using tests and mocks to drive the design of software

Test Driven DesignUsing tests and mocks to drive the design of software

Attila MagyarMicrosec Plc

2012

● End to End– Real environment

● Integration– Against 3rd party API

● Unit– Isolated

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Growing object-oriented software guided by tests: Steve Freeman, Nat Pryce

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Bad design: ● Rigidity● Fragility● Immobility

End to End vs Unit tests

● End to End:● Slow execution and feedback● Sometimes unreliable● Difficult and time-consuming to

write● Difficult to localize bugs● Verifies configuration, integration,

environment

● Unit tests:● Fast execution and feedback● Always deterministic● Easy to write and automate● Easy to localize bugs● Verifies basic correctness

Pyramid

Unit

Integration

End to end

Unit tests:state based testing

// Testlight.switchOn();assertTrue(light.isOn());

light.switchOff();assertFalse(light.isOn())

public class Light { private boolean on; public void switchOn(){ on = true; } public void switchOff(){ on = false; } public boolean isOn(){ return on; }}

Ticket Machineclass TrainTicketMachine implements TicketMachine { private List<Integer> pressedButtons = new ArrayList<Integer>(); [...] public void press(int buttonNumber) { pressedButtons.add(buttonNumber); } public void enter() { ticketReserver.reserve(new ArrayList<Integer>(pressedButtons)); }}

Ticket Machine

Ticket Reserver

Message: reserve([1,2,3])

Ticket Machine

Ticket Reserver

Ticket Persister

public interface TicketReserver { void reserve(List<Integer> ticketCode);}

DB

Ticket Machine

Ticket Reserver

Messages=[reserve(1,2,3), reserve(4,5,6)]

Has received 123?

@Testpublic void reservesTicketUsingEnteredCodes() { // Given Spy spy = new Spy(); ticketMachine = new TrainTicketMachine(spy); // When ticketMachine.press(1); ticketMachine.press(2); ticketMachine.enter(); // Then spy.assertReceived(Arrays.asList(1, 2));}

public class Spy implements TicketReserver { private List<Integer> received = new ArrayList<Integer>(); @Override public void reserve(List<Integer> codes) { received.addAll(codes); } public void assertReceived(List<Integer> codes) { assertThat(received, equalTo(codes)); }}

@RunWith(MockitoJUnitRunner.class)public class TrainTickerReserverTest { TrainTicketMachine ticketMachine; @Mock TicketReserver reserver; @Test public void reservesTicketUsingEnteredCodes() { // Given ticketMachine = new TrainTicketMachine(reserver); // When ticketMachine.press(1); ticketMachine.press(2); ticketMachine.enter(); // Then verify(reserver).reserve(Arrays.asList(1, 2)); }}@RunWith(JMock.class)public class TrainTickerReserverTest { @Mock TicketReserver reserver; Mockery context = new JUnit4Mockery(); TrainTicketMachine ticketMachine; @Test public void reservesTicketUsingEnteredCodes() { context.checking(new Expectations() {{ oneOf(reserver).reserve(Arrays.asList(1, 2)); }}); ticketMachine = new TrainTicketMachine(reserver); ticketMachine.press(1); ticketMachine.press(2); ticketMachine.enter(); }}

(spyito)

Test smells

class TrainTicketMachine implements TicketMachine { private List<Integer> pressedButtons = new ArrayList<Integer>(); public void press(int buttonNumber) { pressedButtons.add(buttonNumber); } public void enter() { TicketReserver.reserve(new ArrayList<Integer>(pressedButtons)); }}

Test smells


class TrainTicketMachine implements TicketMachine { private List<Integer> pressedButtons = new ArrayList<Integer>(); public void press(int buttonNumber) { pressedButtons.add(buttonNumber); } public void enter() { TicketReserver.getInstance().reserve(new ArrayList<Integer>(pressedButtons)); }}

Test smells


class TrainTicketMachine implements TicketMachine { private List<Integer> pressedButtons = new ArrayList<Integer>(); public void press(int buttonNumber) { pressedButtons.add(buttonNumber); } public void enter() { TicketReserver.getInstance().reserve(new ArrayList<Integer>(pressedButtons)); }}

class TrainTicketMachine implements TicketMachine { private List<Integer> pressedButtons = new ArrayList<Integer>(); private TicketReserver ticketReserver = new TicketReserver(); public void press(int buttonNumber) { pressedButtons.add(buttonNumber); } public void enter() { ticketReserver.reserve(new ArrayList<Integer>(pressedButtons)); }}

Test smells

class TrainTicketMachine implements TicketMachine { private List<Integer> pressedButtons = new ArrayList<Integer>(); private TicketReserver ticketReserver; public TrainTicketMachine(TicketReserver ticketReserver) { this.ticketReserver = ticketReserver; } public void press(int buttonNumber) { pressedButtons.add(buttonNumber); } public void enter() { ticketReserver.reserve(new ArrayList<Integer>(pressedButtons)); }}

Solution

Copy component

Dependency inversion

The Dependency Inversion Principle, Robert C. Martin, C++ Report, May 1996

Test smellsPrivate method access

Test smellsPrivate method access

Principles of Object Oriented Design, Robert C. Martin, 2003

Test smellsExcessive test setup

•

• SRP violation• Too much coupling

• (No isolation)

Test smellsExcessive test setup

Test smellsDifficult mocking

(deep stubbing)

• Law of demeter violation


(deep stubbing)

Northeastern University, 1987

dog.getTail().wag()


dog.expressHappiness()


Test smellsAccessing local variable

• SRP violation• Method is too long

Test smellsAccessing local variable

Test smellsDupplication in test and production

• Missing abstraction• Mocking 3rd party components

Test smellsDupplication in test and production

3rd party API mockingpublic class Greetings { [...] public void greetUsers() throws SQLException { Statement stmt = connection.createStatement(); sayHelloTo(stmt.executeQuery("select name from users where type=1 or type=2")); }}

@Test public void saysHelloToUsers() throws SQLException { when(conn.createStatement()).thenReturn(stmt); when(stmt.executeQuery("select name from users where type=1 or type=2")).thenReturn(users); movies.greetUsers(); [...]}

public class Greetings { [...] public void greetUsers() throws SQLException { Statement stmt = connection.createStatement(); sayHelloTo(stmt.executeQuery("select name from users where type=1 or type=2")); }}

@Test public void saysHelloToUsers() throws SQLException { when(conn.createStatement()).thenReturn(stmt); when(stmt.executeQuery("select name from users where type=1 or type=2")).thenReturn(users); movies.greetUsers(); [...]}

Dup

licat

ion

3rd party API mocking

Test smellsToo many changes in test code

Test smellsToo many changes in test code

Meyer, Bertrand (1988). Object-Oriented Software Construction.

Test smellsToo many dependencies

● SRP violation● Missing abstraction

Test smellsToo many dependencies

Test smellsDifficult to instantiate SUT

● Hidden dependency (e.g.: Singleton)

● Insufficient domain separation

Test smellsDifficult to instantiate SUT

Adapter

BL

Adapter

BL

Adapter

BL

msg

3rd party

3rd party3rd party

Application domain

Domain of the outside worldObject must send messages to it peers in terms of its domain language.

Adapter

BL

Adapter

BL

Adapter

BL

msg

3rd party

3rd party3rd party

Application domain

Domain of the outside world

Application wiring- „new” and „set”

- Main method- Spring/Guice- XML/Annotations

MOCK

SUT

Adapter

MOCK

MOCK

BL

3rd party

3rd party3rd party

Application domain



Unit test


SUT

BL

Adapter

BL

Adapter

BL

3rd party

Real3rd party

Application domain



Integration


Adapter

BL

Adapter

BL

Adapter

BL

3rd party

3rd party3rd party

Application domain


Application wiring- „new” and „set”End to end test


How does it work?

● Write a failing test for A

A

How does it work?


● Introduce the interface of a collaborator B

A BInterface discovery

How does it work?


● Introduce the interface of a collaborator B

● Mock the interface● Use the mock to

Finish A

A BInterface discoveryInterface discoveryInterface discovery

How does it work?


● Introduce the interface of a B

● Mock the interface● Use the mock to

finish A

A B

● Write a failing test for B

● Introduce the interface of C

● Mock the interface● "Ensure contract

compliance" between A and B

● Use the mock to

finish B

Interface discovery CInterface discovery

● C is an adapter● Use integration

test for this

Benefits● Early design feedback (SRP, DIP, OCP, etc..)● Mocks encourage the use of „Tell Don't Ask” principle

→ Well encapsulated code● Outside-In approach

● Simpler interface, (and implementation)● No dead code

● Less debugging● More coverage →

● Higher confidence in code and refactoring● Less post-release bugs

DEMO

Cashier service:● Reads barcodes● Queries prices (REST)● Prints receipts

● Commands:– „Command: NewSale”– „Command: EndSale”– „Input: barcode=100008888559”

Command Translator

Sale S

tarted

CashRegister

Product Entered

Sale E

nded

PrinterpriceCalculated

productPrice

RESTCatalog

Money

ReceiptReceiver

ProductCatalog

SaleEventListener

Product

Barcode

Jersey/ApacheHttp client

java.awt.print

CommandListener

Command

References● Growing Object-Oriented Software Guided by Tests

● Steve Freeman, Nat Pryce ● Why You Don't Get Mock Objects

● Gregory Moeck, rubyconf 2011● Using Mocks And Tests To Design Role-Based Objects

● Isaiah Perumalla ● Mock Roles, not Objects

● Steve Freeman, Nat Pryce, Tim Mackinnon, Joe Walnes, High Holborn ● The Deep Synergy Between Testability and Good Design

● Michael Feathers● Surely the Mars Rover Needed Integrated Tests! (Maybe Not?)

● J. B. Rainsberger● Joey Devilla SOLID principle posters

using tests and mocks to drive the design of software

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