Properly guard customization of application context class

SpringApplication wrongly expects spring-web to be on the classpath to
figure out whether or not the web environment should be enabled for a
custom context class.

We now properly guard this check so that the web environment is not
enabled (read: not checked) if `spring-web` is not available.

Closes spring-projectsgh-3856

Author: snicoll

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

@@ -138,6 +138,7 @@
  * @author Dave Syer
  * @author Andy Wilkinson
  * @author Christian Dupuis
+ * @author Stephane Nicoll
  * @see #run(Object, String[])
  * @see #run(Object[], String[])
  * @see #SpringApplication(Object...)
@@ -226,21 +227,12 @@ private void initialize(Object[] sources) {
 		if (sources != null && sources.length > 0) {
 			this.sources.addAll(Arrays.asList(sources));
 		}
-		this.webEnvironment = deduceWebEnvironment();
+		this.webEnvironment = isSpringWebAvailable();
 		setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
 		setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
 		this.mainApplicationClass = deduceMainApplicationClass();
 	}
 
-	private boolean deduceWebEnvironment() {
-		for (String className : WEB_ENVIRONMENT_CLASSES) {
-			if (!ClassUtils.isPresent(className, null)) {
-				return false;
-			}
-		}
-		return true;
-	}
-
 	private Class<?> deduceMainApplicationClass() {
 		try {
 			StackTraceElement[] stackTrace = new RuntimeException().getStackTrace();
@@ -872,7 +864,8 @@ public void setResourceLoader(ResourceLoader resourceLoader) {
 	public void setApplicationContextClass(
 			Class<? extends ConfigurableApplicationContext> applicationContextClass) {
 		this.applicationContextClass = applicationContextClass;
-		if (!WebApplicationContext.class.isAssignableFrom(applicationContextClass)) {
+		if (isSpringWebAvailable() && !WebApplicationContext.class.isAssignableFrom(
+				applicationContextClass)) {
 			this.webEnvironment = false;
 		}
 	}
@@ -935,6 +928,15 @@ public Set<ApplicationListener<?>> getListeners() {
 		return asUnmodifiableOrderedSet(this.listeners);
 	}
 
+	private boolean isSpringWebAvailable() {
+		for (String className : WEB_ENVIRONMENT_CLASSES) {
+			if (!ClassUtils.isPresent(className, null)) {
+				return false;
+			}
+		}
+		return true;
+	}
+
 	/**
 	 * Static helper that can be used to run a {@link SpringApplication} from the
 	 * specified source using default settings.