DAIMI(c) Henrik Bærbak Christensen1 Test Doubles: Stubs, Spies, Mocks, etc.

DAIMI(c) Henrik Bærbak Christensen2 Motivation Thorough testing requires objects to be tested in isolation – to create a harness/environment where defects/complexity in other objects do not invalidate/complicate our testing. We have discussed drivers and stubs for this purpose. –The stub notion is actually rather broad –A classic natural science approach: Define terminology much more precisely Get a deeper insight into the subject matter as we analyze and clarify terminology...

DAIMI(c) Henrik Bærbak Christensen3 Stubs and Stubs? Definition 1 (My book) –Focus: Just get the TDD iteration 1 cycle to compile and get a red bar... Definition 2 (Burnstein) –Stub: auxiliary code representing modules a UUT calls. “display a message that it has been called; return a value from a table,...”

DAIMI(c) Henrik Bærbak Christensen4 Stubs and Stubs Thus stubs is used in different contexts to mean different things –“minimally compilable” or “do something test related” In practice there are a lot of different things you would like a stub to do. Gerard Meszaros defines a clearer terminology by classifying the various uses of stubs...

DAIMI(c) Henrik Bærbak Christensen5 xUnit Pattern: Test Double “Superclass”: Test Double –SUT: System under test (=UUT) –DOC: Depended-on Component When? –Slow tests –DOC is not available not under test control has side-effects Compare :UUT :driver :stub 1. 2.

DAIMI(c) Henrik Bærbak Christensen6 Terminology indirect output –the output a UUT generates, not visible by our driver, but passed as parameters, protocols used, etc., to the DOCs indirect input –the input a Unit Under Test receives, not by parameter passing, instance variables, etc., but from results computed by DOCs. Indirect Input Indirect Output

DAIMI(c) Henrik Bærbak Christensen7 Test Double Solution: –Replace DOC with a double like stunt doubles in movies... –Requires: that this is possible!!! GoF’s 1st principle: Program to an interface... - Parameterize object creation!

DAIMI(c) Henrik Bærbak Christensen8 Double classification Meszaros classify several types of doubles according to the specific testing perspective

DAIMI(c) Henrik Bærbak Christensen9 Test Stub

DAIMI(c) Henrik Bærbak Christensen10 Test Stub Context –In many circumstances, the environment or context in which the system under test (SUT) operates very much influences the behavior of the SUT. To get good enough control over the indirect inputs of the SUT, we may have to replace some of the context with something we can control, a Test Stub. system under test (SUT)SUT indirect inputsSUT Examples: –?

DAIMI(c) Henrik Bærbak Christensen11 Test Stub Examples Typical examples are –Stubbing sensors or hardware In a meteorological system it is important to test wind calculations over north when the wind direction changes from 359 degrees to 0 degrees. –Stubbing random behaviour A dice must be put under test control

DAIMI(c) Henrik Bærbak Christensen12 Stub variations Responder –used to inject valid indirect inputs: happy paths Saboteur –used to inject invalid indirect inputs Temporary Test Stub –a stand in for a not-yet-implemented DOC – the first TDD production code implementation is always of this kind. this is what I called a stub in my book... Entity Chain Snipping –replace a network of objects with a single one

DAIMI(c) Henrik Bærbak Christensen13 Test Stub Conclusion: –the primary purpose of the stub is –to control the UUT’s input space that is –we get testing control over the input space + environment in order to specify test cases = (input, environment, expected output). –usually has methods/means for the test to specify the returned indirect inputs

DAIMI(c) Henrik Bærbak Christensen14 Test Spy

DAIMI(c) Henrik Bærbak Christensen15 Test Spy Context: –In many circumstances, the environment or context in which the SUT operates very much influences the behavior of the SUT. To get good enough visibility of the indirect outputs of the SUT, we may have to replace some of the context with something we can use to capture these outputs of the SUT. SUT indirect outputsSUT

DAIMI(c) Henrik Bærbak Christensen16 Test Spy Examples A Test Spy can –record the parameters passed to it verify indirect computed output equals expected –the order in which DOC methods were called verify the protocol between UUT and DOC A Test Spy does not fail, it merely records interaction. The Spy is inspected after the test execution in order to verify that indirect output was correct.

DAIMI(c) Henrik Bærbak Christensen17 Implementation notes Test Spy inspection variations: Retrieval interface: –The spy must have additional methods to extract the stored indirect output Self Shunt: –The test case class itself implements the DOC interface and is thus feed the indirect output directly to be cached in local variables Inner Test Double –use an inner anonymous class as self shunt

DAIMI(c) Henrik Bærbak Christensen18 Examples A classic example is an abstraction that communicates state changes via the Observer pattern –observer notification is an (important!) side-effect of state-changing method calls, but it is not externally visible: it is indirect output –register a SpyListener that counts the number of observer updates received.

DAIMI(c) Henrik Bærbak Christensen19 Examples

DAIMI(c) Henrik Bærbak Christensen20 Examples Gerry: An artificial Backgammon player –for all valid moves given board and dice make move, compute value of board if (value > bestvalue) { remember this move; } But does it compute the proper moves? Is it really the best move that is taken? movehook.considerMove(move); normally considerMove is the empty method.

DAIMI(c) Henrik Bærbak Christensen21 Fake Object

DAIMI(c) Henrik Bærbak Christensen22 Fake Object Context: –The SUT often depend on other components or systems. The interactions with these other components may be necessary but the side-effects of these interactions as implemented by the real depended- on component (DOC), may be unnecessary or even detrimental. A Fake Object is a much simpler and lighter weight implementation of the functionality provided by the DOC without the side effects we choose to do without.SUTdepended- on component (DOC)DOC

DAIMI(c) Henrik Bærbak Christensen23 Fake Object Stub versus Fake Object? –Fake Object has “realistic” behaviour –Fake Object is not instrumented with the indirect inputs, it defines them itself. –Less focus on testing aspects of the SUT, more focus on making it work.

DAIMI(c) Henrik Bærbak Christensen24 Examples Fake Database –replace database with in-memory HashTables In-Memory Database –semi-real database but not disk-based Fake Web Service –hard-coded or table-driven web server Fake Service Layer –fake the domain layer

DAIMI(c) Henrik Bærbak Christensen25 Examples The TestGame uses a LargeAreaTestImpl instance that is a fake object (fake domain layer) of an Area.

DAIMI(c) Henrik Bærbak Christensen26 Mock Object

DAIMI(c) Henrik Bærbak Christensen27 Mock Object Context: –A test double that verifies the indirect outputs

DAIMI(c) Henrik Bærbak Christensen28 Mock Object Workings Mock objects are somewhat more complex to define but are powerful to verify UUT behaviour with respect to the DOC. –Define Mock object with same interface as DOC –Configure mock with expectations values to return (like test stub) the methods that must be called –including sequence/protocol and call count –expected parameters –The mock will fail if these expectations are not met fail fast! –Thus test driver needs not verify anything!

DAIMI(c) Henrik Bærbak Christensen29 Mock Test appearance Thus test cases using mock objects look rather different than traditional JUnit tests. Example: –TestGame needs to define 5 Loader doubles (Area-, Actor-, etc.) to test the Game implementation in isolation. Exercise: Are they Stubs, Fake Objects, Spies, Mocks?

DAIMI(c) Henrik Bærbak Christensen30 Example Assume that we want to ensure that Game calls the ActorLoader in the proper way (verify indirect outputs): –That load is invoked with valid argument only once –That getHero is invoked one or several times –That getHero is not invoked before load –And we do not want to implement all the house keeping ourselves!

DAIMI(c) Henrik Bærbak Christensen31 jMock jMock will generate Mock Objects dynamically! You simply ask it to –generate an object given any interface –tell it which method + parameters to expect –tell it what objects/values to return and off you go!

DAIMI(c) Henrik Bærbak Christensen32 ActorLoader Mocked

DAIMI(c) Henrik Bærbak Christensen33 Example Expect method “load” to be executed once only, with a BufferedReader as parameter, and return the string “Actor loaded” when invoked. Expect method “getHero” to be executed at least once, with no parameters, after load has been executed, and return the hero object reference.

DAIMI(c) Henrik Bærbak Christensen34 Mock types Mock objects may use strict or lenient policy for protocol checking: –strict: All methods must be invoked in exact same sequence as when mock is “programmed” (expectations set) –lenient (nice): Methods may be invoked in any sequence. jMock defaults to lenient but the after method allows protocol conventions to be checked, strict policy.

DAIMI(c) Henrik Bærbak Christensen35 Considerations Mock objects must be programmed in advance –thus we must be able to predict UUT indirect output in advance – a hard-core whitebox requirement... Mock objects are responsible for failing –thus exceptions thrown must be able to pass out of the UUT may not be possible if it is embedded in a EJB container or similar...

DAIMI(c) Henrik Bærbak Christensen36 TDD with Mocking Freeman et al. argue that Mock object TDD unit tests follow a certain template/pattern: –create mock instance –set expectations –invoke UUT with mock –verify mock consistency

DAIMI(c) Henrik Bærbak Christensen37 Mock Roles, Not Objects I believe in designs based upon the principles of –Role and responsibility driven designs think roles with sets of responsibilities and protocols of collaboration –Compositional designs behaviour arise from highly focused objects collaborating which leads to –shallow class hierarchies –strong focus on interfaces for expressing roles –factor out object creation Freeman et al. comes to the same conclusions...

DAIMI(c) Henrik Bærbak Christensen38 Mocks out there Many applications use third-party API’s that would be nice to mock: –Databases: A Mocked JDBC –WebServers: A Mocked service API

DAIMI(c) Henrik Bærbak Christensen39 Example: JDBC Mocking public void testTransferOk() throws SQLException { prepareResultSet(); Bank bank = new Bank(); bank.connect(); bank.transfer(1, 2, 5000); bank.disconnect(); verifySQLStatementExecuted("select balance"); verifySQLStatementExecuted("update account"); verifySQLStatementParameter("update account", 0, 1, new Integer(-5000)); verifySQLStatementParameter("update account", 0, 2, new Integer(1)); verifySQLStatementParameter("update account", 1, 1, new Integer(5000)); verifySQLStatementParameter("update account", 1, 2, new Integer(2)); verifyCommitted(); verifyNotRolledBack(); verifyAllResultSetsClosed(); verifyAllStatementsClosed(); verifyConnectionClosed(); } MockRunner provides mock objects for a JDBC connection as well as J2EE abstractions.

DAIMI(c) Henrik Bærbak Christensen40 Summary Testing units in isolation is important –unit and integration testing –test-driven development Units have more inputs and outputs than visible from the parameter list and instances variables –especially true in object-oriented programming –indirect inputs: data from DOCs –indirect output: data to DOCs

DAIMI(c) Henrik Bærbak Christensen41 Summary Test Doubles allow you to get access to, inspect, and verify indirect input and output Stub: focus on indirect input Spy: focus on indirect output (record/verify) Mock: focus on indirect output (fail fast) –frameworks to generate doubles dynamically Fake object: light-weight semi-realistic behaviour