Tag Archives: junit

A dive into Mockito – 3

Part 3 - Partial Mocks using Mockito and Limitations of Mockito

  • Partial Mocks – with mocks and spies
  • Create a mock of the class
  • Spy the class
  • Which method to use – mock or spy?
  • Limitations of Mockito

Full Series

Partial Mocks – with mocks and spies

Partial mocks are created when only some methods of a class need to be stubbed, and the real methods must be called for others.

Partial mocks can be created in two ways:

  1. Create a mock of the class
    This would create mock implementation of all the methods. For calling the real method, we must use thenCallRealMethod()
  2. Spy the class
    This is the opposite of creating mocks. This will call real methods, unless otherwise specified by stubs.

Let us consider EmailManager.java having a method: String getEmail(String name)

Create a mock of the class

Partial mocks can be created in the following manner by creating mocks:

EmailManager managerMock = Mockito.mock(EmailManager.class);

This will call the real implementation of getEmail() method in the EmailManager. The mock implementations will be called for all other methods.

Spy the class

This is the method to create spies of real objects:

EmailManager manager = new EmailManager();
EmailManager managerSpy = Mockito.spy(manager);

This will call the stubbed implementation of getEmail() method in the EmailManager. The real implementation will be called for all other methods

Exception when spying:

While spying, remember that the actual methods are called. This can cause a lot of side effects, and sometimes Exceptions.

The primary reason for this is that the method call inside when() actually gets executed. So, even if the method is stubbed, it can internally call another method of the class which might not be stubbed, and the real method will be called.

The below code would fail:

List list = new LinkedList();
List spy = Mockito.spy(list);

A closer look at the stacktrace shows something like this:

Exception in thread "main" java.lang.IndexOutOfBoundsException: Index: 0, Size: 0
  at java.util.LinkedList.entry(LinkedList.java:368)
  at java.util.LinkedList.get(LinkedList.java:313)

The get(0) internally calls entry() method(), which is not stubbed, causing the real method to be called.

To prevent this error, an alternative syntax must be used:


Which method to use – mock or spy?

To decide which method to use, take a look at how many methods need to be stubbed.

  • If more real methods need to be called, then use spy.
  • Otherwise, use mock

New, well-designed and test-driven code should not use partial mocks. Using partial mocks means that the actual code is badly designed. But that should not stop us from writing unit tests.

A bad unit test is better than no unit test.

Limitations of Mockito

Mockito has a few limitations

  • Cannot mock final classes
  • Cannot mock static methods
  • Cannot mock final methods – their real behaviour is executed without any exception. Mockito cannot warn you about mocking final methods, so be vigilant.
  • Cannot mock equals(), hashCode()

Future articles in the series:

  • Part 4 – Some complex examples and common mistakes in using Mockito

A dive into Mockito – 2

Part 2 – Argument Matchers and Stubbing Void Methods

  • Using Argument Matchers
  • A simple use of any() matcher
  • Writing Custom Argument Matchers
  • Stubbing void methods

Full Series

Using Argument Matchers

Mockito argument matchers can be used to specify the method arguments for stubbing. If the method has arguments but you really don’t care what gets passed or cannot predict it, argument matchers can be used.

Argument Matchers provide us a lot of flexibility in stubbing.

Argument Matchers use the equals() method for comparisons.

A simple use of any() matcher

Let us assume we have an EmailService that is trying to save an email ID using an EmailManager. This means that the save() method of the EmailService calls the save() method of the EmailManager, which in turn saves the email to the database. A simple unit test for the save() method of the EmailService can test whether the save() on the Manager was called.

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

public class EmailServiceTest {
	public void testSaveEmail() {
		EmailManager manager = Mockito.mock(EmailManager.class);
		EmailService service = new EmailService(manager);



Note the use of the argument matcher in line 12:


We used the matcher because, we really do not care what parameter was passed on the save().

The generic any() matcher:

Mockito provides a matcher that matches any Object. Some valid variants of the above are:


The cast from Object to String is required in the first line, the lack of which will cause compilation error.

The second line shown above is most useful when we need to match custom classes we create in our application.
Assuming save() method saves a Person object, we would write:


While using argument matchers in stubbing, all arguments of a method must be matchers.

If our save method also takes in an integer, this line would throw error:

Mockito.verify(manager).save(12, Mockito.any(Person.class));

To fix the above scenario, we can write:

Mockito.verify(manager).save(Mockito.eq(12), Mockito.any(Person.class));

Some common Argument Matchers

any() or any(Class clazz)Any object (or object of a given class) or null

Any object of given type or null
eqDefault Matcher. Compares using equals() method
sameObject argument that is the same as the given value. Compares using ==
refEqObject argument that is reflection-equal to the given value. This Matcher can be used when equals() is not implemented on compared objects. Matcher uses Java reflection API to compare fields of wanted and actual objects.
Argument is null/not null
contains(String substring)
matches(String regex)
endsWith(String suffix)
startsWith(String prefix)
String-specific argument matchers

Writing Custom Argument Matchers

Consider this scenario. We have the following classes:

  • Person – A class with two variables – name, email id
  • PersonService – A service class that creates a Person object and calls savePerson() method on the manager
  • PersonManager – The manager class that needs to be mocked

We need to write test case for personLogic() method in PersonService


public class PersonService {
	private final PersonManager manager;

	public PersonService(PersonManager manager) {
		this.manager = manager;

	public void personLogic() {
		Person person = new Person("Karthik", "abc@xyz.com");

The below test will fail:

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

public class PersonServiceTest {
	public void testPersonLogic() {
		PersonManager manager = Mockito.mock(PersonManager.class);
		PersonService service = new PersonService(manager);


		Person verifyPerson = new Person("Karthik", "abc@xyz.com");

Reason: The expected object, with reference verifyPerson is entirely different from the actual object, and the default equals() comparison which argument matchers use, fails.

Use Custom Argument Matcher

In this scenario, using custom argument matchers come in handy.

Lines 12 and 13 above can be replaced with this:

				Mockito.argThat(new ArgumentMatcher() {

					public boolean matches(Object argument) {
						Person person = (Person) argument;
						return person.getName().equals("Karthik")
								&& person.getEmail().equals("abc@xyz.com") ? true
								: false;

Alternate Solution – 1

Use the refEq matcher to compare the fields of the objects in hand:

Person verifyPerson = new Person("Karthik", "abc@xyz.com");

Alternate Solution – 2

Override the equals() method in Person class to compare objects.

Stubbing void methods

We face a lot of scenarios in our applications where we need to stub void methods. With void methods, stubbing is slightly different. We cannot use void methods in the when() call.

The alternative syntax is:


We can also test exception handling. How would my code handle if an exception is thrown by void methods

Mockito.doThrow(new RuntimeException()).when(mock).setAttribute(Mockito.anyString());

We can also set the behaviour of a void method to do nothing when called. This is not possible for non-void methods:


Why when() does not work for void methods:

Take a look at the syntax of when():


Notice that the method is called inside brackets. Compiler does not like void method inside brackets!!

The two ‘when()’ are different syntax.

Notice that the when() for a non-void method stubbing takes in a methodCall as parameter, whereas the when() in void method stubbing takes in a mock object.

doPrimarily used for stubbing void methods
doThrow(Throwable... throwables).when(mock).method();
doReturn(T value).when(mock).method();
doAnswer(Answer answer).when(mock).method();

Future articles in the series:

  • Part 3 – Partial Mocks using Mockito and Limitations of Mockito
  • Part 4 – Some complex examples and common mistakes in using Mockito

A dive into Mockito – 1

Part 1 – Getting Started with Mockito

  • Mocking Frameworks – Mockito
  • A simple use of Mockito
  • Verifying Interactions with the mock
  • Stubbing methods using when()

Full Series

Mocking Frameworks

Mocking Frameworks allow us to test the code you want, without its dependencies. In a unit test, mock objects can simulate the behaviour of complex objects that are impractical or impossible to incorporate into a unit test.

Mock objects isolate the unit of code you are testing.


This is a framework used for test stubbing and interaction testing.

Let us assume that we have a simple servlet, and a method testCheckFormat() that checks the format of the email id submitted in the HTML form. It is impractical to create an actual HttpServletRequest object for unit test. So we mock this object, in order to test the code we are interested in.

A simple use of Mockito

import javax.servlet.http.HttpServletRequest;
import org.junit.Assert;
import org.junit.Test;
import org.mockito.Mockito;

public class ServletFacadeTest {

	public void testCheckFormat() {
		HttpServletRequest requestMock = Mockito.mock(HttpServletRequest.class);

		ServletFacade servletFacade = new ServletFacade();
		boolean result = servletFacade.checkFormat(requestMock);

		Assert.assertEquals(true, result);

There are three fundamental steps in writing JUnit test cases with Mockito:

  1. Create a mock of the object on which the method under test is dependent [line 10]
  2. Specify behaviors on the mock object (stub) [line 11]
  3. Assert/verify test scenario [lines 14-16]

Verifying Interactions with the mock

One of the most important capabilities of Mockito is to verify whether the mock had any interactions. A mock object will remember all its invocations.

The following test would have passed in the above case:


It is also easy to ensure that there were no more interactions with our mock apart from getting the email:


Let’s assume that our logic now tries retrieving two details from the request, namely email and name. To test two invocations, we now can use:


We can write this slightly differently, using argument matchers provided by Mockito:

Mockito.verify(requestMock, Mockito.times(2)).getAttribute(Mockito.anyString());

Verifying a stubbed invocation is unnecessary and often leads to just redundant tests!

verify( T mock).someMethod();
verify( T mock, times(int n) ).someMethod();
verify( T mock, atLeast(int n) ).someMethod();
verify( T mock, atLeastOnce() ).someMethod();
verify( T mock, atMost(int n) ).someMethod();
verify( T mock, never() ).someMethod();
Verify that someMethod was called specified number of times. Default is once.
verifyZeroInteractions( Object... mocks );Verify that there have been no method calls on the mock object(s)
verifyNoMoreInteractions( Object... mocks )Verify that there have been no additional method calls on the mock object(s) besides what has already been verified.

Stubbing methods using when()

Stubbing is used to specify method behaviors on mock objects.
We have already seen an example of stubbing:


Argument matchers can be used in stubs to match the invocation with any specified argument:
In the below example, any invocation of getAttribute() with a String parameter will return the string “xyz@abc.com”.


Sometimes, we might have to test exception handling. How would my code handle if an exception is thrown by getAttribute()

Mockito.when(requestMock.getAttribute(Mockito.anyString())).thenThrow(new RuntimeException());

when() would work only for non-void methods!

whenOnly for non-void methods
when( mock.method() ).thenCallRealMethod();
when( mock.method() ).thenReturn(T value);
when( mock.method() ).thenReturn(T value, T... values);
when( mock.method() ).thenThrow(Throwable... throwables);
when( mock.method() ).thenAnswer(Answer answer);

This should get you started with some basic unit tests with Mockito. Have fun !! :)

Future articles in the series:

  • Part 2 – Argument Matchers and stubbing void methods
  • Part 3 – Partial Mocks using Mockito and Limitations of Mockito
  • Part 4 – Some complex examples and common mistakes in using Mockito