Allow @RegisterExtension fields to be private

Prior to this commit, an exception was thrown if a @RegisterExtension
field was declared private.

This commit removes this restriction.

Closes #2688
See #864, #2680

documentation/src/docs/asciidoc/user-guide/extensions.adoc

@@ -125,8 +125,8 @@ registered last and _after_ callback extensions to be registered first, relative
 programmatically registered extensions.
 ====
 
-NOTE: `@RegisterExtension` fields must not be `private` or `null` (at evaluation time) but
-may be either `static` or non-static.
+NOTE: `@RegisterExtension` fields must not be `null` (at evaluation time) but may be
+either `static` or non-static.
 
 [[extensions-registration-programmatic-static-fields]]
 ===== Static Fields
@@ -159,9 +159,9 @@ include::{testDir}/example/registration/WebServerDemo.java[tags=user_guide]
 
 The Kotlin programming language does not have the concept of a `static` field. However,
 the compiler can be instructed to generate static fields using annotations. Since, as
-stated earlier, `@RegisterExtension` fields must not be `private` nor `null`, one
-**cannot** use the `@JvmStatic` annotation in Kotlin as it generates `private` fields.
-Rather, the `@JvmField` annotation must be used.
+stated earlier, `@RegisterExtension` fields must not be `null`, one **cannot** use the
+`@JvmStatic` annotation in Kotlin as it generates `private` fields. Rather, the
+`@JvmField` annotation must be used.
 
 The following example is a version of the `WebServerDemo` from the previous section that
 has been ported to Kotlin.

junit-jupiter-api/src/main/java/org/junit/jupiter/api/extension/RegisterExtension.java

@@ -30,8 +30,8 @@
  * to pass arguments to the extension's constructor, {@code static} factory
  * method, or builder API.
  *
- * <p>{@code @RegisterExtension} fields must not be {@code private} or
- * {@code null} (when evaluated) but may be either {@code static} or non-static.
+ * <p>{@code @RegisterExtension} fields must not be {@code null} (when evaluated)
+ * but may be either {@code static} or non-static.
  *
  * <h3>Static Fields</h3>
  *

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

@@ -16,7 +16,6 @@
 import static org.junit.platform.commons.util.ReflectionUtils.HierarchyTraversalMode.TOP_DOWN;
 import static org.junit.platform.commons.util.ReflectionUtils.findFields;
 import static org.junit.platform.commons.util.ReflectionUtils.getDeclaredConstructor;
-import static org.junit.platform.commons.util.ReflectionUtils.isNotPrivate;
 import static org.junit.platform.commons.util.ReflectionUtils.tryToReadFieldValue;
 
 import java.lang.reflect.AnnotatedElement;
@@ -108,9 +107,6 @@ static void registerExtensionsFromFields(ExtensionRegistrar registrar, Class<?>
 		fields.stream()
 			.filter(field -> isAnnotated(field, RegisterExtension.class))
 			.forEach(field -> {
-				Preconditions.condition(isNotPrivate(field), () -> String.format(
-					"Failed to register extension via @RegisterExtension field [%s]: field must not be private.",
-					field));
 				tryToReadFieldValue(field, instance).ifSuccess(value -> {
 					Preconditions.condition(value instanceof Extension, () -> String.format(
 						"Failed to register extension via @RegisterExtension field [%s]: field value's type [%s] must implement an [%s] API.",

junit-jupiter-engine/src/test/java/org/junit/jupiter/engine/extension/ExtensionRegistrationViaParametersAndFieldsTests.java

@@ -533,7 +533,7 @@ private static TestTemplateInvocationContext emptyTestTemplateInvocationContext(
 	static class StaticFieldTestCase {
 
 		@MagicField
-		static String staticField1;
+		private static String staticField1;
 
 		@MagicField
 		static String staticField2;
@@ -560,7 +560,7 @@ static class InstanceFieldTestCase {
 		String instanceField1;
 
 		@MagicField
-		String instanceField2;
+		private String instanceField2;
 
 		@Test
 		void test() {
@@ -606,15 +606,15 @@ static class AllInOneWithTestInstancePerMethodTestCase {
 
 		@RegisterExtension
 		@Order(1)
-		static Extension classLevelExtension1 = new ClassLevelExtension1();
+		private static Extension classLevelExtension1 = new ClassLevelExtension1();
 
 		@RegisterExtension
 		@Order(2)
 		static Extension classLevelExtension2 = new ClassLevelExtension2();
 
 		@RegisterExtension
 		@Order(1)
-		Extension instanceLevelExtension1 = new InstanceLevelExtension1();
+		private Extension instanceLevelExtension1 = new InstanceLevelExtension1();
 
 		@RegisterExtension
 		@Order(2)

junit-jupiter-engine/src/test/java/org/junit/jupiter/engine/extension/ProgrammaticExtensionRegistrationTests.java

@@ -167,8 +167,7 @@ void classLevelWithFieldThatDoesNotImplementAnExtensionApi() {
 	@Test
 	void instanceLevelWithPrivateField() {
 		Class<?> testClass = InstanceLevelExtensionRegistrationWithPrivateFieldTestCase.class;
-		executeTestsForClass(testClass).testEvents().assertThatEvents().haveExactly(1, finishedWithFailure(
-			instanceOf(PreconditionViolationException.class), message(expectedNotPrivateMessage(testClass))));
+		executeTestsForClass(testClass).testEvents().assertStatistics(stats -> stats.succeeded(1));
 	}
 
 	/**
@@ -177,8 +176,7 @@ void instanceLevelWithPrivateField() {
 	@Test
 	void classLevelWithPrivateField() {
 		Class<?> testClass = ClassLevelExtensionRegistrationWithPrivateFieldTestCase.class;
-		executeTestsForClass(testClass).containerEvents().assertThatEvents().haveExactly(1, finishedWithFailure(
-			instanceOf(PreconditionViolationException.class), message(expectedNotPrivateMessage(testClass))));
+		executeTestsForClass(testClass).testEvents().assertStatistics(stats -> stats.succeeded(1));
 	}
 
 	@Test
@@ -219,11 +217,6 @@ void classLevelWithNonExtensionFieldValue() {
 			instanceOf(PreconditionViolationException.class), message(expectedMessage(testClass, String.class))));
 	}
 
-	private String expectedNotPrivateMessage(Class<?> testClass) {
-		return "Failed to register extension via @RegisterExtension field [" + field(testClass)
-				+ "]: field must not be private.";
-	}
-
 	private String expectedMessage(Class<?> testClass, Class<?> valueType) {
 		return "Failed to register extension via @RegisterExtension field [" + field(testClass)
 				+ "]: field value's type [" + (valueType != null ? valueType.getName() : null) + "] must implement an ["
@@ -602,14 +595,16 @@ void test() {
 	static class InstanceLevelExtensionRegistrationWithPrivateFieldTestCase extends AbstractTestCase {
 
 		@RegisterExtension
-		private Extension extension;
+		private Extension extension = new Extension() {
+		};
 
 	}
 
 	static class ClassLevelExtensionRegistrationWithPrivateFieldTestCase extends AbstractTestCase {
 
 		@RegisterExtension
-		private static Extension extension;
+		private static Extension extension = new Extension() {
+		};
 
 	}