From time to time I want to be able to reuse already finished unit test. e.g. when I do a new implementation of an interface. In most cases, I did write an abstract base fixture, that I extend as needed:
public abstract class IterableFixture<T> {
protected abstract Iterable<T> create();
@Test
public void iterationDoesSomething(){
assertThat(…);
}
}
Whenever I need to write an Iterable<T> now, I can reuse the generic test like this:
public class MyIterableTest extends IterableFixture<Integer> {
@Override
protected Iterable<Integer> create(){
return new MyIterable<Integer>(…);
}
@Test
public void otherTest(){
assertThat(…);
}
}
Over the time, multiple base fixtures emerged, e.g. for equals/hashcode implementations, iteration, collections, serializing and so on. When new classes needed multiple of these tests, I was forced to implement multiple test classes, e.g. MyFooIsSerializableTest, MyFooImplementsIterableTest, MyFooEqualsTest,… Although this was simple and practical, I never really liked that I have too check for multiple green bubbles inside my IDE’s test runner for one class under test. I also did not like that I could not see in one spot what features my classes are implementing. This was extremely true for collection implementations, as they can be un/modifiable, im/mutable and fixed/variable-size. Not to speak about allowing null or similar variants.
What I really wanted, was to run my single test for my class and have each feature nicely listed, in a similar manner as JUnit Suites do:
Just using a Suite did not help much, because I still had to extend my base fixtures. Additionally I had to add them to the Suites parameter list.
One of the reason that it was so clumsy to work with this approach are the plethora of derived classes - one for each feature that was implemented. So I decided, that using A factory that gets injected into the test classes constructor will simplify the approach. Then I looked at JUnits Parameterized runner, as it has the similar task to instantiate test fixture with constructor argument. The JUnit extension mechanism for custom runners showed to be very straight forward, so I decided to build my own Features Runner.
While thinking about how the using this runner I tried different variant on where to place which annotation. My requirements were:
I have spiked several versions. Here is an example of the final one. A class testing a collection for the features of being iterable and unmodifiable.
@RunWith(Features.class)
public class ArrayAdapterTestSuite {
@Feature(Unmodifiable.class)
public static Unmodifiable.Factory<Integer> unmodifiableFeature() {
return new Unmodifiable.Factory<Integer>() {
@Override
public Collection<Integer> createCollection() {
return asList(0, 1, 2);
}
@Override
public Integer createUniqueItem(int id) {
return id;
}
};
}
@Feature(Iterable.class)
public static Iterable.Factory<Integer> iterableFeature() {
return new Iterable.Factory<Integer>() {
@Override
public java.lang.Iterable<Integer> createIterable() {
return asList(0, 1, 2);
}
};
}
}
The feature fixture is implemented like this:
public class Unmodifiable<T> implements FeatureFixture {
private final Factory<T> factory;
public Unmodifiable(Factory<T> factory) {
this.factory = factory;
}
public interface Factory<T> {
Collection<T> createCollection();
T createUniqueItem(int id);
}
@Test(expected = UnsupportedOperationException.class)
public void addIsUnsupported() {
Collection<T> unmodifiable = factory.createCollection();
unmodifiable.add(factory.createUniqueItem(42));
}
}
The feature test is self contained. Only a factory needs to be supplied. The features are assembled by tagging a public static method creating a factory or a public static field with a @Feature annotations. The runner will ensure that the types of the factory and the annotated feature do match. This is a runtime check, as there is no way to achieve this at compile time because of java’s type erasure.
The Features Suite runner will get executed because the @RunsWith annotation on top of your test class. JUnit will instantiate it and pass it the Class<?> descriptor of your test. It will get inspected in the same way JUnit does it by using reflection. The gained information will be used to construct explicit FeatureRunner with the feature test Class<?> descriptor and the extracted factory.
Because the FeatureRunner would execute a group of test cases, I decided to use Suite as a base class for the implementation:
public class Features extends Suite {
public Features(Class<?> klass) throws InitializationError {
super(klass, extractAndCreateRunners(klass));
}
private static List<Runner> extractAndCreateRunners(Class<?> klass)
throws InitializationError {
List<Runner> runners = new ArrayList<Runner>();
for (FeatureAccessor field : extractFieldsWithTest(klass)) {
Class<? extends FeatureFixture> test = field.getFeature();
runners.add(new FeatureRunner(test, field.getFactory()));
}
addSuiteIfItContainsTests(klass, runners);
return runners;
}
private static void addSuiteIfItContainsTests(Class<?> klass,
List<Runner> runners) {
try {
runners.add(new BlockJUnit4ClassRunner(klass));
} catch (InitializationError e) {// do nothing, no tests
}
}
private static abstract class FeatureAccessor<TField …> {
private final TField field;
static <TField extends …> boolean isValid(TField field) {
return Modifier.isPublic(field.getModifiers())
&& Modifier.isStatic(field.getModifiers())
&& field.isAnnotationPresent(Feature.class);
}
static <TField …> FeatureAccessor<?> createFrom(final TField field) {
…
}
Class<? extends FeatureFixture> getFeature() {
return field.getAnnotation(Feature.class).value();
}
Object getFactory() {
…
return this.field.invoke(null);
}
}
private static List<FeatureAccessor> extractFieldsWithTest(Class<?> klass) {
List<FeatureAccessor> factoryFieldsWithProperty =
new ArrayList<FeatureAccessor>();
for (Field field : klass.getFields()) {
if (!FeatureAccessor.isValid(field)) {
continue;
}
factoryFieldsWithProperty.add(FeatureAccessor.createFrom(field));
}
…
return factoryFieldsWithProperty;
}
}
The FeatureRunner simply extends JUnits default runner in passing the factory instance into the test constructor:
class FeatureRunner extends BlockJUnit4ClassRunner {
private final Object factory;
FeatureRunner(Class<?> klass, Object factory)
throws InitializationError {
super(klass);
if(factory == null){
throw new InitializationError(…);
}
this.factory = factory;
}
protected Object createTest() throws Exception {
return getTestClass().getOnlyConstructor().newInstance(factory);
}
…
}
Changing my base class fixtures to FeatureFixtures was extremely simple and composing standard unit test bits really is fun now! If you are interested in this solution, the source code is on google-code. I hope it will help you as much as it does help me!
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