Servlet Container Beans (i.e. Filter, Servlet, etc) are instantiated too soon

[rwinch]

Spring Boot 1.4.0.M3 (and prior versions) initialize Servlet Filters too eagerly. This can lead to inconsistencies between running tests and integration tests (i.e. spring-projects/spring-data-examples#189). It can also lead to ordering issues when working with AOP (i.e. #2578 behaves differently between test and integration test).

The problem is cause by:

• AbstractApplicationContext invokes onRefresh
• EmbeddedWebApplicationContext implements onRefresh by invoking createEmbeddedServletContainer
• When using Tomcat, TomcatEmbeddedServletContainer will start Tomcat
• This causes all the Servlet Container Beans (i.e. Filter, Servlet, etc) to be instantiated eagerly in getServletContextInitializerBeans.

This is not ideal because these Beans might depend on a lot of other Beans (i.e. Servlet initializes all of Spring MVC, Filter initializes Spring Security, etc) and cause all sorts of ordering issues due to the eager initialization of the Beans. It should be noted that while DelegatingFilterProxy is used, the servlet container initializes the DelegatingFilterProxy which in turn initializes the Filter that is marked as a Bean.

As it stands onRefresh initializes every Bean rather than "special Beans" (as is intended).

What is expected is that the Servlet Container Beans would be initialized in finishBeanFactoryInitialization.

[philwebb]

It's unfortunately not possible to call ServletInitializer beans in finishBeanFactoryInitialization because it's too late at that point. Tomcat throws the following exception:

java.lang.IllegalStateException: Servlets cannot be added to context  as the context has been initialised
    at org.apache.catalina.core.ApplicationContext.addServlet(ApplicationContext.java:843) ~[tomcat-embed-core-8.5.3.jar:8.5.3]

[philwebb]

I've made a couple of changes which when combined with these updates to the sample application appear to fix the problem. I'm not keen to backport this to 1.3, so we'll probably leave it as a 1.4 only fix.