codecholeric commented on a change in pull request #17133:
URL: https://github.com/apache/flink/pull/17133#discussion_r753306333



##########
File path: 
flink-architecture-tests/src/test/java/org/apache/flink/architecture/common/Conditions.java
##########
@@ -0,0 +1,143 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you 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
+ *
+ *     http://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.apache.flink.architecture.common;
+
+import com.tngtech.archunit.base.DescribedPredicate;
+import com.tngtech.archunit.core.domain.JavaClass;
+import com.tngtech.archunit.core.domain.JavaMethod;
+import com.tngtech.archunit.core.domain.JavaParameterizedType;
+import com.tngtech.archunit.core.domain.JavaType;
+import com.tngtech.archunit.core.domain.properties.HasName;
+import com.tngtech.archunit.lang.ArchCondition;
+import com.tngtech.archunit.lang.ConditionEvents;
+import com.tngtech.archunit.lang.SimpleConditionEvent;
+
+import java.util.Collections;
+import java.util.List;
+import java.util.stream.Collectors;
+import java.util.stream.Stream;
+
+import static 
org.apache.flink.architecture.common.SourcePredicates.isJavaClass;
+
+/** Common conditions for architecture tests. */
+public class Conditions {
+
+    /** Generic condition to check fulfillment of a predicate. */
+    public static <T extends HasName> ArchCondition<T> 
fulfill(DescribedPredicate<T> predicate) {
+        return new ArchCondition<T>(predicate.getDescription()) {
+            @Override
+            public void check(T item, ConditionEvents events) {
+                if (!predicate.apply(item)) {
+                    final String message =
+                            String.format(
+                                    "%s does not satisfy: %s",
+                                    item.getName(), 
predicate.getDescription());
+                    events.add(SimpleConditionEvent.violated(item, message));
+                }
+            }
+        };
+    }
+
+    /**
+     * Tests leaf return types of a method against the given predicate.
+     *
+     * <p>Given some {@link JavaType}, "leaf" types are recursively determined 
as follows:
+     *
+     * <ul>
+     *   <li>If the type is an array type, check its base component type.
+     *   <li>If the type is a generic type, check the type itself and all of 
its type arguments.
+     *   <li>Otherwise, check just the type itself.
+     * </ul>
+     */
+    public static ArchCondition<JavaMethod> haveLeafReturnTypes(

Review comment:
       Originally within ArchUnit I had also attempted to unit test these 
things with sort of stub imported objects, but eventually I switched over to 
simply creating test examples and using the real `ClassFileImporter` for 
imports. It's just more blackbox and the tests are still super fast. If you 
really want to mock-unit-test something like this without creating an example 
class you could ponder about making the signature
   
   ```
   public static <T extends HasName.AndFullName & HasReturnType> 
ArchCondition<T> haveLeafReturnTypes(...)
   ```
   
   Then you could simply mock the interface. But typically within ArchUnit we 
use tests with local classes where possible, like
   
   ```
   @Test
   void example() {
     class SomeExample {...}
   
     new ClassFileImporter().importClass(SomeExample.class);
   }
   ```




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