Changing the default XML namespace prefix generated with JAXWS

Maybe it's late for you and I'm not sure if this may work, but you can try.

First you need to implement a SoapHandler and, in the handleMessage method you can modify the SOAPMessage. I'm not sure if you can modify directly that prefix but you can try:

public class MySoapHandler implements SOAPHandler<SOAPMessageContext>
{

  @Override
  public boolean handleMessage(SOAPMessageContext soapMessageContext)
  {
    try
    {
      SOAPMessage message = soapMessageContext.getMessage();
      // I haven't tested this
      message.getSOAPHeader().setPrefix("soapenv");
      soapMessageContext.setMessage(message);
    }
    catch (SOAPException e)
    {
      // Handle exception
    }

    return true;
  }

  ...
}

Then you need to create a HandlerResolver:

public class MyHandlerResolver implements HandlerResolver
{
  @Override
  public List<Handler> getHandlerChain(PortInfo portInfo)
  {
    List<Handler> handlerChain = Lists.newArrayList();
    Handler soapHandler = new MySoapHandler();
    String bindingID = portInfo.getBindingID();

    if (bindingID.equals("http://schemas.xmlsoap.org/wsdl/soap/http"))
    {
      handlerChain.add(soapHandler);
    }
    else if (bindingID.equals("http://java.sun.com/xml/ns/jaxws/2003/05/soap/bindings/HTTP/"))
    {
      handlerChain.add(soapHandler);
    }

    return handlerChain;
  }
}

And finally you'll have to add your HandlerResolver to your client service:

Service service = Service.create(wsdlLoc, serviceName);
service.setHandlerResolver(new MyHandlerResolver());

This post, while likely too late for the original poster, is intended to help others who may run into this. I had to solve this problem in the last few days. In particular, I needed to change the prefixes used in the SOAP envelope because the service provider required that the namespace prefixes conform to a very specific pattern. Conforming to this pattern required changing the namespace prefix for the envelope, header and body, and body elements (from the standard ones put in by JAX-WS). Here is an outline of the solution I used:

import javax.xml.namespace.QName;
import javax.xml.soap.SOAPBody;
import javax.xml.soap.SOAPElement;
import javax.xml.soap.SOAPEnvelope;
import javax.xml.soap.SOAPException;
import javax.xml.soap.SOAPHeader;
import javax.xml.soap.SOAPMessage;
import javax.xml.ws.handler.MessageContext;
import javax.xml.ws.handler.soap.SOAPHandler;
import javax.xml.ws.handler.soap.SOAPMessageContext;
import java.util.HashSet;
import java.util.Iterator;
import java.util.Set;

public class MyMessageNamespaceMapper implements SOAPHandler<SOAPMessageContext> {

  @Override
  public Set<QName> getHeaders() {
    return null;
  }

  @Override
  public boolean handleMessage(SOAPMessageContext context) {
    final Boolean outbound = (Boolean) context.get(MessageContext.MESSAGE_OUTBOUND_PROPERTY);
    // only process outbound messages
    if (outbound) {
      try {
        final SOAPMessage soapMessage = context.getMessage();
        final SOAPEnvelope soapEnvelope = soapMessage.getSOAPPart().getEnvelope();
        final SOAPHeader soapHeader = soapMessage.getSOAPHeader();
        final SOAPBody soapBody = soapMessage.getSOAPBody();

        // STEP 1: add new prefix/namespace entries
        soapEnvelope.addNamespaceDeclaration("S1", "http://schemas.xmlsoap.org/soap/envelope/");
        soapEnvelope.addNamespaceDeclaration("FOO-1", "http://foo1.bar.com/ns");

        // STEP 2: set desired namespace prefixes
        // set desired namespace prefix for the envelope, header and body
        soapEnvelope.setPrefix("S1");
        soapHeader.setPrefix("S1");
        soapBody.setPrefix("S1");
        addDesiredBodyNamespaceEntries(soapBody.getChildElements());

        // STEP 3: remove prefix/namespace entries entries added by JAX-WS
        soapEnvelope.removeNamespaceDeclaration("S");
        soapEnvelope.removeNamespaceDeclaration("SOAP-ENV");
        removeUndesiredBodyNamespaceEntries(soapBody.getChildElements());

        // IMPORTANT! "Save" the changes
        soapMessage.saveChanges();
      }
      catch (SOAPException e) {
        // handle the error
      }
    }

    return true;
  }

  private void addDesiredBodyNamespaceEntries(Iterator childElements) {
    while (childElements.hasNext()) {
      final Object childElementNode = childElements.next();
      if (childElementNode instanceof SOAPElement) {
        SOAPElement soapElement = (SOAPElement) childElementNode;

        // set desired namespace body element prefix
        soapElement.setPrefix("FOO-1");

        // recursively set desired namespace prefix entries in child elements
        addDesiredBodyNamespaceEntries(soapElement.getChildElements());
      }
    }
  }

  private void removeUndesiredBodyNamespaceEntries(Iterator childElements) {
    while (childElements.hasNext()) {
      final Object childElementNode = childElements.next();
      if (childElementNode instanceof SOAPElement) {
        SOAPElement soapElement = (SOAPElement) childElementNode;

        // we remove any prefix/namespace entries added by JAX-WS in the body element that is not the one we want
        for (String prefix : getNamespacePrefixList(soapElement.getNamespacePrefixes())) {
          if (prefix != null && ! "FOO-1".equals(prefix)) {
            soapElement.removeNamespaceDeclaration(prefix);
          }
        }

        // recursively remove prefix/namespace entries in child elements
        removeUndesiredBodyNamespaceEntries(soapElement.getChildElements());
      }
    }
  }

  private Set<String> getNamespacePrefixList(Iterator namespacePrefixIter) {
    Set<String> namespacePrefixesSet = new HashSet<>();
    while (namespacePrefixIter.hasNext()) {
      namespacePrefixesSet.add((String) namespacePrefixIter.next());
    }
    return namespacePrefixesSet;
  }

  @Override
  public boolean handleFault(SOAPMessageContext context) {
    return true;
  }

  @Override
  public void close(MessageContext context) {
  }
}

Setting the handler resolver above on an instance of the service class (generated by JAX-WS/wsimport) looks like this:

yourWebServiceClient.setHandlerResolver(new HandlerResolver() {
  @Override
  public List<Handler> getHandlerChain(PortInfo portInfo) {
    return Arrays.asList(new MyMessageNamespaceMapper());
  }
});