WebAsyncTask returns empty page/response

[cdalexndr]

Spring boot 2.2.0.RELEASE

Sample project: https://github.com/cdalexndr/spring-boot-issue-18931

[wilkinsona]

Thanks for the sample. The fact that the controller method you are testing is asynchronous needs to be taken into consideration in your test. Please see the Spring Framework documentation for how to do that.

[cdalexndr]

@wilkinsona Indeed, fixed test.

The problem occurred while running a larger spring boot app. After more debugging, it seems the culprit is ShallowEtagHeaderFilter.
The tests now are green, but running the application and requesting http://localhost:8080/async results in hanged request. Request to http://localhost:8080/normal works fine as expected.

Updated sample repo.
Run gradlew bootRun then navigate to http://localhost:8080/async

[wilkinsona]

ShallowEtagHeaderFilter is misconfigured. By default, it will participate in an async dispatch and only update the header once the async dispatch is complete. You haven't configured the filter's dispatcher types so it uses the default of DispatcherType.REQUEST. As a result, the filter is never called when the dispatch completes so it never gets a chance to update the header and write out the response.

If you want the filter to participate in async dispatches, you need to use a FilterRegistrationBean to configure the required dispatcher types. Replace your @Bean method for the filter with something like this:

@Bean
public FilterRegistrationBean<ShallowEtagHeaderFilter> etagFilter() {
    FilterRegistrationBean<ShallowEtagHeaderFilter> registration = new FilterRegistrationBean<>(new ShallowEtagHeaderFilter());
    registration.setDispatcherTypes(EnumSet.of(DispatcherType.REQUEST, DispatcherType.ASYNC));
    return registration;
}

If you have any further questions, please follow up on Stack Overflow or Gitter. As mentioned in the guidelines for contributing, we prefer to use GitHub issues only for bugs and enhancements.

[cdalexndr]

@wilkinsona the fact that by adding ShallowEtagHeaderFilter with default configuration it breaks all async requests, doesn't this qualify as a bug?
Instead of breaking requests because of default configuration, I was expecting to ignore the filter if it wasn't configured for async requests.

As stated in previous comment, to reproduce the invalid behaviour, pull the latest change in the sample repository, run gradlew bootRun, then navigate to http://localhost:8080/async and notice that the response hangs for 60secs then an empty response is received(bug?).

The tests are all green probably because mock mvc uses a diffferent mechanism for async requests than normal (production) running app.

[wilkinsona]

the fact that by adding ShallowEtagHeaderFilter with default configuration it breaks all async requests, doesn't this qualify as a bug?

Unfortunately, this is out of Spring Boot's control as ShallowEtagHeaderFilter is part of Spring Framework and its the Servlet spec that dictates that the default filter dispatcher types only include REQUEST.

If you'd like to pursue this, you may want to open a Spring Framework issue suggesting a change to the default value of shouldNotFilterAsyncDispatch or some improvements to the relevant section of the documentation.

It would be nice if the filter itself could detect the mis-configuration. Unfortunately the Servlet API does not provide a way for a filter to get the dispatcher types with which it has been registered.

and notice that the response hangs for 60secs then an empty response is received(bug?)

No, this isn't a bug. As I said above, the filter is never called when the dispatch completes so it doesn't get a chance to write the response. As a result, the client eventually times out waiting for one. If you correct the dispatcher types as I described above this will not happen.

[cdalexndr]

@wilkinsona thanks for detailed response.

[wilkinsona]

I've opened #18953 to see if there's any room for improvement in Boot on the back of the documentation updates that have just been made in Framework.