Home > Articles > Software Development & Management

  • Print
  • + Share This
This chapter is from the book Mock Object

Mock Object

How do we implement Behavior Verification for indirect outputs of the SUT?

How can we verify logic independently when it depends on indirect inputs from other software components?

We replace an object on which the SUT depends on with a test-specific object that verifies it is being used correctly by the SUT.

In many circumstances, the environment or context in which the SUT operates very much influences the behavior of the SUT. In other cases, we must peer “inside”[2] the SUT to determine whether the expected behavior has occurred.

A Mock Object is a powerful way to implement Behavior Verification while avoiding Test Code Duplication between similar tests. It works by delegating the job of verifying the indirect outputs of the SUT entirely to a Test Double.

How It Works

First, we define a Mock Object that implements the same interface as an object on which the SUT depends. Then, during the test, we configure the Mock Object with the values with which it should respond to the SUT and the method calls (complete with expected arguments) it should expect from the SUT. Before exercising the SUT, we install the Mock Object so that the SUT uses it instead of the real implementation. When called during SUT execution, the Mock Object compares the actual arguments received with the expected arguments using Equality Assertions (see Assertion Method ) and fails the test if they don’t match. The test need not make any assertions at all!

When to Use It

We can use a Mock Object as an observation point when we need to do Behavior Verification to avoid having an Untested Requirement (see Production Bugs on page 268) caused by our inability to observe the side effects of invoking methods on the SUT. This pattern is commonly used during endoscopic testing [ET] or need-driven development [MRNO]. Although we don’t need to use a Mock Object when we are doing State Verification , we might use a Test Stub or Fake Object. Note that test drivers have found other uses for the Mock Object toolkits, but many of these are actually examples of using a Test Stub rather than a Mock Object.

To use a Mock Object, we must be able to predict the values of most or all arguments of the method calls before we exercise the SUT. We should not use a Mock Object if a failed assertion cannot be reported back to the Test Runner effectively. This may be the case if the SUT runs inside a container that catches and eats all exceptions. In these circumstances, we may be better off using a Test Spy instead.

Mock Objects (especially those created using dynamic mocking tools) often use the equals methods of the various objects being compared. If our test-specific equality differs from how the SUT would interpret equals, we may not be able to use a Mock Object or we may be forced to add an equals method where we didn’t need one. This smell is called Equality Pollution (see Test Logic in Production). Some implementations of Mock Objects avoid this problem by allowing us to specify the “comparator” to be used in the Equality Assertions.

Mock Objects can be either “strict” or “lenient” (sometimes called “nice”). A “strict” Mock Object fails the test if the calls are received in a different order than was specified when the Mock Object was programmed. A “lenient” Mock Object tolerates out-of-order calls.

Implementation Notes

Tests written using Mock Objects look different from more traditional tests because all the expected behavior must be specified before the SUT is exercised. This makes the tests harder to write and to understand for test automation neophytes. This factor may be enough to cause us to prefer writing our tests using Test Spies.

The standard Four-Phase Test is altered somewhat when we use Mock Objects. In particular, the fixture setup phase of the test is broken down into three specific activities and the result verification phase more or less disappears, except for the possible presence of a call to the “final verification” method at the end of the test.

Fixture setup:

  • Test constructs Mock Object.
  • Test configures Mock Object. This step is omitted for Hard-Coded Test Doubles.
  • Test installs Mock Object into SUT.

Exercise SUT:

  • SUT calls Mock Object; Mock Object does assertions.

Result verification:

  • Test calls “final verification” method.

Fixture teardown:

  • No impact.

Let’s examine these differences a bit more closely:


As part of the fixture setup phase of our Four-Phase Test, we must construct the Mock Object that we will use to replace the substitutable dependency. Depending on which tools are available in our programming language, we can either build the Mock Object class manually, use a code generator to create a Mock Object class, or use a dynamically generated Mock Object.

Configuration with Expected Values

Because the Mock Object toolkits available in many members of the xUnit family typically create Configurable Mock Objects , we need to configure the Mock Object with the expected method calls (and their parameters) as well as the values to be returned by any functions. (Some Mock Object frameworks allow us to disable verification of the method calls or just their parameters.) We typically perform this configuration before we install the Test Double.

This step is not needed when we are using a Hard-Coded Test Double such as an Inner Test Double (see Hard-Coded Test Double).


Of course, we must have a way of installing a Test Double into the SUT to be able to use a Mock Object. We can use whichever substitutable dependency pattern the SUT supports. A common approach in the test-driven development community is Dependency Injection ; more traditional developers may favor Dependency Lookup.


When the SUT calls the methods of the Mock Object, these methods compare the method call (method name plus arguments) with the expectations. If the method call is unexpected or the arguments are incorrect, the assertion fails the test immediately. If the call is expected but came out of sequence, a strict Mock Object fails the test immediately; by contrast, a lenient Mock Object notes that the call was received and carries on. Missed calls are detected when the final verification method is called.

If the method call has any outgoing parameters or return values, the Mock Object needs to return or update something to allow the SUT to continue executing the test scenario. This behavior may be either hard-coded or configured at the same time as the expectations. This behavior is the same as for Test Stubs, except that we typically return happy path values.

Final Verification

Most of the result verification occurs inside the Mock Object as it is called by the SUT. The Mock Object will fail the test if the methods are called with the wrong arguments or if methods are called unexpectedly. But what happens if the expected method calls are never received by the Mock Object? The Mock Object may have trouble detecting that the test is over and it is time to check for unfulfilled expectations. Therefore, we need to ensure that the final verification method is called. Some Mock Object toolkits have found a way to invoke this method automatically by including the call in the tearDown method.[3] Many other toolkits require us to remember to call the final verification method ourselves.

Motivating Example

The following test verifies the basic functionality of creating a flight. But it does not verify the indirect outputs of the SUT—namely, the SUT is expected to log each time a flight is created along with the date/time and username of the requester.

   public void testRemoveFlight() throws Exception {
      // setup
      FlightDto expectedFlightDto = createARegisteredFlight();
      FlightManagementFacade facade = new FlightManagementFacadeImpl();
      // exercise
      // verify
      assertFalse("flight should not exist after being removed",
                  facade.flightExists( expectedFlightDto.

Refactoring Notes

Verification of indirect outputs can be added to existing tests by using a Replace Dependency with Test Double refactoring. This involves adding code to the fixture setup logic of our test to create the Mock Object; configuring the Mock Object with the expected method calls, arguments, and values to be returned; and installing it using whatever substitutable dependency mechanism is provided by the SUT. At the end of the test, we add a call to the final verification method if our Mock Object framework requires one.

Example: Mock Object (Hand-Coded)

In this improved version of the test, mockLog is our Mock Object. The method setExpectedLogMessage is used to program it with the expected log message. The statement facade.setAuditLog(mockLog) installs the Mock Object using the Setter Injection (see Dependency Injection) test double-installation pattern. Finally, the verify() method ensures that the call to logMessage() was actually made.

   public void testRemoveFlight_Mock() throws Exception {
      // fixture setup
      FlightDto expectedFlightDto = createAnonRegFlight();
      // mock configuration
      ConfigurableMockAuditLog mockLog =
         new ConfigurableMockAuditLog();
      // mock installation
      FlightManagementFacade facade = new FlightManagementFacadeImpl();
      // exercise
      // verify
      assertFalse("flight still exists after being removed",
                  facade.flightExists( expectedFlightDto.

This approach was made possible by use of the following Mock Object. Here we have chosen to use a hand-built Mock Object. In the interest of space, just the logMessage method is shown:

   public void logMessage( Date actualDate,
                           String actualUser,
                           String actualActionCode,
                           Object actualDetail) {

      Assert.assertEquals("date", expectedDate, actualDate);
      Assert.assertEquals("user", expectedUser, actualUser);
      Assert.assertEquals("action code",
      Assert.assertEquals("detail", expectedDetail,actualDetail);

The Assertion Methods are called as static methods. In JUnit, this approach is required because the Mock Object is not a subclass of TestCase; thus it does not inherit the assertion methods from Assert. Other members of the xUnit family may provide different mechanisms to access the Assertion Methods. For example, NUnit provides them only as static methods on the Assert class, so even Test Methods need to access the Assertion Methods this way. Test::Unit, the xUnit family member for the Ruby programming language, provides them as mixins; as a consequence, they can be called in the normal fashion.

Example: Mock Object (Dynamically Generated)

The last example used a hand-coded Mock Object. Most members of the xUnit family, however, have dynamic Mock Object frameworks available. Here’s the same test rewritten using JMock:

   public void testRemoveFlight_JMock() throws Exception {
      // fixture setup
      FlightDto expectedFlightDto = createAnonRegFlight();
      FlightManagementFacade facade = new FlightManagementFacadeImpl();
      // mock configuration
      Mock mockLog = mock(AuditLog.class);
      // mock installation
      facade.setAuditLog((AuditLog) mockLog.proxy());
      // exercise
      // verify
      assertFalse("flight still exists after being removed",
                  facade.flightExists( expectedFlightDto.
      // verify() method called automatically by JMock

Note how JMock provides a “fluent” Configuration Interface (see Configurable Test Double) that allows us to specify the expected method calls in a fairly readable fashion. JMock also allows us to specify the comparator to be used by the assertions; in this case, the calls to eq cause the default equals method to be called.

Further Reading

Almost every book on automated testing using xUnit has something to say about Mock Objects, so I won’t list those resources here. As you are reading other books, keep in mind that the term Mock Object is often used to refer to a Test Stub and sometimes even to Fake Objects. Mocks, Fakes, Stubs, and Dummies (in Appendix B) contains a more thorough comparison of the terminology used in various books and articles.

  • + Share This
  • 🔖 Save To Your Account

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020