public SOAPEncoder build() {
      if (jaxbContextFactory == null) {
        throw new IllegalStateException("JAXBContextFactory must be non-null");
      }
      return new SOAPEncoder(this);
    }
The protocol used to create message factory. Default is "SOAP 1.1 Protocol".

@param soapProtocol a string constant representing the MessageFactory protocol.
@see SOAPConstants#SOAP_1_1_PROTOCOL
@see SOAPConstants#SOAP_1_2_PROTOCOL
@see SOAPConstants#DYNAMIC_SOAP_PROTOCOL
@see MessageFactory#newInstance(String)
soap-jakarta/src/main/java/feign/soap/SOAPEncoder.java