`HalFormsAdaptingResponseBodyAdvice` throws `NullPointerException` if body is `null`

[vierbergenlars]

This piece of code to get the affordances may result in a NullPointerException in case body is null.

spring-data-rest/spring-data-rest-webmvc/src/main/java/org/springframework/data/rest/webmvc/config/HalFormsAdaptingResponseBodyAdvice.java

Lines 77 to 78 in 3eed1c2

	boolean hasAffordances = body.getLinks().stream()
	.anyMatch(it -> !it.getAffordances().isEmpty());

In the ResponseBodyAdvice interface from spring-webmvc, the body parameter is annotated as @Nullable, an annotation that was not taken into account in HalFormsAdaptingResponseBodyAdvice.

While developing a spring-data-rest application, I encountered a case where the body might be null, in the case that a 304 Not Modified response is being sent.
A Not Modified status can be sent by spring-data-rest when there is an entity with a @LastModifiedDate-annotated property, and an If-Modified-Since request header is sent by the client.
In case the date from If-Modified-Since is after the date from the @LastModifiedDate property, the 304 Not Modified response is sent with a null body, resulting in a NullPointerException from HalFormsAdaptingResponseBodyAdvice, which always results in an Internal Server Error.

[odrotbohm]

This should be fixed in all maintained snapshots (4.0-3.5.x).