Avoid JupiterTestEngine crash if Hamcrest is not on the classpath

Prior to this commit, the JupiterTestEngine would crash without
executing any tests if JUnit 4 was on the classpath but Hamcrest was
not.

This commit fixes this bug by ensuring that initialization of the
OpenTest4JAndJUnit4AwareThrowableCollector class no longer fails if the
org.junit.internal.AssumptionViolatedException class cannot be loaded
from the classpath -- for example, due to a missing Hamcrest dependency.

Fixes #2004

documentation/src/docs/asciidoc/release-notes/release-notes-5.5.2.adoc

@@ -21,7 +21,11 @@ No changes.
 
 ==== Bug Fixes
 
-* ❓
+* The `JupiterTestEngine` no longer crashes without executing any tests if JUnit 4 is on
+  the classpath but Hamcrest is not. Specifically, initialization of the
+  `OpenTest4JAndJUnit4AwareThrowableCollector` class no longer fails if the
+  `org.junit.internal.AssumptionViolatedException` class cannot be loaded from the
+  classpath due to a missing Hamcrest dependency.
 
 
 [[release-notes-5.5.2-junit-vintage]]

junit-jupiter-engine/src/main/java/org/junit/jupiter/engine/support/OpenTest4JAndJUnit4AwareThrowableCollector.java

@@ -12,6 +12,7 @@
 
 import java.util.function.Predicate;
 
+import org.junit.platform.commons.util.BlacklistedExceptions;
 import org.junit.platform.commons.util.ReflectionUtils;
 import org.junit.platform.engine.support.hierarchical.ThrowableCollector;
 import org.opentest4j.TestAbortedException;
@@ -37,10 +38,16 @@ private static Predicate<? super Throwable> createAbortedExecutionPredicate() {
 		Predicate<Throwable> otaPredicate = TestAbortedException.class::isInstance;
 
 		// Additionally support JUnit 4's AssumptionViolatedException?
-		Class<?> clazz = ReflectionUtils.tryToLoadClass(
-			"org.junit.internal.AssumptionViolatedException").toOptional().orElse(null);
-		if (clazz != null) {
-			return otaPredicate.or(clazz::isInstance);
+		try {
+			Class<?> clazz = ReflectionUtils.tryToLoadClass("org.junit.internal.AssumptionViolatedException").get();
+			if (clazz != null) {
+				return otaPredicate.or(clazz::isInstance);
+			}
+		}
+		catch (Throwable throwable) {
+			BlacklistedExceptions.rethrowIfBlacklisted(throwable);
+			// Otherwise ignore it since it's likely a ClassNotFoundException,
+			// NoClassDefFoundError, or similar.
 		}
 
 		// Else just OTA's TestAbortedException

junit-jupiter-engine/src/test/java/org/junit/jupiter/engine/support/OpenTest4JAndJUnit4AwareThrowableCollectorTests.java

@@ -0,0 +1,80 @@
+/*
+ * Copyright 2015-2019 the original author or authors.
+ *
+ * All rights reserved. This program and the accompanying materials are
+ * made available under the terms of the Eclipse Public License v2.0 which
+ * accompanies this distribution and is available at
+ *
+ * https://www.eclipse.org/legal/epl-v20.html
+ */
+
+package org.junit.jupiter.engine.support;
+
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertThrows;
+
+import java.net.URL;
+import java.net.URLClassLoader;
+
+import org.junit.internal.AssumptionViolatedException;
+import org.junit.jupiter.api.Test;
+import org.junit.platform.commons.util.ReflectionUtils;
+
+/**
+ * Unit tests for {@link OpenTest4JAndJUnit4AwareThrowableCollector}.
+ *
+ * @since 5.6
+ */
+class OpenTest4JAndJUnit4AwareThrowableCollectorTests {
+
+	@Test
+	void simulateHamcrestNotInTheClasspath() throws Exception {
+		ClassLoader originalClassLoader = Thread.currentThread().getContextClassLoader();
+		try {
+			HamcrestHidingClassLoader classLoader = new HamcrestHidingClassLoader();
+
+			// We have to set our custom ClassLoader as the TCCL so that
+			// ReflectionUtils uses it (indirectly via ClassLoaderUtils).
+			Thread.currentThread().setContextClassLoader(classLoader);
+
+			// Ensure that our custom ClassLoader actually throws a NoClassDefFoundError
+			// when attempting to load the AssumptionViolatedException class.
+			assertThrows(NoClassDefFoundError.class,
+				() -> ReflectionUtils.tryToLoadClass(AssumptionViolatedException.class.getName()));
+
+			Class<?> clazz = classLoader.loadClass(OpenTest4JAndJUnit4AwareThrowableCollector.class.getName());
+			assertNotNull(ReflectionUtils.newInstance(clazz));
+		}
+		finally {
+			Thread.currentThread().setContextClassLoader(originalClassLoader);
+		}
+	}
+
+	private static class HamcrestHidingClassLoader extends URLClassLoader {
+
+		HamcrestHidingClassLoader() {
+			super(new URL[] {
+					OpenTest4JAndJUnit4AwareThrowableCollector.class.getProtectionDomain().getCodeSource().getLocation() },
+				getSystemClassLoader());
+		}
+
+		@Override
+		public Class<?> loadClass(String name) throws ClassNotFoundException {
+
+			// Load a new instance of the OpenTest4JAndJUnit4AwareThrowableCollector class
+			if (name.equals(OpenTest4JAndJUnit4AwareThrowableCollector.class.getName())) {
+				return findClass(name);
+			}
+
+			// Simulate that Hamcrest is not in the classpath when loading AssumptionViolatedException
+			if (name.equals(AssumptionViolatedException.class.getName())) {
+				throw new NoClassDefFoundError("org/hamcrest/SelfDescribing");
+			}
+
+			// Else
+			return super.loadClass(name);
+		}
+
+	}
+
+}