Fix stack overflow in SpringBootMockResolver

Closes gh-32632

Author: mhalbritter

spring-boot-project/spring-boot-test/src/main/java/org/springframework/boot/test/mock/mockito/SpringBootMockResolver.java

@@ -18,12 +18,14 @@
 
 import org.mockito.plugins.MockResolver;
 
-import org.springframework.test.util.AopTestUtils;
+import org.springframework.aop.TargetSource;
+import org.springframework.aop.framework.Advised;
+import org.springframework.aop.support.AopUtils;
+import org.springframework.util.Assert;
 
 /**
- * A {@link MockResolver} for testing Spring Boot applications with Mockito. Resolves
- * mocks by returning the {@link AopTestUtils#getUltimateTargetObject(Object) ultimate
- * target object} of the instance.
+ * A {@link MockResolver} for testing Spring Boot applications with Mockito. It resolves
+ * mocks by walking the proxy chain until the target or a non-static proxy is found.
  *
  * @author Andy Wilkinson
  * @since 2.4.0
@@ -32,7 +34,28 @@ public class SpringBootMockResolver implements MockResolver {
 
 	@Override
 	public Object resolve(Object instance) {
-		return AopTestUtils.getUltimateTargetObject(instance);
+		return getUltimateTargetObject(instance);
+	}
+
+	@SuppressWarnings("unchecked")
+	private static <T> T getUltimateTargetObject(Object candidate) {
+		Assert.notNull(candidate, "Candidate must not be null");
+		try {
+			if (AopUtils.isAopProxy(candidate) && candidate instanceof Advised) {
+				Advised advised = (Advised) candidate;
+				TargetSource targetSource = advised.getTargetSource();
+				if (targetSource.isStatic()) {
+					Object target = targetSource.getTarget();
+					if (target != null) {
+						return getUltimateTargetObject(target);
+					}
+				}
+			}
+		}
+		catch (Throwable ex) {
+			throw new IllegalStateException("Failed to unwrap proxied object", ex);
+		}
+		return (T) candidate;
 	}
 
 }

spring-boot-project/spring-boot-test/src/test/java/org/springframework/boot/test/mock/mockito/SpringBootMockResolverTests.java

@@ -0,0 +1,66 @@
+/*
+ * Copyright 2012-2020 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.springframework.boot.test.mock.mockito;
+
+import org.junit.jupiter.api.Test;
+
+import org.springframework.aop.SpringProxy;
+import org.springframework.aop.framework.ProxyFactory;
+import org.springframework.aop.target.HotSwappableTargetSource;
+import org.springframework.aop.target.SingletonTargetSource;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link SpringBootMockResolver}.
+ *
+ * @author Moritz Halbritter
+ */
+class SpringBootMockResolverTests {
+
+	@Test
+	void testStaticTarget() {
+		MyServiceImpl myService = new MyServiceImpl();
+		MyService proxy = ProxyFactory.getProxy(MyService.class, new SingletonTargetSource(myService));
+		Object target = new SpringBootMockResolver().resolve(proxy);
+		assertThat(target).isInstanceOf(MyServiceImpl.class);
+	}
+
+	@Test
+	void testNonStaticTarget() {
+		MyServiceImpl myService = new MyServiceImpl();
+		MyService proxy = ProxyFactory.getProxy(MyService.class, new HotSwappableTargetSource(myService));
+		Object target = new SpringBootMockResolver().resolve(proxy);
+		assertThat(target).isInstanceOf(SpringProxy.class);
+	}
+
+	private interface MyService {
+
+		int a();
+
+	}
+
+	private static class MyServiceImpl implements MyService {
+
+		@Override
+		public int a() {
+			return 1;
+		}
+
+	}
+
+}