谷歌 Scott-lessons learned in testability

Lessons Learned in Testability

Scott McMaster

Google

Kirkland, Washington USA

About Me

• Software Design Engineer @ Google.

– Building high-traffic web frontends and services in Java.

– AdWords, Google Code

• Ph.D. in Computer Science, U. of Maryland.

• Formerly of Amazon.com (2 years), Lockheed Martin (2 years), Microsoft

(7 years), and some small startups.

• Frequent adjunct professor @ Seattle University, teaching software

design, architecture, and OO programming.

• Author of technical blog at http://www.scottmcmaster365.com.

Testing and Me

• Doing automated testing since 1995.

• Ph.D. work in test coverage and test suite maintenance.

• Champion of numerous unit, system, and performance

testing tools and techniques.

• Co-founder of WebTestingExplorer open-source automated

web testing framework (www.webtestingexplorer.org).

Agenda

• What is Testability?

• Testability Sins

– Statics and singletons

– Mixing Business and Presentation Logic

– Breaking the Law of Demeter

• Testability Solutions

– Removing singletons.

– Asking for Dependencies

– Dependency Injection

– Mocks and Fakes

– Refactoring to UI Patterns

Testability: Formal Definition

• Wikipedia: “the degree to which a software

artifact (i.e. a software system, software

module, requirements- or design document)

supports testing in a given test context.”

http://en.wikipedia.org/wiki/Software_testability

Some Aspects of Testability

• Controllable: We can put the software in a state to begin

testing.

• Observable: We can see things going right (or wrong).

• Isolatable: We can test classes/modules/systems apart from

others.

• Automatable: We can write or generate automated tests.

– Requires each of the previous three to some degree.

Testability: More Practical Definition

• Testability is a function of your testing goals.

• Our primary goal is to write or generate automated tests.

• Therefore, testability is the ease with which we can write:

– Unit tests

– System tests

– End-to-end tests

Testers and Testability• At Google, test engineers:

– Help ensure that developers build testable

software.

– Provide guidance to developers on best practices

for unit and end-to-end testing.

– May participate in refactoring production code for

testability.

Example: Weather App

Weather App Architecture

Rich Browser UI (GWT)

Frontend Server (GWT RPC servlet)

Remote Web Service (XML-over-

HTTP)User Database

Original Weather App Design

Testability Problems?

• Can’t test without calling the cloud service.

– Slow to run, unstable.

• Can’t test any client-side components without

loading a browser or browser emulator.

– Slow to develop, slow to run, perhaps unstable.

Mission #1: Unit Tests for WeatherServiceImpl• Problem: Uses static singleton reference to

GlobalWeatherService, can’t be tested in isolation.

• Solution:

– Eliminate the static singleton.

– Pass a mock or stub to the WeatherServiceImpl

constructor at test-time.

WeatherServiceImpl: Beforeprivate private private private static static static static GlobalWeatherServiceGlobalWeatherServiceGlobalWeatherServiceGlobalWeatherService service = new service = new service = new service = new GlobalWeatherServiceGlobalWeatherServiceGlobalWeatherServiceGlobalWeatherService();();();();

public public public public List<String> List<String> List<String> List<String> getCitiesForCountrygetCitiesForCountrygetCitiesForCountrygetCitiesForCountry(String (String (String (String countryNamecountryNamecountryNamecountryName) {) {) {) { try try try try {{{{ if if if if ((((countryNamecountryNamecountryNamecountryName == null || == null || == null || == null || countryName.isEmptycountryName.isEmptycountryName.isEmptycountryName.isEmpty()) {()) {()) {()) { return return return return new new new new ArrayListArrayListArrayListArrayList<String>();<String>();<String>();<String>(); } return return return return service.getCitiesForCountryservice.getCitiesForCountryservice.getCitiesForCountryservice.getCitiesForCountry((((countryNamecountryNamecountryNamecountryName))));;;; } catch (Exception e) {catch (Exception e) {catch (Exception e) {catch (Exception e) { throw throw throw throw new new new new RuntimeExceptionRuntimeExceptionRuntimeExceptionRuntimeException(e);(e);(e);(e); }}

What if we try to test this in its current form?1. GlobalWeatherService gets loaded at classload-time.

1. This itself could be slow or unstable depending on the implementation.2. When we call getCititesForCountry(“China”), a remote web service call gets made.3. This remote web service call may:

1. Fail.2. Be really slow.3. Not return predictable results.� Any of these things can make our test “flaky”.

Proposed Solution

• First we need to get rid of the static singleton.

• Then we need something that:

– Behaves like GlobalWebService.

– Is fast and predictable.

– Can be inserted into WeatherServiceImpl at test-

time.

A Word About Static Methods and Singletons• Never use them!

• They are basically global variables (and we’ve all

been taught to avoid those).

• They are hard to replace with alternative

implementations, mocks, and stubs/fakes.

– They make automated unit testing extremely difficult.

Scott’s Rules About Static Methods and Singletons

1. Avoid static methods.

2. For classes that are logically “singleton”, make

them non-singleton instances and manage

them in a dependency injection container (more

on this shortly).

Singleton Removal

public class public class public class public class WeatherServiceImplWeatherServiceImplWeatherServiceImplWeatherServiceImpl extends extends extends extends RemoteServiceServletRemoteServiceServletRemoteServiceServletRemoteServiceServlet implements implements implements implements WeatherService {

private private private private final final final final GlobalWeatherServiceGlobalWeatherServiceGlobalWeatherServiceGlobalWeatherService service; service; service; service;

public public public public WeatherServiceImplWeatherServiceImplWeatherServiceImplWeatherServiceImpl((((GlobalWeatherServiceGlobalWeatherServiceGlobalWeatherServiceGlobalWeatherService service) { service) { service) { service) { this.servicethis.servicethis.servicethis.service = service;= service;= service;= service; } ...

• Also, make GlobalWeatherService into an interface.• Now we can pass in a special implementation for unit

testing.• But we have a big problem…

We’ve Broken Our Service!

• The servlet container does not understand

how to create WeatherServiceImpl anymore.

– Its constructor takes a funny parameter.

• The solution?

Dependency Injection• Can be a little complicated, but here is what you

need to know here:

• Accept your dependencies, don’t ask for them.

– Then your dependencies can be replaced (generally, with

simpler implementations) at test time.

– In production, your dependencies get inserted by a

dependency injection container.

• In Java, this is usually Spring or Google Guice.

Dependency Injection with Google Guice

• Google Guice: A Dependency Injection framework.

• When properly set up, it will create your objects and pass

them to the appropriate constructors at runtime, freeing you

up to do other things with the constructors at test-time.

• Setting up Guice is outside the scope of this talk.

– This will get you started: http://code.google.com/p/google-

guice/wiki/Servlets

Fixing WeatherServiceImpl (1)

•Configure our servlet to use Guice and tell it about our objects:

•When someone asks for a “GlobalWeatherService”, Guice will

give it an instance of GlobalWeatherServiceImpl.

public class public class public class public class WeatherAppModuleWeatherAppModuleWeatherAppModuleWeatherAppModule extends extends extends extends AbstractModuleAbstractModuleAbstractModuleAbstractModule { { { { @Override protected protected protected protected void configure() {void configure() {void configure() {void configure() { bind(WeatherServiceImpl.classclassclassclass);););); bind(GlobalWeatherService.classclassclassclass).to().to().to().to(GlobalWeatherServiceImpl.classGlobalWeatherServiceImpl.classGlobalWeatherServiceImpl.classGlobalWeatherServiceImpl.class);););); }}

Fixing WeatherServiceImpl (2)

• At runtime, Guice will create our servlet and the object(s) it

needs:

@Singletonpublic class public class public class public class WeatherServiceImplWeatherServiceImplWeatherServiceImplWeatherServiceImpl extends extends extends extends RemoteServiceServletRemoteServiceServletRemoteServiceServletRemoteServiceServlet implements implements implements implements WeatherService {

private private private private final final final final GlobalWeatherServiceGlobalWeatherServiceGlobalWeatherServiceGlobalWeatherService service; service; service; service;

@Inject public public public public WeatherServiceImplWeatherServiceImplWeatherServiceImplWeatherServiceImpl((((GlobalWeatherServiceGlobalWeatherServiceGlobalWeatherServiceGlobalWeatherService service) { service) { service) { service) { this.servicethis.servicethis.servicethis.service = service;= service;= service;= service; } ...

The “@Inject” constructor parameters is how we ask Guice for instances.

After Testability Refactoring #1

Finally! We Can Test!• But how?

• We want to test WeatherServiceImpl in

isolation.

⇒For GlobalWeatherService, we only care about

how it interacts with WeatherServiceImpl.

⇒To create the proper interactions, a mock object mock object mock object mock object

is ideal

Mock Object Testing

• Mock objects simulate real objects in ways specified by the tester.

• The mock object framework verifies these interactions occur as expected.

– A useful consequence of this: If appropriate, you can verify that an application

is not making more remote calls than expected.

– Another useful consequence: Mocks make it easy to test exception handling.

• Common mocking frameworks (for Java):

– Mockito

– EasyMock

• I will use this.

Using Mock Objects1. Create a mock object.

2. Set up expectations:

1. How we expect the class-under-test to call it.

2. What we want it to return.

3. “Replay” the mock.

4. Invoke the class-under-test.

5. “Verify” the mock interactions were as-expected.

Testing with a Mock Objectprivate private private private GlobalWeatherServiceGlobalWeatherServiceGlobalWeatherServiceGlobalWeatherService globalWeatherServiceglobalWeatherServiceglobalWeatherServiceglobalWeatherService;;;;private private private private WeatherServiceImplWeatherServiceImplWeatherServiceImplWeatherServiceImpl weatherServiceweatherServiceweatherServiceweatherService;;;;

@@@@BeforeBeforeBeforeBeforepublic public public public void void void void setUpsetUpsetUpsetUp() {() {() {() { globalWeatherServiceglobalWeatherServiceglobalWeatherServiceglobalWeatherService = = = = EasyMock.createMockEasyMock.createMockEasyMock.createMockEasyMock.createMock((((GlobalWeatherService.classGlobalWeatherService.classGlobalWeatherService.classGlobalWeatherService.class);););); weatherServiceweatherServiceweatherServiceweatherService = new = new = new = new WeatherServiceImplWeatherServiceImplWeatherServiceImplWeatherServiceImpl((((globalWeatherServiceglobalWeatherServiceglobalWeatherServiceglobalWeatherService););););}}}}

@@@@TestTestTestTestpublic public public public void void void void testGetCitiesForCountry_nonEmptytestGetCitiesForCountry_nonEmptytestGetCitiesForCountry_nonEmptytestGetCitiesForCountry_nonEmpty() throws Exception {() throws Exception {() throws Exception {() throws Exception { EasyMock.expectEasyMock.expectEasyMock.expectEasyMock.expect((((globalWeatherService.getCitiesForCountryglobalWeatherService.getCitiesForCountryglobalWeatherService.getCitiesForCountryglobalWeatherService.getCitiesForCountry("china"))("china"))("china"))("china")) . . . .andReturnandReturnandReturnandReturn((((ImmutableList.ofImmutableList.ofImmutableList.ofImmutableList.of("("("("beijingbeijingbeijingbeijing", "shanghai"));", "shanghai"));", "shanghai"));", "shanghai")); EasyMock.replayEasyMock.replayEasyMock.replayEasyMock.replay((((globalWeatherServiceglobalWeatherServiceglobalWeatherServiceglobalWeatherService);););); List List List List<String> cities = <String> cities = <String> cities = <String> cities = weatherService.getCitiesForCountryweatherService.getCitiesForCountryweatherService.getCitiesForCountryweatherService.getCitiesForCountry("china");("china");("china");("china"); assertEqualsassertEqualsassertEqualsassertEquals(2, (2, (2, (2, cities.sizecities.sizecities.sizecities.size());());());()); assertTrueassertTrueassertTrueassertTrue((((cities.containscities.containscities.containscities.contains("("("("beijingbeijingbeijingbeijing"));"));"));")); assertTrueassertTrueassertTrueassertTrue((((cities.containscities.containscities.containscities.contains("shanghai"));("shanghai"));("shanghai"));("shanghai")); EasyMock.verifyEasyMock.verifyEasyMock.verifyEasyMock.verify((((globalWeatherServiceglobalWeatherServiceglobalWeatherServiceglobalWeatherService););););}}}}

Observe:• How we take advantage of the new WeatherServiceImpl constructor.• How we use the mock GlobalWeatherService.

Mission #2: Unit Tests for GlobalWeatherService

• Problem: Talks to external web service, does non-trivial XML

processing that we want to test.

• Solution:

– Split the remote call from the XML processing.

– Wrap external web service in an object with a known interface.

– Pass an instance to the GlobalWeatherServiceImpl constructor.

– Use dependency injection to create the real object at runtime, use a

fake at test-time.

After Testability Refactoring #2

Fakes vs. Mocks

• Mocks

– Verifies behavior (expected calls).

– Implementation usually generated by a mock object framework.

– Often only usable in a single test case.

– Often fragile as the implementation changes.

• Fakes

– Contains a simplified implementation of the real thing (perhaps using static data, an in-memory

database, etc.).

– Implementation usually generated by hand.

– Often reusable across test cases and test suites if carefully designed.

• Mocks and Fakes

– Either can often be used in a given situation.

– But some situations lend themselves to one more than the other.

Use a Fake, or Use a Mock?• Problem: Setting up a mock object for

GlobalWeatherDataAccess that returns static XML is

possible, but ugly (and perhaps not very reusable).

• Idea: Create a fakefakefakefake implementation of

GlobalWeatherDataAccess.

– We can give the fake object the capability to return

different XML in different test circumstances.

Implementing the Fake Objectpublic class public class public class public class FakeGlobalWeatherDataAccessFakeGlobalWeatherDataAccessFakeGlobalWeatherDataAccessFakeGlobalWeatherDataAccess implements implements implements implements GlobalWeatherDataAccessGlobalWeatherDataAccessGlobalWeatherDataAccessGlobalWeatherDataAccess { { { {

/ / / // Try http:/// Try http:/// Try http:/// Try http://www.htmlescape.netwww.htmlescape.netwww.htmlescape.netwww.htmlescape.net////javaescape_tool.htmljavaescape_tool.htmljavaescape_tool.htmljavaescape_tool.html to generate these. to generate these. to generate these. to generate these. private private private private static final String CHINA_CITIES = "<?xml version=\"1.0\" encoding=\"utf-8\"?>\n<string static final String CHINA_CITIES = "<?xml version=\"1.0\" encoding=\"utf-8\"?>\n<string static final String CHINA_CITIES = "<?xml version=\"1.0\" encoding=\"utf-8\"?>\n<string static final String CHINA_CITIES = "<?xml version=\"1.0\" encoding=\"utf-8\"?>\n<string xmlnsxmlnsxmlnsxmlns=\"http://=\"http://=\"http://=\"http://www.webserviceX.NETwww.webserviceX.NETwww.webserviceX.NETwww.webserviceX.NET\"><\"><\"><\"><NewDataSetNewDataSetNewDataSetNewDataSet>\n <Table>\n <Country>China</Country>\n <City>Beijing</City>\n </Table>\n <Table>\n <Country>China</Country>\n <City>Shanghai</City>\n </Table>\n</>\n <Table>\n <Country>China</Country>\n <City>Beijing</City>\n </Table>\n <Table>\n <Country>China</Country>\n <City>Shanghai</City>\n </Table>\n</>\n <Table>\n <Country>China</Country>\n <City>Beijing</City>\n </Table>\n <Table>\n <Country>China</Country>\n <City>Shanghai</City>\n </Table>\n</>\n <Table>\n <Country>China</Country>\n <City>Beijing</City>\n </Table>\n <Table>\n <Country>China</Country>\n <City>Shanghai</City>\n </Table>\n</NewDataSetNewDataSetNewDataSetNewDataSet></string>";></string>";></string>";></string>"; private private private private static final String BEIJING_WEATHER = "<?xml version=\"1.0\" encoding=\"utf-8\"?>\n<string static final String BEIJING_WEATHER = "<?xml version=\"1.0\" encoding=\"utf-8\"?>\n<string static final String BEIJING_WEATHER = "<?xml version=\"1.0\" encoding=\"utf-8\"?>\n<string static final String BEIJING_WEATHER = "<?xml version=\"1.0\" encoding=\"utf-8\"?>\n<string xmlnsxmlnsxmlnsxmlns=\"http://=\"http://=\"http://=\"http://www.webserviceX.NETwww.webserviceX.NETwww.webserviceX.NETwww.webserviceX.NET\">&\">&\">&\">&ltltltlt;?xml version=\"1.0\" encoding=\"utf-16\"?&;?xml version=\"1.0\" encoding=\"utf-16\"?&;?xml version=\"1.0\" encoding=\"utf-16\"?&;?xml version=\"1.0\" encoding=\"utf-16\"?&gtgtgtgt;\;\;\;\n&lt;CurrentWeather&gtn&lt;CurrentWeather&gtn&lt;CurrentWeather&gtn&lt;CurrentWeather&gt;\n &;\n &;\n &;\n &lt;Location&gt;Beijinglt;Location&gt;Beijinglt;Location&gt;Beijinglt;Location&gt;Beijing, China (ZBAA) 39-56N 116-17E 55M&lt;/, China (ZBAA) 39-56N 116-17E 55M&lt;/, China (ZBAA) 39-56N 116-17E 55M&lt;/, China (ZBAA) 39-56N 116-17E 55M&lt;/Location&gtLocation&gtLocation&gtLocation&gt;\n &;\n &;\n &;\n &lt;Time&gt;Octlt;Time&gt;Octlt;Time&gt;Octlt;Time&gt;Oct 27, 2012 - 04:00 PM EDT / 2012.10.27 2000 27, 2012 - 04:00 PM EDT / 2012.10.27 2000 27, 2012 - 04:00 PM EDT / 2012.10.27 2000 27, 2012 - 04:00 PM EDT / 2012.10.27 2000 UTC&ltUTC&ltUTC&ltUTC&lt;/;/;/;/Time&gtTime&gtTime&gtTime&gt;\n &;\n &;\n &;\n &lt;Wind&gtlt;Wind&gtlt;Wind&gtlt;Wind&gt; from the N (010 degrees) at 9 MPH (8 KT):0&lt;/; from the N (010 degrees) at 9 MPH (8 KT):0&lt;/; from the N (010 degrees) at 9 MPH (8 KT):0&lt;/; from the N (010 degrees) at 9 MPH (8 KT):0&lt;/Wind&gtWind&gtWind&gtWind&gt;\n ;\n ;\n ;\n &&&&lt;Visibility&gtlt;Visibility&gtlt;Visibility&gtlt;Visibility&gt; greater than 7 mile(s):0&lt;/; greater than 7 mile(s):0&lt;/; greater than 7 mile(s):0&lt;/; greater than 7 mile(s):0&lt;/Visibility&gtVisibility&gtVisibility&gtVisibility&gt;\n &;\n &;\n &;\n &lt;Temperature&gtlt;Temperature&gtlt;Temperature&gtlt;Temperature&gt; 39 F (4 C)&; 39 F (4 C)&; 39 F (4 C)&; 39 F (4 C)&ltltltlt;/;/;/;/Temperature&gtTemperature&gtTemperature&gtTemperature&gt;\n &;\n &;\n &;\n &lt;DewPoint&gtlt;DewPoint&gtlt;DewPoint&gtlt;DewPoint&gt; 28 F (-2 C)&; 28 F (-2 C)&; 28 F (-2 C)&; 28 F (-2 C)&ltltltlt;/;/;/;/DewPoint&gtDewPoint&gtDewPoint&gtDewPoint&gt;\n &;\n &;\n &;\n &lt;RelativeHumidity&gtlt;RelativeHumidity&gtlt;RelativeHumidity&gtlt;RelativeHumidity&gt; 64%&; 64%&; 64%&; 64%&ltltltlt;/;/;/;/RelativeHumidity&gtRelativeHumidity&gtRelativeHumidity&gtRelativeHumidity&gt;\n &;\n &;\n &;\n &lt;Pressure&gtlt;Pressure&gtlt;Pressure&gtlt;Pressure&gt; 30.30 in. Hg (1026 ; 30.30 in. Hg (1026 ; 30.30 in. Hg (1026 ; 30.30 in. Hg (1026 hPahPahPahPa)&)&)&)&ltltltlt;/;/;/;/Pressure&gtPressure&gtPressure&gtPressure&gt;\n &;\n &;\n &;\n &lt;Status&gt;Success&ltlt;Status&gt;Success&ltlt;Status&gt;Success&ltlt;Status&gt;Success&lt;/;/;/;/Status&gtStatus&gtStatus&gtStatus&gt;\;\;\;\n&ltn&ltn&ltn&lt;/;/;/;/CurrentWeather&gtCurrentWeather&gtCurrentWeather&gtCurrentWeather&gt;</string>";;</string>";;</string>";;</string>"; private private private private static final String NO_CITIES = "<?xml version=\"1.0\" encoding=\"utf-8\"?>\n<string static final String NO_CITIES = "<?xml version=\"1.0\" encoding=\"utf-8\"?>\n<string static final String NO_CITIES = "<?xml version=\"1.0\" encoding=\"utf-8\"?>\n<string static final String NO_CITIES = "<?xml version=\"1.0\" encoding=\"utf-8\"?>\n<string xmlnsxmlnsxmlnsxmlns=\"http://=\"http://=\"http://=\"http://www.webserviceX.NETwww.webserviceX.NETwww.webserviceX.NETwww.webserviceX.NET\"><\"><\"><\"><NewDataSetNewDataSetNewDataSetNewDataSet /></string>"; /></string>"; /></string>"; /></string>";

@ @ @ @OverrideOverrideOverrideOverride public public public public String String String String getCitiesForCountryXmlgetCitiesForCountryXmlgetCitiesForCountryXmlgetCitiesForCountryXml(String (String (String (String countryNamecountryNamecountryNamecountryName) throws Exception {) throws Exception {) throws Exception {) throws Exception { if if if if ("("("("china".equalschina".equalschina".equalschina".equals((((countryName.toLowerCasecountryName.toLowerCasecountryName.toLowerCasecountryName.toLowerCase())) {())) {())) {())) { return return return return CHINA_CITIES;CHINA_CITIES;CHINA_CITIES;CHINA_CITIES; } } } } return return return return NO_CITIES;NO_CITIES;NO_CITIES;NO_CITIES; } } } }

@ @ @ @OverrideOverrideOverrideOverride public public public public String String String String getWeatherForCityXmlgetWeatherForCityXmlgetWeatherForCityXmlgetWeatherForCityXml(String (String (String (String countryNamecountryNamecountryNamecountryName, String , String , String , String cityNamecityNamecityNamecityName)))) throws throws throws throws Exception {Exception {Exception {Exception { return return return return BEIJING_WEATHER;BEIJING_WEATHER;BEIJING_WEATHER;BEIJING_WEATHER; } } } }}}}}

Testing with a Fake Objectprivate private private private GlobalWeatherServiceImplGlobalWeatherServiceImplGlobalWeatherServiceImplGlobalWeatherServiceImpl globalWeatherServiceglobalWeatherServiceglobalWeatherServiceglobalWeatherService;;;;private private private private FakeGlobalWeatherDataAccessFakeGlobalWeatherDataAccessFakeGlobalWeatherDataAccessFakeGlobalWeatherDataAccess dataAccessdataAccessdataAccessdataAccess;;;;

@Beforepublic public public public void void void void setUpsetUpsetUpsetUp() {() {() {() { dataAccess = new new new new FakeGlobalWeatherDataAccessFakeGlobalWeatherDataAccessFakeGlobalWeatherDataAccessFakeGlobalWeatherDataAccess();();();(); globalWeatherService = new new new new GlobalWeatherServiceImplGlobalWeatherServiceImplGlobalWeatherServiceImplGlobalWeatherServiceImpl((((dataAccessdataAccessdataAccessdataAccess););););}

@Testpublic public public public void void void void testGetCitiesForCountry_nonEmptytestGetCitiesForCountry_nonEmptytestGetCitiesForCountry_nonEmptytestGetCitiesForCountry_nonEmpty() throws Exception {() throws Exception {() throws Exception {() throws Exception { List<String> cities = globalWeatherService.getCitiesForCountry("china"); assertEquals(2, cities.size()); assertTrue(cities.contains("beijing")); assertTrue(cities.contains("shanghai"));}

@Testpublic public public public void void void void testGetCitiesForCountry_emptytestGetCitiesForCountry_emptytestGetCitiesForCountry_emptytestGetCitiesForCountry_empty() throws Exception {() throws Exception {() throws Exception {() throws Exception { List<String> cities = globalWeatherService.getCitiesForCountry("nowhere"); assertTrue(cities.isEmpty());}

The fake keeps the tests short, simple, and to-the-point!

Mission #3: Unit Tests for WeatherHome

• Problem: UI and business logic / service calls all mixed

together.

– The view layer is difficult and slow to instantiate at unit test-time.

– But we need to unit test the business logic.

• Solution:

– Refactor to patterns -- Model-View-Presenter (MVP).

– Write unit tests for the Presenter using a mock or stub View.

Mixing Business and Presentation@UiHandler("login")void void void void onLoginonLoginonLoginonLogin((((ClickEventClickEventClickEventClickEvent e) { e) { e) { e) { weatherService.getWeatherForUser(userName.getText(), new new new new AsyncCallbackAsyncCallbackAsyncCallbackAsyncCallback<Weather>() {<Weather>() {<Weather>() {<Weather>() {

@Override public public public public void void void void onFailureonFailureonFailureonFailure((((ThrowableThrowableThrowableThrowable caught) caught) caught) caught) {{{{ Window.alert("oops");

}

@Override public public public public voidvoidvoidvoid onSuccessonSuccessonSuccessonSuccess((((WeatherWeatherWeatherWeather weatherweatherweatherweather) {) {) {) { if if if if (weather != null) {(weather != null) {(weather != null) {(weather != null) { fillWeather(weather); unknownUser.setVisible(false);false);false);false); } elseelseelseelse { { { { unknownUser.setVisible(true);true);true);true); } } });}

How NOT to write a UI event handler for maximum testability:• Have tight coupling between the UI event, processing a remote service call, and

updating the UI.

Model-View-Presenter (MVP)

• UI pattern that separates business and

presentation logic.

• Makes the View easier to modify.

• Makes the business logic easier to test by

isolating it in the Presenter.

Model-View-Presenter Overview

Model-View-Presenter Responsibilities

• Presenter uses the View interface to

manipulate the UI.

• View delegates UI event handling back to the

Presenter via an event bus or an interface.

• Presenter handles all service calls and

reading/updating of the Model.

Passive View MVP• A particular style of MVP where the View is

completely passive, only defining and layout and

exposing its widgets for manipulation by the

controller.

– In practice, you sometimes don’t quite get here, but this is

the goal.

• Especially if you use this style, you can skip testing

the View altogether.

After Testability Refactoring #3

Presenter Unit Test Using EasyMock

@Testpublic public public public void void void void testOnLogin_unknownUsertestOnLogin_unknownUsertestOnLogin_unknownUsertestOnLogin_unknownUser() {() {() {() { weatherService.expectGetWeatherForUser("unknown"); EasyMock.expect(weatherView.getUserName()).andReturn("unknown"); weatherView.setUnknownUserVisible(true);true);true);true); EasyMock.expectLastCall(); weatherView.setEventHandler(EasyMock.anyObject(WeatherViewEventHandler.classclassclassclass));));));)); EasyMock.expectLastCall();

EasyMock.replay(weatherView);

WeatherHomePresenter presenter = new new new new WeatherHomePresenterWeatherHomePresenterWeatherHomePresenterWeatherHomePresenter((((weatherServiceweatherServiceweatherServiceweatherService, , , , weatherViewweatherViewweatherViewweatherView);););); presenter.onLogin();

EasyMock.verify(weatherView); weatherService.verify();}

This test uses a manually created mock to make handling the async callback easier.

Question

• Why does the View make the Presenter do

this:weatherView.setUnknownUserVisible(true);;;;

• Instead of this:weatherView.getUnknownUser().setVisible(true)

Answer

weatherView.getUnknownUser().setVisible(true)

• Is hard to test because it is hard to mock:

–To mock this, we would have to mock not only the

WeatherView, but also the UnknownUser Label inside

of it.

• The above code is “talking to strangers”.

Law of Demeter

• Also known as the “Don’t talk to strangers” rule.

• It says:

–Only have knowledge of closely collaborating objects.

–Only make calls on immediate friends.

• Look out for long chained “get”-style calls (and don’t do them:

a.getB().getC().getD()

• Your system will be more testable (and maintainable,

because you have to rework calling objects less often).

What’s the Point?• To write good unit tests, we need to be able to insert mocks and fakes

into the code.

• Some things that help us do that:

– Eliminating static methods and singletons.

– Asking for dependencies instead of creating them.

– Using design patterns that promote loose coupling, especially between

business and presentation logic.

– Obeying the Law of Demeter.

• Code that does not do these things will often have poor test coverage.

What Can I Do?• Developers:

– Follow these practices!

• Testers:

– Educate your developers.

– Jump into the code and drive testability improvements.

• A good way to motivate this is to track test coverage metrics.

Questions?Scott McMaster

Google

Kirkland, Washington USA

scott.d.mcmaster (at) gmail.com

Bonus Slides

Model-View-Controller (MVC)

Model-View-Controller (MVC)

• View directly accesses the Model and fires

events to the Controller.

• Controller performs operations on the Model.

• Controller doesn’t really know about the View

other than selecting the View to render.

Which to Use?

• Many web MVC frameworks exist (Struts,

Rails, ASP.NET MVC).

• But these days, we work more with MVP.

Manually Created Mockpublic class public class public class public class MockWeatherServiceAsyncMockWeatherServiceAsyncMockWeatherServiceAsyncMockWeatherServiceAsync implements implements implements implements WeatherServiceAsyncWeatherServiceAsyncWeatherServiceAsyncWeatherServiceAsync { { { {

private private private private List<String> List<String> List<String> List<String> expectGetWeatherForUserCallsexpectGetWeatherForUserCallsexpectGetWeatherForUserCallsexpectGetWeatherForUserCalls = = = = Lists.Lists.Lists.Lists.newArrayListnewArrayListnewArrayListnewArrayList();();();(); private private private private List<String> List<String> List<String> List<String> observeGetWeatherForUserCallsobserveGetWeatherForUserCallsobserveGetWeatherForUserCallsobserveGetWeatherForUserCalls = = = = Lists.Lists.Lists.Lists.newArrayListnewArrayListnewArrayListnewArrayList();();();();

// More @Overrides not shown on the slide.

@Override public public public public void void void void getWeatherForUsergetWeatherForUsergetWeatherForUsergetWeatherForUser(String (String (String (String userNameuserNameuserNameuserName, , , , AsyncCallbackAsyncCallbackAsyncCallbackAsyncCallback<Weather> callback) {<Weather> callback) {<Weather> callback) {<Weather> callback) { observeGetWeatherForUserCalls.add(userName); if if if if ("("("("scottscottscottscott".equals(".equals(".equals(".equals(userNameuserNameuserNameuserName)) {)) {)) {)) { callback.onSuccess(new Weather());new Weather());new Weather());new Weather()); } elseelseelseelse { { { { callback.onSuccess(nullnullnullnull);););); } }

public public public public voidvoidvoidvoid expectGetWeatherForUserexpectGetWeatherForUserexpectGetWeatherForUserexpectGetWeatherForUser((((StringStringStringString userNameuserNameuserNameuserName) {) {) {) { expectGetWeatherForUserCalls.add(userName); }

public public public public voidvoidvoidvoid verifyverifyverifyverify() {() {() {() { assertEquals(expectGetWeatherForUserCalls, observeGetWeatherForUserCalls); expectGetWeatherForUserCalls.clear(); observeGetWeatherForUserCalls.clear(); }}

2012-12-20