I am using WebServiceGatewaySupport class as a support for sending SOAP requests

@Service
public class SoapClient extends WebServiceGatewaySupport implements BeanNameAware {

   private static final Logger LOG = LoggerFactory.getLogger(SoapClient.class);

   
   public <T, U> JAXBElement<T> getSoapResponse(JAXBElement<U> requestObj) {
      WebServiceTemplate webServiceTemplate = getWebServiceTemplate();
      webServiceTemplate.setMessageSender(getWebServiceMessageSender());
      String defaultUri = webServiceTemplate.getDefaultUri();
      try {
         return (JAXBElement<T>) webServiceTemplate.marshalSendAndReceive(requestObj);
      } catch (Exception ex) {
         LOG.error("Exception during soap call");
      }
   }

I am also extending HttpUrlConnectionMessageSender and overriding prepareConnection method for adding custom headers.

public class CustomWebServiceMessageSender extends HttpUrlConnectionMessageSender {

   @Override
   protected void prepareConnection(HttpURLConnection connection) throws IOException {
      addApiKeyHeaders(connection);
      super.prepareConnection(connection);
   }

   private void addApiKeyHeaders(HttpURLConnection connection) {
         connection.setRequestProperty("customHeaderKey", "customHeaderValue");
      }
   }

}

I need to write a test that will check that for any SOAP request the customHeader is added. How can I test that?

Test expectation:
Before adding header:

<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/">
    <SOAP-ENV:Header>
        <!-- Other SOAP header elements -->
    </SOAP-ENV:Header>
    <SOAP-ENV:Body>
        <!-- SOAP body content -->
    </SOAP-ENV:Body>
</SOAP-ENV:Envelope>

With header:

POST /some-endpoint HTTP/1.1
Host: example.com
Content-Type: text/xml; charset=utf-8
Content-Length: xxx
Custom-Header: CustomValue

<SOAP-ENV:Envelope xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/">
    <SOAP-ENV:Header>
        <!-- Other SOAP header elements -->
    </SOAP-ENV:Header>
    <SOAP-ENV:Body>
        <!-- SOAP body content -->
    </SOAP-ENV:Body>
</SOAP-ENV:Envelope>