Issue #1227: Fix module testing not working due to call to JUnit internal API

spock-core/src/main/java/org/spockframework/runtime/SpockNode.java

@@ -2,13 +2,13 @@
 
 import org.spockframework.runtime.model.*;
 import org.spockframework.runtime.model.parallel.ResourceAccessMode;
+import org.spockframework.util.ClassUtil;
 import org.spockframework.util.ExceptionUtil;
 import spock.config.RunnerConfiguration;
 
 import java.util.*;
 import java.util.stream.Collectors;
 
-import org.junit.platform.commons.util.ClassUtils;
 import org.junit.platform.engine.*;
 import org.junit.platform.engine.support.descriptor.*;
 import org.junit.platform.engine.support.hierarchical.*;
@@ -91,7 +91,7 @@ public Set<ExclusiveResource> getExclusiveResources() {
   protected static MethodSource featureToMethodSource(FeatureInfo info) {
     return MethodSource.from(info.getSpec().getBottomSpec().getReflection().getName(),
       info.getName(),
-      ClassUtils.nullSafeToString(info.getFeatureMethod().getReflection().getParameterTypes()) // TODO replace internal API
+      ClassUtil.allNullSafeToString(info.getFeatureMethod().getReflection().getParameterTypes())
     );
   }
 

spock-core/src/main/java/org/spockframework/util/ClassUtil.java

@@ -0,0 +1,42 @@
+/*
+ * Copyright 2025 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.spockframework.util;
+
+import org.jetbrains.annotations.NotNull;
+
+import java.util.Arrays;
+import static java.util.stream.Collectors.joining;
+
+public class ClassUtil {
+  public static @NotNull String nullSafeToString(@Nullable Class<?> clazz) {
+    if (clazz != null)
+      return clazz.toString();
+    else
+      return "null";
+  }
+
+  // not an overload because I couldn't fix
+  // ambiguous overload errors in the spec
+  public static @NotNull String allNullSafeToString(@Nullable Class<?>... clazzes) {
+    if (clazzes != null)
+      return Arrays.stream(clazzes)
+             .map(ClassUtil::nullSafeToString)
+             .collect(joining(", "));
+    else
+      return "";
+  }
+}

spock-specs/src/test/groovy/org/spockframework/util/ClassUtilSpec.groovy

@@ -0,0 +1,61 @@
+package org.spockframework.util
+
+import spock.lang.Specification
+
+class ClassUtilSpec extends Specification {
+  def 'get string representation of non-null class'() {
+    expect:
+    ClassUtil.nullSafeToString(clazz) == clazz.toString()
+    where:
+    clazz << [Set.class, Tuple.class, ClassUtil.class]
+  }
+
+  def 'get string representation of null class'() {
+    when:
+    String result = ClassUtil.nullSafeToString(null)
+    then:
+    notThrown(NullPointerException)
+    result == 'null'
+  }
+
+  def 'get string representation of multiple potentially null classes'(Class<?>... clazzes) {
+    given:
+    def clazzStrings = []
+    for (clazz in clazzes) {
+      clazzStrings.add(ClassUtil.nullSafeToString(clazz))
+    }
+    when:
+    String result = ClassUtil.allNullSafeToString(clazzes)
+    then: 'result is names from single class overload separated by a comma and space each'
+    notThrown(NullPointerException)
+    result == String.join(', ', clazzStrings)
+    where:
+    clazzes << [[List.class, Set.class, Queue.class, Map.class],
+                [File.class, null, URL.class],
+                [null, null]]
+  }
+
+  def 'get string representation of one class not known to be just one'() {
+    given:
+    Class<?>[] singleton = [Specification.class]
+    Class<?>[] singletonOfNull = [null]
+    expect:
+    ClassUtil.allNullSafeToString(singleton) == ClassUtil.nullSafeToString(Specification.class)
+    ClassUtil.allNullSafeToString(singletonOfNull) == ClassUtil.nullSafeToString(null)
+  }
+
+  def 'get string representation of no classes'() {
+    expect:
+    ClassUtil.allNullSafeToString() == ''
+  }
+
+  def 'get string representation of null array of classes'() {
+    given:
+    Class<?>[] nullArray = null
+    when:
+    String result = ClassUtil.allNullSafeToString(nullArray)
+    then:
+    notThrown(NullPointerException)
+    result == ''
+  }
+}