SpringBoot Difficult to reason about application context type in SpringApplication

I wanted to write a listener that ensured the application context type in a SpringApplication was not annotation-driven (the default setting). It almost works to simply inspect the webApplicationType (which has a public accessor) because the default application context type is strongly correlated with that, but not quite. It doesn't work in tests that expect a mock server (the default) because the test context loader sets the webApplicationType to REACTIVE (until recently it was NONE by accident), and then you have to guess that the user wants to run a real server and get it wrong (the server fails to start, but it's not supposed to even be created in the test).

We could fix this (for me) by simply providing a public accessor for the applicationContextType in SpringApplication. I have worked around it using reflection to access that field. I'm open to other ideas.

My listener currently, for reference, in case it helps:

if (event instanceof ApplicationEnvironmentPreparedEvent) {
    ApplicationEnvironmentPreparedEvent prepared = (ApplicationEnvironmentPreparedEvent) event;
    if (!isEnabled(prepared.getEnvironment())) {
            return;
    }
    SpringApplication application = prepared.getSpringApplication();
    WebApplicationType type = application.getWebApplicationType();
    Class<?> contextType = getApplicationContextType(application);
    if (type == WebApplicationType.NONE) {
        if (contextType == AnnotationConfigApplicationContext.class) {
            application
                    .setApplicationContextClass(GenericApplicationContext.class);
        }
    }
    else if (type == WebApplicationType.REACTIVE) {
        if (contextType == AnnotationConfigReactiveWebApplicationContext.class) {
            application.setApplicationContextClass(
                    ReactiveWebServerApplicationContext.class);
        }
    }
    else if (type == WebApplicationType.SERVLET) {
        if (contextType == AnnotationConfigServletWebServerApplicationContext.class) {
            application.setApplicationContextClass(
                    ServletWebServerApplicationContext.class);
        }
    }
}

Comment From: snicoll

In the case of a test it is not that simple. There are actually more flavours of this as indicated by the WebEnvironment enum:

• No web support at all
• Mocking the web environment (with support for both Servlet and Reactive)
• Starting a full web environment, either on a defined port or a random port (ditto)

The real problem IMO is that we still have a few places where the logic is handled manually rather than derive from something that was set upstream. I wonder now if changing that NONE to REACTIVE doesn't lead to a regression in integration tests.

Comment From: dsyer

I think that REACTIVE is still the best setting for that property in the test (and it works in regular Spring Boot apps). The problem is really that the complete state of SpringApplication is not knowable from outside, and sometimes you need to reason about it. We could either add more public accessors and let users figure out the state themselves, or, as you say, make the state more expressive.

Comment From: andresetevejob

I want to work on this issue

Comment From: snicoll

@andresetevejob thanks for the offer but with RSocket support this will have to be revisited one way or the other (ping @bclozel). I've added pending-design-work to make it clear we need to make up our mind on it.

Comment From: bclozel

I've closed #16412 as we don't need to do that anymore. The REACTIVE type might not be strictly necessary, so we still need to make up our mind about it - this time with the fact that it seems that RSocket apps don't really need their own application type to work.

Comment From: philwebb

We're cleaning out the issue tracker and closing issues that we've not seen much demand to fix. Feel free to comment with additional justifications if you feel that this one should not have been closed.