Do not register shutdownHook for WAR deployments

The application context shutdownHook is not needed for WAR deployments,
and we should let the Servlet container handle the application lifecycle
here.

Closes gh-19398

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/web/servlet/support/SpringBootServletInitializer.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * 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.
@@ -131,6 +131,7 @@ public abstract class SpringBootServletInitializer implements WebApplicationInit
 		if (this.registerErrorPageFilter) {
 			application.addPrimarySources(Collections.singleton(ErrorPageFilterConfiguration.class));
 		}
+		application.setRegisterShutdownHook(false);
 		return run(application);
 	}
 

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/web/servlet/support/SpringBootServletInitializerTests.java

 /*
- * Copyright 2012-2019 the original author or authors.
+ * 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.
@@ -100,6 +100,12 @@ class SpringBootServletInitializerTests {
 				WithConfigurationAnnotation.class);
 	}
 
+	@Test
+	void shutdownHookIsNotRegistered() {
+		new WithConfigurationAnnotation().createRootApplicationContext(this.servletContext);
+		assertThat(this.application).hasFieldOrPropertyWithValue("registerShutdownHook", false);
+	}
+
 	@Test
 	void errorPageFilterRegistrationCanBeDisabled() {
 		WebServer webServer = new UndertowServletWebServerFactory(0).getWebServer((servletContext) -> {