by stubbed methods. If a stubbed method was not called, while being important from the test perspective, something else should break in a test. Mockito's philosophy allows the test writer to focus on interesting behaviors in the test for the SUT and his collaborators.

Mockito enables you to verify if interactions with a mock were performed in a given order using the InOrder API. It is possible to create a group of mocks and verify the call order of all calls within that group.

@Test
    publicvoidshouldVerifyInOrderThroughDifferentMocks(){
    WaterSourcewaterSource1=mock(WaterSource.class);
    WaterSourcewaterSource2=mock(WaterSource.class);

    waterSource1.doSelfCheck();
    waterSource2.getWaterPressure();
    waterSource1.getWaterTemperature();

    InOrderinOrder=inOrder(waterSource1,waterSource2);
    inOrder.verify(waterSource1).doSelfCheck();
    inOrder.verify(waterSource2).getWaterPressure();
    inOrder.verify(waterSource1).getWaterTemperature();
    }

Warning: The need to use a custom answer may indicate that tested code is too complicated and should be re-factored.

During a verification of an interaction, Mockito uses equals () methods on the passed arguments. This is usually enough. It is a l s o possible to use the standard matchers, described earlier about stubbing, as well as custom matchers. However, in some situations it may be helpful to keep the actual argument value to make custom assertions on it. Mockito offers an ArgumentCaptor class, enabling us to retrieve the argument passed to a mock.

//when
    flowerSearcherMock.findMatching(searchCriteria);

    //then
    ArgumentCaptor<SearchCriteria>captor=ArgumentCaptor.
    forClass(SearchCriteria.class);
    Verify(flowerSearcherMock).findMatching(captor.capture()
    );
    SearchCriteriausedSearchCriteria=captor.getValue();s
    assertEquals(usedSearchCriteria.getColor(),"yellow");s
    assertEquals(usedSearchCriteria.getNumberOfBuds(),3);

ArgumentCaptor can be also created using @Captor annotation (see appropriate section with annotations).

Warning: It is recommended to use ArgumentCaptor with verification, but not with stubbing. Creating and using a captor in two different test blocks can decrease test readability. In addition to a situation when a stubbed method is not called, no argument is captured, which can be confusing.

Warning: When an SUT internally uses the same object reference for multiple calls on a mock, every time changing its internal state (e.g., adding elements to the same list) captor . getAllValues () will return the same object in a state for the last call.

Mockito lets you verify interactions within a specified time frame. It causes a verify() method to wait for a specified period of time for a requested interaction rather than fail immediately if that had not already happened. It can be useful while testing multi-threaded systems.

@Test
    publicvoidshouldFailForLateCall(){
    WaterSourcewaterSourceMock=mock(WaterSource.class);
    Threadt=waitAndCallSelfCheck(40,waterSourceMock);

    t.start();

    verify(waterSourceMock,never()).doSelfCheck();
    try{
    verify(waterSourceMock,timeout(20)).doSelfCheck();
    fail("verificationshouldfail");
    }catch(MockitoAssertionErrore){
    //expected
    }
    }

Warning: Currently, verifying with timeout doesn't work with inOrder verification.

Warning: All the multi-thread tests can become non-deterministic (e.g., under heavy load).

With Mockito, you can use real objects instead of mocks by replacing only some of their methods with the stubbed ones. Usually there is no reason to spy on real objects, and it can be a sign of a code smell, but in some situations (like working with legacy code and IoC containers) it allows us to test things impossible to test with pure mocks.

@Test
    publicvoidshouldStubMethodAndCallRealNotStubbedMethod(){
    FlowerrealFlower=newFlower();
    realFlower.setNumberOfLeafs(ORIGINAL_NUMBER_OF_LEAFS);
    FlowerflowerSpy=spy(realFlower);
    willDoNothing().given(flowerSpy).setNumberOfLeafs(anyInt());

    flowerSpy.setNumberOfLeafs(NEW_NUMBER_OF_LEAFS);//stubbed‚àíshoulddonothing

    verify(flowerSpy).setNumberOfLeafs(NEW_NUMBER_OF_LEAFS);
    assertEquals(flowerSpy.getNumberOfLeafs(),ORIGINAL_NUMBER_OF_LEAFS);//valuewasnot
    changed
    }

Warning: While spying, Mockito creates a copy of a real object, and therefore all interactions should be passed using the created spy.

Mockito offers three annotations–@Mock, @Spy, @Captor– to simplify the process of creating relevant objects using static methods. @InjectMocks annotation simplifies mock and spy injection. It can inject objects using constructor injection, setter injection or field injection.

//with constructor: PlantWaterer(WaterSource waterSource,
    // WateringScheduler wateringScheduler) {...}

    public class MockInjectingTest {
    @Mock
    private WaterSource waterSourceMock;

    @Spy
    private WateringScheduler wateringSchedulerSpy;

    @InjectMocks
    private PlantWaterer plantWaterer;

    @BeforeMethod
    public void init() {
    MockitoAnnotations.initMocks(this);
    }

    @Test
    public void shouldInjectMocks() {
    assertNotNull(plantWaterer.getWaterSource());
    assertNotNull(plantWaterer.getWateringScheduler());
    }
    }

Mockito enables us to redefine a default value returned from non-stubbed methods

Warning: The last three default answers should not be needed when working with well-crafted, testable code. The behavior can be configured per mock during its creation or globally for all tests using GlobalConfiguration mechanism (it helps to use RETURNS_SMART_ NULLS by default).

PlantWatererplantWatererMock=mock(PlantWaterer.class,
    Mockito.RETURNS_DEEP_STUBS);
    given(plantWatererMock.getWaterSource().getWaterPressure()).willReturn(5);

    @Mock(answer=Answers.RETURNS_SMART_NULLS)
    privatePlantWatererplantWatererMock;

Sample verbose exception received using SmartNull:

org.mockito.exceptions.verification.SmartNullPointerException:
    You have a NullPointerException here:
    -> at PlantWaterer.generateNPE(PlantWaterer.java:24)
    because this method call was *not* stubbed correctly:
    -> at PlantWaterer.generateNPE(PlantWaterer.java:24)
    wateringScheduler.returnNull();

    at PlantWaterer.generateNPE(PlantWaterer.java:24)
    at DefaultValuesTest.shouldReturnNicerErrorMessageOnNPE(DefaultValuesTest.java:64)

In some rare cases (like using a mock as a bean in an IoC container) you may need to reset a mock using the Mockito. reset (T ... mocks) static method. This causes the mock to forget all previous behavior and interactions.

Warning: In most cases, using the reset method in a test is a code smell and should be avoided. Splitting a large test into smaller ones with mocks created for each can be helpful.

Mockito has a few limitations worth remembering. They are generally technical restrictions, but Mockito authors believe using hacks to work around them would encourage people to write poorly testable code. Mockito cannot:

• mock final classes
• mock enums
• mock final methods
• mock static methods
• mock private methods
• mock hashCode() and equals()

Nevertheless, when working with a badly written legacy code, remember that some of the mentioned limitations can be mitigated using the PowerMock or JMockit libraries.

• http://blog.solidsoft.info/mockito-docs/- official Mockito documentation (redirect to the current version)
• http://blog.solidsoft.info/beyond-the-mockito-refcard/ - a series of articles extending information contained In this Refcard