[chuuminggg]

When modifyHeaders() sets a header and a later modification adds or removes a value of the same header, preprocessing fails with an UnsupportedOperationException:

modifyHeaders().set("a", "alpha").add("a", "avocado");
// java.lang.UnsupportedOperationException
//     at java.base/java.util.AbstractList.add(AbstractList.java:155)
//     at java.base/java.util.AbstractList.add(AbstractList.java:113)
//     at org.springframework.util.MultiValueMapAdapter.add(MultiValueMapAdapter.java:67)
//     at org.springframework.http.HttpHeaders.add(HttpHeaders.java:1796)

modifyHeaders().set("a", "alpha", "avocado").remove("a", "alpha");
// java.lang.UnsupportedOperationException: remove
//     at java.base/java.util.Iterator.remove(Iterator.java:102)
//     at java.base/java.util.AbstractCollection.remove(AbstractCollection.java:285)
//     at org.springframework.restdocs.operation.preprocess.HeadersModifyingOperationPreprocessor$RemoveValueHeaderModification.applyTo(HeadersModifyingOperationPreprocessor.java:182)

set(String, String...) stores its values as Arrays.asList(values), and SetHeaderModification puts that fixed-size list into the headers as it is. The add modification then calls HttpHeaders.add, which appends to that list. The remove-value modification calls List.remove on it.

This PR makes SetHeaderModification put a copy of the values into the headers. The list in the headers can then be modified. Each request and response also gets its own list, so modifying one doesn't change the values that the preprocessor sets on the next one.

The new tests cover adding a value to a header that was set and removing a value from it, for both a request and a response.

[wilkinsona]

Thanks for the suggestion but I consider that to be a misuse of the API.