Tukukkk

WebSecurityConfigurerAdapter offers two overrides as follows:

protected void configure(AuthenticationManagerBuilder auth)

and

protected void configure(HttpSecurity http)

Both HttpSecurity and AuthenticationManagerBuilder offer registration for authenticationProviders. Is there any difference between registering my providers with one vs the other?

I'm also using Spring boot 2.1 with @SpringBootApplication(exclude = SecurityAutoConfiguration.class) to turn off their autoconfig completely.

From Spring Security Architecture

The main strategy interface for authentication is
AuthenticationManager [...]

The most commonly used implementation of AuthenticationManager is
ProviderManager, which delegates to a chain of
AuthenticationProvider instances. An AuthenticationProvider is a
bit like an AuthenticationManager [...]

A ProviderManager can support multiple different authentication
mechanisms in the same application by delegating to a chain of
AuthenticationProviders. If a ProviderManager doesn’t recognise a
particular Authentication instance type it will be skipped.

A ProviderManager has an optional parent, which it can consult if
all providers return null. If the parent is not available then a null
Authentication results in an AuthenticationException.

Generally speaking WebSecurityConfigurerAdapter provides configuration for HttpSecurity apart from Filter's configuration (like UsernamePasswordAuthenticationFilter, LogoutFilter etc.) it's also creates and configures (adding AuthenticationProviders and parent AuthenticationManager) AuthenticationManagers in HttpSecurity by using AuthenticationManagerBuilder.

WebSecurityConfigurerAdapter will create only one AuthenticationManager for HttpSecurity. However AuthenticationManager has its own AuthenticationProviders and its own optional parent AuthenticationProvider. When you are doing http.authenticationProvider(...) you are adding new AuthenticationProvider to the AuthenticationManager which belong to that http. By using configure(AuthenticationManagerBuilder auth) you are configuring AuthenticationManager which is the parent of the AuthenticationManager which belongs to that particular HttpSecurity.

Spring is providing default configuration for the parent of that particular AuthenticationManager, but by using configure(AuthenticationManagerBuilder auth) you are rejecting spring's configuration in favour of your (auth).