Improve class condition check

This commit improves a class condition check on the actual
implementation rather than the general purpose interface.

Closes gh-11608

spring-boot/src/main/java/org/springframework/boot/liquibase/LiquibaseServiceLocatorApplicationListener.java

 /*
- * Copyright 2012-2016 the original author or authors.
+ * Copyright 2012-2018 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.
@@ -40,7 +40,8 @@ public class LiquibaseServiceLocatorApplicationListener
 
 	@Override
 	public void onApplicationEvent(ApplicationStartingEvent event) {
-		if (ClassUtils.isPresent("liquibase.servicelocator.ServiceLocator", null)) {
+		if (ClassUtils.isPresent("liquibase.servicelocator.CustomResolverServiceLocator",
+				event.getSpringApplication().getClassLoader())) {
 			new LiquibasePresent().replaceServiceLocator();
 		}
 	}

spring-boot/src/test/java/org/springframework/boot/liquibase/LiquibaseServiceLocatorApplicationListenerTests.java

 /*
- * Copyright 2012-2016 the original author or authors.
+ * Copyright 2012-2018 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.
@@ -17,7 +17,13 @@
 package org.springframework.boot.liquibase;
 
 import java.lang.reflect.Field;
+import java.net.URL;
+import java.net.URLClassLoader;
+import java.util.Arrays;
+import java.util.List;
 
+import liquibase.servicelocator.CustomResolverServiceLocator;
+import liquibase.servicelocator.DefaultPackageScanClassResolver;
 import liquibase.servicelocator.ServiceLocator;
 import org.junit.After;
 import org.junit.Test;
@@ -25,6 +31,7 @@ import org.junit.Test;
 import org.springframework.boot.SpringApplication;
 import org.springframework.context.ConfigurableApplicationContext;
 import org.springframework.context.annotation.Configuration;
+import org.springframework.core.io.DefaultResourceLoader;
 import org.springframework.util.ReflectionUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
@@ -33,6 +40,7 @@ import static org.assertj.core.api.Assertions.assertThat;
  * Tests for {@link LiquibaseServiceLocatorApplicationListener}.
  *
  * @author Phillip Webb
+ * @author Stephane Nicoll
  */
 public class LiquibaseServiceLocatorApplicationListenerTests {
 
@@ -46,15 +54,33 @@ public class LiquibaseServiceLocatorApplicationListenerTests {
 	}
 
 	@Test
-	public void replacesServiceLocator() throws Exception {
+	public void replacesServiceLocator() throws IllegalAccessException {
 		SpringApplication application = new SpringApplication(Conf.class);
 		application.setWebEnvironment(false);
 		this.context = application.run();
+		Object resolver = getServiceLocator();
+		assertThat(resolver).isInstanceOf(SpringPackageScanClassResolver.class);
+	}
+
+	@Test
+	public void replaceServiceLocatorBacksOffIfNotPresent()
+			throws IllegalAccessException {
+		SpringApplication application = new SpringApplication(Conf.class);
+		application.setWebEnvironment(false);
+		DefaultResourceLoader resourceLoader = new DefaultResourceLoader();
+		resourceLoader.setClassLoader(new ClassHidingClassLoader(
+				CustomResolverServiceLocator.class));
+		application.setResourceLoader(resourceLoader);
+		this.context = application.run();
+		Object resolver = getServiceLocator();
+		assertThat(resolver).isInstanceOf(DefaultPackageScanClassResolver.class);
+	}
+
+	private Object getServiceLocator() throws IllegalAccessException {
 		ServiceLocator instance = ServiceLocator.getInstance();
 		Field field = ReflectionUtils.findField(ServiceLocator.class, "classResolver");
 		field.setAccessible(true);
-		Object resolver = field.get(instance);
-		assertThat(resolver).isInstanceOf(SpringPackageScanClassResolver.class);
+		return field.get(instance);
 	}
 
 	@Configuration
@@ -62,4 +88,32 @@ public class LiquibaseServiceLocatorApplicationListenerTests {
 
 	}
 
+	private final class ClassHidingClassLoader extends URLClassLoader {
+
+		private final List<Class<?>> hiddenClasses;
+
+		private ClassHidingClassLoader(Class<?>... hiddenClasses) {
+			super(new URL[0], LiquibaseServiceLocatorApplicationListenerTests.class.getClassLoader());
+			this.hiddenClasses = Arrays.asList(hiddenClasses);
+		}
+
+		@Override
+		public Class<?> loadClass(String name) throws ClassNotFoundException {
+			if (isHidden(name)) {
+				throw new ClassNotFoundException();
+			}
+			return super.loadClass(name);
+		}
+
+		private boolean isHidden(String name) {
+			for (Class<?> hiddenClass : this.hiddenClasses) {
+				if (hiddenClass.getName().equals(name)) {
+					return true;
+				}
+			}
+			return false;
+		}
+
+	}
+
 }