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• What are Mock Objects • Case Study • Mock Objects pattern • Why use Mock Objects? Błaej Pietrzak [email protected] • Mock Objects Limitations • Mock Objects Frameworks
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„A Mock Object is a substitute • Mock Objects is a development technique that: – Lets you unit test classes that you didn’t think you could implementation to emulate or instrument – Helps you write better code whilst doing so
other domain code. It should be simpler • Mock Objects is based on two key concepts: – Replace everything except the code under test with mock than the real code, not duplicate its implementations that emulate the rest of the environment implementation, and allow you to – Put the test assertions inside those mock implementations so that you can validate the interactions between the objects set up private state to aid in testing.” and your test case Endo-Testing: Unit Testing with Mock Objects • Mock Objects that become too complex to manage Tim Mackinnon, Steve Freeman, Philip Craig suggest that their domain clients are candidates for refactoring, and we avoid chaining Mock Objects.
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Testing a component that directs robots through a warehouse. Testing a component that directs robots through a warehouse. public class TestRobot { public class TestRobot { ...... public void setUp() { Did thepu rbolbicotv omidovset Uinp (t)his{ test? robot = new Robot(); Don’t knowro.b..o.t = new Robot(); } } public void testGotoSamePlace() { public void testGotoSamePlace() { final Position POSITION = new Position(1, 1); final Position POSITION = new Position(1, 1); robot.setCurrentPosition(POSITION); robot.setCurrentPosition(POSITION); robot.goTo(POSITION); robot.goTo(POSITION);
assertEquals("Should be same", POSITION, assertEquals("Should be same", POSITION, robot.getPosition()); robot.getPosition()); } } } }
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One option is to store and retrieve the route it took after each Next test is to move one point on the grid. goto public void testMoveOnePoint() { public void testGotoSamePlace() { final Position POSITION = new Position(1, 1); final Position DESTINATION = new Position(1, 0); robot.setCurrentPosition(POSITION); robot.setCurrentPosition(new Position(0, 0)); robot.goTo(POSITION); robot.goTo(DESTINATION);
assertEquals("Should be empty", 0, assertEquals("Should be destination", DESTINATION, robot.getRecentMoveRequests().size()); robot.getPosition()); assertEquals("Should be same", POSITION, List moves = robot.getRecentMoveRequests(); robot.getPosition()); assertEquals("Should be one move", 1, moves.size()); } assertEquals("Should be same move", new MoveRequest(1, MoveRequest.SOUTH), moves.get(1)); }
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Tests become more complex... • Problems with this approach to testing: public void testMoveALongWay() { – If one of these tests fail, one will have to step final Position DESTINATION = new Position(34, 71); through the code to find the problem robot.setCurrentPosition(new Position(0, 0)); because the assertions have been made robot.goTo(DESTINATION); after the call has finished assertEquals("Should be destination", DESTINATION, robot.getPosition()); – Functionality had to be added to the getRecentMovesRequests assertEquals("Should be same moves", production class ( ) makeExpectedLongWayMoves(), robot.getRecentMoves()); }
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testGotoSamePlace() { Is there a better way? Can I find a style that will public void final Position POSITION = new Position(0, 0); give me better error reporting and put robot.setCurrentPosition(POSITION); the behaviour in the right place? Let’s refactor... Motor mockMotor = new Motor() { public interface Motor { public void move(MoveRequest request) { void move(MoveRequest request); fail(„Should be no moves"); } } public class OneSpeedMotor implements Motor { }; public void move(MoveRequest request) { robot.setMotor(mockMotor); turnBy(request.getTurn()); advance(request.getDistance()); robot.goTo(POSITION); } ... assertEquals("Should be same", POSITION, robot.getPosition()); } }
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public class TestRobot { ...... public void testMoveOnePoint() { final Position DESTINATION static final Position ORIGIN = new Position(1, 0); = new Position(0, 0); mockMotor.addExpectedRequest( public void setUp() { new MoveRequest(1, MoveRequest.SOUTH)); mockMotor = new MockMotor(); robot = new Robot(mockMotor); robot.goTo(DESTINATION); robot.setCurrentPosition(ORIGIN); assertEquals("Should be destination", DESTINATION, robot.getPosition()); } mockMotor.verify(); ... }
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... public void testMoveAlongWay() { final Position DESTINATION 1. Create instances of Mock Objects = new Position(34, 71); 2. Set state in the Mock Objects
mockMotor.addExpectedRequests( 3. Set expectations in the Mock Objects makeExpectedLongWayMoveRequests()); (addExpected... methods)
robot.goTo(DESTINATION); 4. Invoke domain code with Mock Objects assertEquals("Should be destination", as parameters DESTINATION, robot.getPosition()); 5. Verify consistency in the Mock Objects mockMotor.verify(); } (verify method) }
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• Adapts to any kind of unit test(Servlet, JDBC etc.) • Localising unit tests – cont. • Localising unit tests – No stone unturned – Some unit tests need to test conditions that are very difficult to reproduce. – Deferring Infrastructure Choices „Not commit to infrastructure before you have it” public void testCloseStream() { eXtreme Programming mock.setExpectedCloseCalls(1); e.g. The real object does not yet exist myApp.setStream(mock); byte[] data = myApp.getData(); – Coping with scale // get data and close The real object is difficult to set up. The setup of mock.verify(); complex state is localised to one Mock Object instead } of scattered throughout many unit tests. – The real object is or has a user interface
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• Better tests • Better tests – Effects of coding style – Failures fail fast – the real object is slow – Developing with Mock Objects reduces the Domain objects often fail some time after an error occurs, need to expose the structure of domain which is one reason that debugging can be so difficult. code. – Refactored assertions – A test knows more about the behavior and • Usually assertions about the domain code are refactored into shared methods, but the developer has to remember less about the structure of tested code. to apply them to new tests. – Unit testing in the presence of singletons • These assertions are built into Mock Objects and so are applied by default whenever the object is used. • The singletons might not have set up the state • New assertions are added once in the Mock Object methods or query methods the results afterwards where they will automatically apply to all previously • Developing with Mock Objects encourages a existing tests. coding style where objects are passed into the code that needs them. This makes substitution possible and reduces the risk of unexpected side- effects.
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• Better tests – Effects of coding style • Better tests – cont. – The code developed with MO tends to – Interface discovery conform to The Law of Demeter • When the tests all run, we can extract the following: • The unit tests push towards to writing domain code public interface PersonHandler { that refers only to local objects as parameters, void name(String name); without an explicit policy to do so. void age(int age); – Preserves encapsulation void telephone(String telephone); – Reduces global dependencies void writeTo(PrintWriter writer); – Clarify interactions between classes } •This approach ensures that the interface will be the minimum that the domain code needs, •following the Extreme Programming principle of not adding features beyond our current understanding.
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• Mock Object might contain errors e.g. returning • In some cases it can be hard to create values in degrees rather than radians. Mock Objects to represent types in a complex external library. • Libraries must define their APIs in terms of – The most difficult aspect is usually the discovery of interfaces rather than classes so that clients of values and structures for parameters that are passed the library can use such techniques. into the domain code – Vector class in version 1 of Java had many final – In an event-based system, the object that represents methods but no interface, making it impossible to an event might be the root of a graph of objects, all of substitute. which need mocking up for the domain code to work – This process can be costly and sometimes must be weighed against the benefit of having the unit tests
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• Force the developer to refactor his/her code • Mock Libraries Note that if you need to implement tests for existing code it can easily become a nightmare ... • Static Java Mock Generators • Using MO, it is not sure the classes will run (tags in javadoc, command line params correctly in the chosen environment. etc.) • Using MO is not simple. It needs some discipline /** and some experience. * • In some cases MO mandates creating API that * Desription ... are not normally needed. * @mock • Also the code may become more complex */ (even if more flexible) because of the need for public class Person testing and not because of business requirements
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Interface to be mocked • Creating mocks on the fly using Java’s dynamic org.easymock.samples; proxy mechanism (J2SE >= 1.3) package • Hand writing classes for Mock Objects is not public interface Collaborator { needed void documentAdded(String title); • Supports return values and exceptions void documentChanged(String title); • Supports changing results for the same method void documentRemoved(String title); call byte voteForRemoval(String title); • Supports call count checking byte voteForRemovals(String[] title); • Supports default values for method calls } • Supports checking the order of method calls
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Class to be tested... A sample test case... public class ClassUnderTest { protected void setUp() { public void addListener(Collaborator control = listener) { ... } MockControl.createControl(Collaborator.class); mock = (Collaborator) control.getMock(); public void addDocument(String title, classUnderTest = new ClassUnderTest(); byte[] document) { ... } classUnderTest.addListener(mock); public boolean removeDocument(String } title) { ... } public boolean removeDocuments(String[] public void testAddDocument() { titles) { ... } mock.documentAdded("Wrong title"); } control.replay(); classUnderTest.addDocument("New Document", new byte[0]); control.verify(); }
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Test result... Expecting an explicit number of calls junit.framework.AssertionFailedError: public void testAddAndChangeDocument() { Unexpected method call mock.documentAdded("Document"); documentAdded("Wrong title"): mock.documentChanged("Document"); documentAdded("Wrong title"): mock.documentChanged("Document"); mock.documentChanged("Document"); expected: 0, actual: 1 control.replay(); documentAdded("New Document"): cut.addDocument("Document", new byte[0]); expected: 1, actual: 0 cut.addDocument("Document", new byte[0]); ... cut.addDocument("Document", new byte[0]); cut.addDocument("Document", new byte[0]); control.verify(); }
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Expecting an explicit number of calls (equivalent) public void testAddAndChangeDocument() { MockControl.createControl(...); mock.documentAdded("Document"); The order of method calls is not checked. mock.documentChanged("Document"); control.setVoidCallable(3); control.replay(); MockControl.createStrictControl(...); cut.addDocument("Document", new byte[0]); The order of method calls is checked cut.addDocument("Document", new byte[0]); cut.addDocument("Document", new byte[0]); cut.addDocument("Document", new byte[0]); control.verify(); }
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Specifying return values and exceptions • Various Authors, SingletonsAreEvil http://c2.wiki.com/cgi/wiki?SingletonsAreEvil mock.voteForRemoval("Document"); • Erich Gamma, Richard Helm, Ralph Johnson, and John control.setReturnValue(42, 3); // 3 times Vlissides. Design patterns: elements of reusable object- control.setThrowable(new RuntimeException(), 4); //4x oriented software. Reading, Mass.: Addison-Wesley, 1994. control.setReturnValue(-42, MockControl.ZERO_OR_MORE); • Karl J. Lieberherr and Ian M. Holland. Assuring good style for object-oriented programs. IEEE Software 6(5):38-49, Values returned by voteForRemoval: September 1989. 42, 42, 42, RuntimeException, RuntimeException, • Tim Mackinnon, Steve Freeman, Philip Craig. Endo-Testing: RuntimeException, RuntimeException, -42, .., -42 Unit Testing with Mock Objects http://www.mockobjects.com/wiki/MocksObjectsPaper?a ction=AttachFile&do=get&target=mockobjects.pdf • Steve Freeman. How Mock Objects Happened http://www.mockobjects.com/wiki/HowMockObjectsHap pened
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• Mock Objects – site and mock library for J2SE Thank You for your attention J and J2EE • What is your general http://www.mockobjects.com/ impression (1-6) • Easy Mock – dynamic mock creator http://www.easymock.org/ • Was it too slow or too fast? • Mock Maker – static mock object creator • What important did you learn http://www.mockmaker.org/ during the lecture? • MockDoclet – static mock object creator • What to improve and how? http://joe.truemesh.com/mockdoclet/
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