Polish and add tests for PR #2974

Author: juliette-derancourt

junit-jupiter-api/src/main/java/org/junit/jupiter/api/DisplayNameGenerator.java

@@ -74,7 +74,7 @@ public interface DisplayNameGenerator {
 	/**
 	 * Generate a display name for the given top-level or {@code static} nested test class.
 	 *
-	 * If {@code null} is generated, the default generator will be used to generate the display name.
+	 * <p>If it returns {@code null}, the default display name generator will be used instead.
 	 *
 	 * @param testClass the class to generate a name for; never {@code null}
 	 * @return the display name for the class; never blank
@@ -84,7 +84,7 @@ public interface DisplayNameGenerator {
 	/**
 	 * Generate a display name for the given {@link Nested @Nested} inner test class.
 	 *
-	 * If {@code null} is generated, the default generator will be used to generate the display name.
+	 * <p>If it returns {@code null}, the default display name generator will be used instead.
 	 *
 	 * @param nestedClass the class to generate a name for; never {@code null}
 	 * @return the display name for the nested class; never blank
@@ -94,7 +94,7 @@ public interface DisplayNameGenerator {
 	/**
 	 * Generate a display name for the given method.
 	 *
-	 * If {@code null} is generated, the default generator will be used to generate the display name.
+	 * <p>If it returns {@code null}, the default display name generator will be used instead.
 	 *
 	 * @implNote The class instance supplied as {@code testClass} may differ from
 	 * the class returned by {@code testMethod.getDeclaringClass()} &mdash; for

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/descriptor/DisplayNameUtils.java

@@ -106,21 +106,20 @@ static Supplier<String> createDisplayNameSupplierForNestedClass(Class<?> testCla
 			generator -> generator.generateDisplayNameForNestedClass(testClass));
 	}
 
-	static Supplier<String> createDisplayNameSupplierForMethod(Class<?> testClass, Method testMethod,
+	private static Supplier<String> createDisplayNameSupplierForMethod(Class<?> testClass, Method testMethod,
 			JupiterConfiguration configuration) {
 		return createDisplayNameSupplier(testClass, configuration,
 			generator -> generator.generateDisplayNameForMethod(testClass, testMethod));
 	}
 
 	private static Supplier<String> createDisplayNameSupplier(Class<?> testClass, JupiterConfiguration configuration,
 			Function<DisplayNameGenerator, String> generatorFunction) {
-		Preconditions.notNull(generatorFunction, "DisplayNameGenerator function must not be null");
-		return () -> getOptionalDisplayNameGenerator(testClass) //
+		return () -> findDisplayNameGenerator(testClass) //
 				.map(generatorFunction) //
 				.orElseGet(() -> generatorFunction.apply(configuration.getDefaultDisplayNameGenerator()));
 	}
 
-	private static Optional<DisplayNameGenerator> getOptionalDisplayNameGenerator(Class<?> testClass) {
+	private static Optional<DisplayNameGenerator> findDisplayNameGenerator(Class<?> testClass) {
 		Preconditions.notNull(testClass, "Test class must not be null");
 
 		return AnnotationUtils.findAnnotation(testClass, DisplayNameGeneration.class, true) //

junit-jupiter-engine/src/test/java/org/junit/jupiter/engine/descriptor/DisplayNameUtilsTests.java

@@ -101,6 +101,15 @@ void shouldGetDisplayNameFromDefaultDisplayNameGenerator() {
 
 				assertThat(displayName.get()).isEqualTo("class-display-name");
 			}
+
+			@Test
+			void shouldFallbackOnDefaultDisplayNameGeneratorWhenNullIsGenerated() {
+				when(configuration.getDefaultDisplayNameGenerator()).thenReturn(new CustomDisplayNameGenerator());
+				Supplier<String> displayName = DisplayNameUtils.createDisplayNameSupplierForClass(
+					NullDisplayNameTestCase.class, configuration);
+
+				assertThat(displayName.get()).isEqualTo("class-display-name");
+			}
 		}
 	}
 
@@ -126,6 +135,15 @@ void shouldGetDisplayNameFromDefaultDisplayNameGenerator() {
 
 			assertThat(displayName.get()).isEqualTo("nested-class-display-name");
 		}
+
+		@Test
+		void shouldFallbackOnDefaultDisplayNameGeneratorWhenNullIsGenerated() {
+			when(configuration.getDefaultDisplayNameGenerator()).thenReturn(new CustomDisplayNameGenerator());
+			Supplier<String> displayName = DisplayNameUtils.createDisplayNameSupplierForNestedClass(
+				NullDisplayNameTestCase.NestedTestCase.class, configuration);
+
+			assertThat(displayName.get()).isEqualTo("nested-class-display-name");
+		}
 	}
 
 	@Nested
@@ -153,6 +171,17 @@ void shouldGetDisplayNameFromDefaultNameGenerator() throws Exception {
 
 			assertThat(displayName).isEqualTo("method-display-name");
 		}
+
+		@Test
+		void shouldFallbackOnDefaultDisplayNameGeneratorWhenNullIsGenerated() throws Exception {
+			Method method = NullDisplayNameTestCase.class.getDeclaredMethod("test");
+			when(configuration.getDefaultDisplayNameGenerator()).thenReturn(new CustomDisplayNameGenerator());
+
+			String displayName = DisplayNameUtils.determineDisplayNameForMethod(NullDisplayNameTestCase.class, method,
+				configuration);
+
+			assertThat(displayName).isEqualTo("method-display-name");
+		}
 	}
 
 	private LogRecord firstWarningLogRecord(LogRecordListener listener) throws AssertionError {
@@ -188,4 +217,36 @@ static class NotDisplayNameTestCase {
 	class NestedTestCase {
 	}
 
+	@DisplayNameGeneration(value = NullDisplayNameGenerator.class)
+	static class NullDisplayNameTestCase {
+
+		@Test
+		void test() {
+		}
+
+		@Nested
+		class NestedTestCase {
+		}
+
+	}
+
+	static class NullDisplayNameGenerator implements DisplayNameGenerator {
+
+		@Override
+		public String generateDisplayNameForClass(Class<?> testClass) {
+			return null;
+		}
+
+		@Override
+		public String generateDisplayNameForNestedClass(Class<?> nestedClass) {
+			return null;
+		}
+
+		@Override
+		public String generateDisplayNameForMethod(Class<?> testClass, Method testMethod) {
+			return null;
+		}
+
+	}
+
 }