Analogy
A mock object can be useful to the software tester like a car designer uses aMotivation
In a unit test, mock objects can simulate the behavior of complex, real objects and are therefore useful when a real object is impractical or impossible to incorporate into a unit test. If an object has any of the following characteristics, it may be useful to use a mock object in its place: * it supplies non-deterministic results (e.g. the current time or the current temperature) * it has states that are difficult to create or reproduce (e.g. a network error) * it is slow (e.g. a completeTechnical details
Mock objects have the same interface as the real objects they mimic, allowing a client object to remain unaware of whether it is using a real object or a mock object. Many available mock object frameworks allow the programmer to specify which methods will be invoked on a mock object, in what order, what parameters will be passed to them, and what values will be returned. Thus, the behavior of a complex object such as a network socket can be mimicked by a mock object, allowing the programmer to discover whether the object being tested responds appropriately to the wide variety of states such mock objects may be in.Mocks, fakes or stubs
The definitions of mock, fake and stub are not consistent across the literature. Nonetheless, all represent a production object in a testing environment by exposing the same interface. Regardless of name, the simplest form returns pre-arranged responses (as in a method stub) and the most complex form imitates a production object's complete logic. Such a test object might contain assertions to examine the context of each call. For example, a mock object might assert the order in which its methods are called, or assert consistency of data across method calls. In the book '' The Art of Unit Testing'' mocks are described as a fake object that helps decide whether a test failed or passed by verifying whether an interaction with an object occurred. Everything else is defined as a stub. In that book, ''fakes'' are anything that is not real, which, based on their usage, can be either ''stubs'' or ''mocks''.Setting expectations
Consider an example where an authorization subsystem has been mocked. The mock object implements anisUserAllowed(task : Task) : boolean method to match that in the real authorization class. Many advantages follow if it also exposes an isAllowed : boolean property, which is not present in the real class. This allows test code to easily set the expectation that a user will, or will not, be granted permission in the next call and therefore to readily test the behavior of the rest of the system in either case.
Similarly, mock-only settings could ensure that subsequent calls to the sub-system will cause it to throw an exception, hang without responding, or return null
Null may refer to:
Science, technology, and mathematics Astronomy
*Nuller, an optical tool using interferometry to block certain sources of light Computing
*Null (SQL) (or NULL), a special marker and keyword in SQL indicating that a data value do ...
etc. Thus, it is possible to develop and test client behaviors for realistic fault conditions in back-end sub-systems, as well as for their expected responses. Without such a simple and flexible mock system, testing each of these situations may be too laborious for them to be given proper consideration.
Writing log strings
A mock database object'ssave(person : Person) method may not contain much (if any) implementation code. It might check the existence and perhaps the validity of the Person object passed in for saving (see fake vs. mock discussion above), but beyond that there might be no other implementation.
This is a missed opportunity. The mock method could add an entry to a public log string. The entry need be no more than "Person saved", or it may include some details from the person object instance, such as a name or ID. If the test code also checks the final contents of the log string after various series of operations involving the mock database, then it is possible to verify that in each case exactly the expected number of database saves have been performed. This can find otherwise invisible performance-sapping bugs, for example, where a developer, nervous of losing data, has coded repeated calls to save() where just one would have sufficed.
Use in test-driven development
Programmers working with theLimitations
The use of mock objects can closely couple the unit tests to the implementation of the code that is being tested. For example, many mock object frameworks allow the developer to check the order of and number of times that mock object methods were invoked by the real object being tested; subsequent refactoring of the code that is being tested could therefore cause the test to fail even though all mocked object methods still obey the contract of the previous implementation. This illustrates that unit tests should test a method's external behavior rather than its internal implementation. Over-use of mock objects as part of a suite of unit tests can result in a dramatic increase in the amount of maintenance that needs to be performed on the tests themselves during system evolution as refactoring takes place. The improper maintenance of such tests during evolution could allow bugs to be missed that would otherwise be caught by unit tests that use instances of real classes. Conversely, simply mocking one method might require far less configuration than setting up an entire real class and therefore reduce maintenance needs. Mock objects have to accurately model the behavior of the object they are mocking, which can be difficult to achieve if the object being mocked comes from another developer or project or if it has not even been written yet. If the behavior is not modelled correctly, then the unit tests may register a pass even though a failure would occur at run time under the same conditions that the unit test is exercising, thus rendering the unit test inaccurate.See also
*References
External links
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