Initializes classes prior to instrumentation to avoid uncontrolled code execution.

src/main/java/org/mockito/internal/creation/bytebuddy/InlineBytecodeGenerator.java

@@ -206,6 +206,7 @@ public <T> Class<? extends T> mockClass(MockFeatures<T> features) {
         Set<Class<?>> types = new HashSet<>();
         types.add(features.mockedType);
         types.addAll(features.interfaces);
+
         synchronized (this) {
             triggerRetransformation(types, false);
         }
@@ -223,17 +224,26 @@ public synchronized void mockClassConstruction(Class<?> type) {
         triggerRetransformation(Collections.singleton(type), false);
     }
 
+    private static void assureInitialization(Class<?> type) {
+        try {
+            Class.forName(type.getName(), true, type.getClassLoader());
+        } catch (Throwable ignore) {
+        }
+    }
+
     private <T> void triggerRetransformation(Set<Class<?>> types, boolean flat) {
         Set<Class<?>> targets = new HashSet<Class<?>>();
 
         for (Class<?> type : types) {
             if (flat) {
                 if (!mocked.contains(type) && flatMocked.add(type)) {
+                    assureInitialization(type);
                     targets.add(type);
                 }
             } else {
                 do {
                     if (mocked.add(type)) {
+                        assureInitialization(type);
                         if (!flatMocked.remove(type)) {
                             targets.add(type);
                         }

subprojects/inline/src/test/java/org/mockitoinline/InitializationTest.java

@@ -0,0 +1,25 @@
+/*
+ * Copyright (c) 2020 Mockito contributors
+ * This program is made available under the terms of the MIT License.
+ */
+package org.mockitoinline;
+
+import org.junit.Test;
+import org.mockito.Mockito;
+
+import static junit.framework.TestCase.assertEquals;
+
+public class InitializationTest {
+
+    @Test
+    public void assure_initialization_prior_to_instrumentation() {
+        @SuppressWarnings("unused")
+        SampleEnum mock = Mockito.mock(SampleEnum.class);
+        SampleEnum[] values = SampleEnum.values();
+        assertEquals("VALUE", values[0].name());
+    }
+
+    public enum SampleEnum {
+        VALUE
+    }
+}

subprojects/inline/src/test/java/org/mockitoinline/PluginTest.java

@@ -7,14 +7,20 @@
 import org.junit.Test;
 import org.mockito.internal.configuration.plugins.Plugins;
 import org.mockito.internal.creation.bytebuddy.InlineByteBuddyMockMaker;
+import org.mockito.internal.util.reflection.ModuleMemberAccessor;
 
 import static org.junit.Assert.*;
 
 public class PluginTest {
 
     @Test
-    public void plugin_type_should_be_inline() throws Exception {
+    public void mock_maker_should_be_inline() throws Exception {
         assertTrue(Plugins.getMockMaker() instanceof InlineByteBuddyMockMaker);
     }
 
+    @Test
+    public void member_accessor_should_be_module() throws Exception {
+        assertTrue(Plugins.getMemberAccessor() instanceof ModuleMemberAccessor);
+    }
+
 }