JacksonXmlEncoder.canEncode incorrectly returns true for String body with application/xml

[yikuo123]

When using Spring Boot 4's WebClient to send an XML request with a String body and Content-Type: application/xml, the request fails with:

Stream decoding is currently not supported

Minimal Reproducer:

webClient
.post()
.uri(url)
.contentType(MediaType.APPLICATION_XML)
.bodyValue("<xml><a>test</a></xml>")
.retrieve();

Root Cause Analysis

The failure is caused by JacksonXmlEncoder#canEncode, which returns true for MediaType.APPLICATION_XML + String.

However, Jackson’s XML encoder does not​ actually support encoding raw String values.

This behavior differs from JacksonJsonEncoder, which explicitly prevents this case via:

@Override
public boolean canEncode(ResolvableType elementType, @Nullable MimeType mimeType) {
    return super.canEncode(elementType, mimeType) && !String.class.isAssignableFrom(elementType.toClass());
}

JacksonXmlEncoder does not​ override canEncode, so it incorrectly signals support for String bodies.

Expected Behavior

JacksonXmlEncoder.canEncode should return false for String types, similar to JacksonJsonEncoder. And falls back to a non-streaming encoder for String bodies with application/xml.