Add an example showing how to use AutoConfigureRestDocs with WebTestClient

@AutoConfigureRestDocs used with WebTestClient causes the following NullPointerException:

java.lang.NullPointerException
	at java.util.HashMap.putMapEntries(HashMap.java:501)
	at java.util.HashMap.<init>(HashMap.java:490)
	at org.springframework.restdocs.webtestclient.WebTestClientRestDocumentation.retrieveConfiguration(WebTestClientRestDocumentation.java:143)
	at org.springframework.restdocs.webtestclient.WebTestClientRestDocumentation.lambda$document$0(WebTestClientRestDocumentation.java:80)
	at org.springframework.test.web.reactive.server.DefaultWebTestClient$DefaultBodyContentSpec.lambda$consumeWith$3(DefaultWebTestClient.java:540)
	at org.springframework.test.web.reactive.server.ExchangeResult.assertWithDiagnostics(ExchangeResult.java:197)
	at org.springframework.test.web.reactive.server.DefaultWebTestClient$DefaultBodyContentSpec.consumeWith(DefaultWebTestClient.java:540)
	at com.example.demo.web.UserControllerWithAutoconfigureDocTest.shouldGetUser(UserControllerWithAutoconfigureDocTest.java:31)

Here’s a complete minimal test case reproducing the problem: https://github.com/jnizet/webtestclientrestdocsissue

See the failing test, with the complete stack trace, in https://github.com/jnizet/webtestclientrestdocsissue/blob/master/src/test/java/com/example/demo/web/UserControllerWithAutoconfigureDocTest.java

The same test not using REST Docs passes: https://github.com/jnizet/webtestclientrestdocsissue/blob/master/src/test/java/com/example/demo/web/UserControllerTest.java

And the same test using REST docs but configuring it manually also passes: https://github.com/jnizet/webtestclientrestdocsissue/blob/master/src/test/java/com/example/demo/web/UserControllerWithApplicationContextDocTest.java

[wilkinsona]

Thanks for the offer. The problem isn’t specific to WebTestClient and REST Docs’ MockMvc and REST Assured integration can fail in a similar way. I think it’s right that it fails and I don’t think that just adding a null check would be an improvement. It may, however, be an improvement to check for the configuration being null and, if it is, throwing something like an IllegalStateException that guides the user towards the integration with whatever testing client is in use being incorrectly configured.

[snicoll]

Closing in favour of PR #15978