This is a test project meant to highlight a difference in filter registration
between a normal Spring Boot application startup and a test annotated with
@AutoConfigureMockMvc.
The setup is roughly the following:
There is an auto configuration SampleFilterAutoConfiguration that creates
a FilterRegistrationBean registering a SampleFilter with order 0. The
assumption is that this would happen inside a library.
There is another (non-auto) configuration ReorderingFilterConfiguration that
aims to override the order of the SampleFilter to
Ordered.HIGHEST_PRECEDENCE by registering another FilterRegistrationBean
with the same filter instance.
On application startup, this leads to a single addition of the filter to the
filter chain. The second addition with the lower order is ignored in
AbstractFilterRegistrationBean because the servlet context returns null due
to the already existing filter.
On test startup, however, the SpringBootMockMvcBuilderCustomizer simply adds
all filters to a list, which leads to a duplicate entry of the same filter to
the mock filter chain. You can see this because the filter logs a message twice
during a single HTTP request, as opposed to sending the same request to the
application started in the normal way.