Guard WebApplicationType detection in case spring web is not present

Closes gh-14589

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/SpringApplication.java

@@ -43,7 +43,6 @@ import org.springframework.boot.Banner.Mode;
 import org.springframework.boot.context.properties.bind.Bindable;
 import org.springframework.boot.context.properties.bind.Binder;
 import org.springframework.boot.context.properties.source.ConfigurationPropertySources;
-import org.springframework.boot.web.reactive.context.ReactiveWebApplicationContext;
 import org.springframework.boot.web.reactive.context.StandardReactiveWebEnvironment;
 import org.springframework.context.ApplicationContext;
 import org.springframework.context.ApplicationContextInitializer;
@@ -76,7 +75,6 @@ import org.springframework.util.ObjectUtils;
 import org.springframework.util.ReflectionUtils;
 import org.springframework.util.StopWatch;
 import org.springframework.util.StringUtils;
-import org.springframework.web.context.WebApplicationContext;
 import org.springframework.web.context.support.StandardServletEnvironment;
 
 /**
@@ -1187,16 +1185,28 @@ public class SpringApplication {
 
 	private WebApplicationType deduceWebApplicationType(
 			Class<?> applicationContextClass) {
-		if (WebApplicationContext.class.isAssignableFrom(applicationContextClass)) {
+		if (safeIsAssignableFrom("org.springframework.web.context.WebApplicationContext",
+				applicationContextClass)) {
 			return WebApplicationType.SERVLET;
 		}
-		if (ReactiveWebApplicationContext.class
-				.isAssignableFrom(applicationContextClass)) {
+		if (safeIsAssignableFrom(
+				"org.springframework.boot.web.reactive.context.ReactiveWebApplicationContext",
+				applicationContextClass)) {
 			return WebApplicationType.REACTIVE;
 		}
 		return WebApplicationType.NONE;
 	}
 
+	private boolean safeIsAssignableFrom(String target, Class<?> type) {
+		try {
+			Class<?> targetClass = ClassUtils.forName(target, getClassLoader());
+			return targetClass.isAssignableFrom(type);
+		}
+		catch (Throwable ex) {
+			return false;
+		}
+	}
+
 	/**
 	 * Sets the {@link ApplicationContextInitializer} that will be applied to the Spring
 	 * {@link ApplicationContext}.

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/SpringApplicationNoWebTests.java

@@ -0,0 +1,69 @@
+/*
+ * 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.
+ * 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.springframework.boot;
+
+import org.junit.After;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+
+import org.springframework.boot.testsupport.runner.classpath.ClassPathExclusions;
+import org.springframework.boot.testsupport.runner.classpath.ModifiedClassPathRunner;
+import org.springframework.context.ConfigurableApplicationContext;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.context.support.StaticApplicationContext;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link SpringApplication} when spring web is not on the classpath.
+ *
+ * @author Stephane Nicoll
+ */
+@RunWith(ModifiedClassPathRunner.class)
+@ClassPathExclusions("spring-web*.jar")
+public class SpringApplicationNoWebTests {
+
+	private ConfigurableApplicationContext context;
+
+	@After
+	public void cleanUp() {
+		if (this.context != null) {
+			this.context.close();
+		}
+	}
+
+	@Test
+	public void detectWebApplicationTypeToNone() {
+		SpringApplication application = new SpringApplication(ExampleConfig.class);
+		assertThat(application.getWebApplicationType())
+				.isEqualTo(WebApplicationType.NONE);
+	}
+
+	@Test
+	public void specificApplicationContextClass() {
+		SpringApplication application = new SpringApplication(ExampleConfig.class);
+		application.setApplicationContextClass(StaticApplicationContext.class);
+		this.context = application.run();
+		assertThat(this.context).isInstanceOf(StaticApplicationContext.class);
+	}
+
+	@Configuration
+	static class ExampleConfig {
+
+	}
+
+}