Assertions dans JUnit 4 et JUnit 5

1. Introduction

Dans cet article, nous allons explorer en détail les assertions disponibles dans JUnit.

Suite à la migration de JUnit 4 vers JUnit 5 et les articles A Guide to JUnit 5, nous allons maintenant entrer dans les détails des différentes assertions disponibles dans JUnit 4 et JUnit 5.

Nous soulignerons également les améliorations apportées aux assertions avec JUnit 5.

2. Assertions

Les assertions sont des méthodes utilitaires pour soutenir l'affirmation de conditions dans les tests ; ces méthodes sont accessibles via la classe Assert , dans JUnit 4, et celle Assertions , dans JUnit 5.

Afin d'augmenter la lisibilité du test et des assertions elles-mêmes, il est toujours recommandé d' importer statiquement la classe respective. De cette manière, nous pouvons nous référer directement à la méthode d'assertion elle-même sans la classe représentant comme préfixe.

Commençons par explorer les assertions disponibles avec JUnit 4.

3. Assertions dans JUnit 4

Dans cette version de la bibliothèque, les assertions sont disponibles pour tous les types primitifs, objets et tableaux (soit des primitives, soit des objets).

L'ordre des paramètres, dans l'assertion, est la valeur attendue suivie de la valeur réelle; facultativement, le premier paramètre peut être un message String qui représente la sortie du message de la condition évaluée.

Il n'y en a qu'une légèrement différente dans la définition des assertions assertThat , mais nous en parlerons plus tard.

Commençons par celui d' assertEquals .

3.1. assertEquals

L' assertion assertEquals vérifie que les valeurs attendues et réelles sont égales:

@Test public void whenAssertingEquality_thenEqual() { String expected = "Baeldung"; String actual = "Baeldung"; assertEquals(expected, actual); }

Il est également possible de spécifier un message à afficher lorsque l'assertion échoue:

assertEquals("failure - strings are not equal", expected, actual);

3.2. assertArrayEquals

Si nous voulons affirmer que deux tableaux sont égaux, nous pouvons utiliser l' assertArrayEquals:

@Test public void whenAssertingArraysEquality_thenEqual() { char[] expected = {'J','u','n','i','t'}; char[] actual = "Junit".toCharArray(); assertArrayEquals(expected, actual); }

Si les deux tableaux sont nuls , l'assertion les considérera égaux:

@Test public void givenNullArrays_whenAssertingArraysEquality_thenEqual() { int[] expected = null; int[] actual = null; assertArrayEquals(expected, actual); }

3.3. assertNotNull et assertNull

Lorsque nous voulons tester si un objet est nul, nous pouvons utiliser l' assertion assertNull :

@Test public void whenAssertingNull_thenTrue() { Object car = null; assertNull("The car should be null", car); }

Dans le cas contraire, si nous voulons affirmer qu'un objet ne doit pas être nul, nous pouvons utiliser l' assertion assertNotNull.

3.4. assertNotSame et assertSame

Avec assertNotSame , il est possible de vérifier si deux variables ne font pas référence au même objet:

@Test public void whenAssertingNotSameObject_thenDifferent() { Object cat = new Object(); Object dog = new Object(); assertNotSame(cat, dog); }

Sinon, lorsque nous voulons vérifier que deux variables font référence au même objet, nous pouvons utiliser l' assertion assertSame .

3.5. assertTrue et assertFalse

Dans le cas où nous souhaitons vérifier qu'une certaine condition est vraie ou fausse , nous pouvons respectivement utiliser l' assertion assertTrue ou assertFalse :

@Test public void whenAssertingConditions_thenVerified() { assertTrue("5 is greater then 4", 5 > 4); assertFalse("5 is not greater then 6", 5 > 6); }

3.6. échouer

L' assertion d' échec échoue à un test lançant une AssertionFailedError . Il peut être utilisé pour vérifier qu'une exception réelle est levée ou lorsque nous voulons faire échouer un test pendant son développement.

Voyons comment nous pouvons l'utiliser dans le premier scénario:

@Test public void whenCheckingExceptionMessage_thenEqual() { try { methodThatShouldThrowException(); fail("Exception not thrown"); } catch (UnsupportedOperationException e) { assertEquals("Operation Not Supported", e.getMessage()); } }

3.7. affirmer que

L' assertThat assertion est la seule dans JUnit 4 qui a un ordre inverse des paramètres par rapport aux autres assertions.

In this case, the assertion has an optional failure message, the actual value, and a Matcher object.

Let's see how we can use this assertion to check if an array contains particular values:

@Test public void testAssertThatHasItems() { assertThat( Arrays.asList("Java", "Kotlin", "Scala"), hasItems("Java", "Kotlin")); } 

Additional information, on the powerful use of the assertThat assertion with Matcher object, is available at Testing with Hamcrest.

4. JUnit 5 Assertions

JUnit 5 kept many of the assertion methods of JUnit 4 while adding few new ones that take advantage of the Java 8 support.

Also in this version of the library, assertions are available for all primitive types, Objects, and arrays (either of primitives or Objects).

The order of the parameters of the assertions changed, moving the output message parameter as the last parameter. Thanks to the support of Java 8, the output message can be a Supplier, allowing lazy evaluation of it.

Let's start reviewing the assertions available also in JUnit 4.

4.1. assertArrayEquals

The assertArrayEquals assertion verifies that the expected and the actual arrays are equals:

@Test public void whenAssertingArraysEquality_thenEqual() { char[] expected = { 'J', 'u', 'p', 'i', 't', 'e', 'r' }; char[] actual = "Jupiter".toCharArray(); assertArrayEquals(expected, actual, "Arrays should be equal"); }

If the arrays aren't equal, the message “Arrays should be equal” will be displayed as output.

4.2. assertEquals

In case we want to assert that two floats are equals, we can use the simple assertEquals assertion:

@Test public void whenAssertingEquality_thenEqual() { float square = 2 * 2; float rectangle = 2 * 2; assertEquals(square, rectangle); }

However, if we want to assert that the actual value differs by a predefined delta from the expected value, we can still use the assertEquals but we have to pass the delta value as the third parameter:

@Test public void whenAssertingEqualityWithDelta_thenEqual() { float square = 2 * 2; float rectangle = 3 * 2; float delta = 2; assertEquals(square, rectangle, delta); }

4.3. assertTrue and assertFalse

With the assertTrue assertion, it's possible to verify the supplied conditions are true:

@Test public void whenAssertingConditions_thenVerified() { assertTrue(5 > 4, "5 is greater the 4"); assertTrue(null == null, "null is equal to null"); }

Thanks to the support of the lambda expression, it's possible to supply a BooleanSupplier to the assertion instead of a boolean condition.

Let's see how we can assert the correctness of a BooleanSupplier using the assertFalse assertion:

@Test public void givenBooleanSupplier_whenAssertingCondition_thenVerified() { BooleanSupplier condition = () -> 5 > 6; assertFalse(condition, "5 is not greater then 6"); }

4.4. assertNull and assertNotNull

When we want to assert that an object is not null we can use the assertNotNull assertion:

@Test public void whenAssertingNotNull_thenTrue() { Object dog = new Object(); assertNotNull(dog, () -> "The dog should not be null"); }

In the opposite way, we can use the assertNull assertion to check if the actual is null:

@Test public void whenAssertingNull_thenTrue() { Object cat = null; assertNull(cat, () -> "The cat should be null"); }

In both cases, the failure message will be retrieved in a lazy way since it's a Supplier.

4.5. assertSame and assertNotSame

When we want to assert that the expected and the actual refer to the same Object, we must use the assertSame assertion:

@Test public void whenAssertingSameObject_thenSuccessfull() { String language = "Java"; Optional optional = Optional.of(language); assertSame(language, optional.get()); }

In the opposite way, we can use the assertNotSame one.

4.6. fail

The fail assertion fails a test with the provided failure message as well as the underlying cause. This can be useful to mark a test when it's development it's not completed:

@Test public void whenFailingATest_thenFailed() { // Test not completed fail("FAIL - test not completed"); }

4.7. assertAll

One of the new assertion introduced in JUnit 5 is assertAll.

This assertion allows the creation of grouped assertions, where all the assertions are executed and their failures are reported together. In details, this assertion accepts a heading, that will be included in the message string for the MultipleFailureError, and a Stream of Executable.

Let's define a grouped assertion:

@Test public void givenMultipleAssertion_whenAssertingAll_thenOK() { assertAll( "heading", () -> assertEquals(4, 2 * 2, "4 is 2 times 2"), () -> assertEquals("java", "JAVA".toLowerCase()), () -> assertEquals(null, null, "null is equal to null") ); }

The execution of a grouped assertion is interrupted only when one of the executables throws a blacklisted exception (OutOfMemoryError for example).

4.8. assertIterableEquals

The assertIterableEquals asserts that the expected and the actual iterables are deeply equal.

In order to be equal, both iterable must return equal elements in the same order and it isn't required that the two iterables are of the same type in order to be equal.

With this consideration, let's see how we can assert that two lists of different types (LinkedList and ArrayList for example) are equal:

@Test public void givenTwoLists_whenAssertingIterables_thenEquals() { Iterable al = new ArrayList(asList("Java", "Junit", "Test")); Iterable ll = new LinkedList(asList("Java", "Junit", "Test")); assertIterableEquals(al, ll); }

In the same way of the assertArrayEquals, if both iterables are null, they are considered equal.

4.9. assertLinesMatch

The assertLinesMatch asserts that the expected list of String matches the actual list.

This method differs from the assertEquals and assertIterableEquals since, for each pair of expected and actual lines, it performs this algorithm:

  1. check if the expected line is equal to the actual one. If yes it continues with the next pair
  2. treat the expected line as a regular expression and performs a check with the String.matches() method. If yes it continues with the next pair
  3. check if the expected line is a fast-forward marker. If yes apply fast-forward and repeat the algorithm from the step 1

Let's see how we can use this assertion to assert that two lists of String have matching lines:

@Test public void whenAssertingEqualityListOfStrings_thenEqual() { List expected = asList("Java", "\\d+", "JUnit"); List actual = asList("Java", "11", "JUnit"); assertLinesMatch(expected, actual); }

4.10. assertNotEquals

Complementary to the assertEquals, the assertNotEquals assertion asserts that the expected and the actual values aren't equal:

@Test public void whenAssertingEquality_thenNotEqual() { Integer value = 5; // result of an algorithm assertNotEquals(0, value, "The result cannot be 0"); }

If both are null, the assertion fails.

4.11. assertThrows

In order to increase simplicity and readability, the new assertThrows assertion allows us a clear and a simple way to assert if an executable throws the specified exception type.

Let's see how we can assert a thrown exception:

@Test void whenAssertingException_thenThrown() { Throwable exception = assertThrows( IllegalArgumentException.class, () -> { throw new IllegalArgumentException("Exception message"); } ); assertEquals("Exception message", exception.getMessage()); }

The assertion will fail if no exception is thrown, or if an exception of a different type is thrown.

4.12. assertTimeout and assertTimeoutPreemptively

In case we want to assert that the execution of a supplied Executable ends before a given Timeout, we can use the assertTimeout assertion:

@Test public void whenAssertingTimeout_thenNotExceeded() { assertTimeout( ofSeconds(2), () -> { // code that requires less then 2 minutes to execute Thread.sleep(1000); } ); }

However, with the assertTimeout assertion, the supplied executable will be executed in the same thread of the calling code. Consequently, execution of the supplier won't be preemptively aborted if the timeout is exceeded.

In case we want to be sure that execution of the executable will be aborted once it exceeds the timeout, we can use the assertTimeoutPreemptively assertion.

Les deux assertions peuvent accepter, au lieu d'un exécutable, un ThrowingSupplier , représentant tout bloc de code générique qui renvoie un objet et qui peut potentiellement lancer un Throwable.

5. Conclusion

Dans ce didacticiel, nous avons couvert toutes les assertions disponibles dans JUnit 4 et JUnit 5.

Nous avons brièvement mis en évidence les améliorations apportées à JUnit 5, avec l'introduction de nouvelles assertions et le support des lambdas.

Comme toujours, le code source complet de cet article est disponible à l'adresse over sur GitHub.