Consume Webservice in java

Hi all , I need to call method from webservice that I have created.
The webservice is deployed in jboss 4.0.4 and the webservice is simple
Hello test.

However , when i tried to call method , it gave error below:

I am new in webservice . Anyone have any ideas or suggestion is much
appreciated. Thanks

serialization error: serialization error:
java.lang.NullPointerException
        at
com.sun.xml.rpc.encoding.DynamicInternalTypeMappingRegistry.getSerializer(DynamicInternalTypeMappingRegistry.java:
196)
        at
com.sun.xml.rpc.encoding.literal.LiteralResponseSerializer.initialize(LiteralResponseSerializer.java:
230)
        at
com.sun.xml.rpc.client.dii.BasicCall.createRpcLiteralResponseSerializer(BasicCall.java:
757)
        at
com.sun.xml.rpc.client.dii.BasicCall.getResponseDeserializer(BasicCall.java:
690)
        at com.sun.xml.rpc.client.dii.BasicCall.invoke(BasicCall.java:
486)
        at com.test.client.TestWebService.main(TestWebService.java:45)

This is the web service client code:

public static void main(String [] args){

        try{

         String wsdlURL = "http://cxrus020:8080/HelloWebService/
HelloWS?wsdl";
         String namespace = "urn:HelloWS/wsdl";
         String serviceName = "HelloWS";
         QName serviceQN = new QName(namespace, serviceName);

         ServiceFactory serviceFactory = ServiceFactory.newInstance();
         Service service = serviceFactory.createService(new
URL(wsdlURL), serviceQN);
         Call call = service.createCall();
         call.setPortTypeName(serviceQN);
         call.setOperationName(new QName(namespace, "greet"));
         call.setProperty(Call.ENCODINGSTYLE_URI_PROPERTY, "");

call.addParameter("string_1",serviceQN,javax.xml.rpc.ParameterMode.IN);
         call.setReturnType(serviceQN);
         Object[] inParams = new Object[]{"weetat is testing"};
         String ret = (String) call.invoke(inParams);
         System.out.println("ret:" + ret);

        }catch(Exception ex){
            ex.printStackTrace();
        }

The wsdl file for your reference :

<definitions name='HelloWS' targetNamespace='urn:HelloWS/wsdl'
xmlns='http://schemas.xmlsoap.org/wsdl/' xmlns:soap='http://
schemas.xmlsoap.org/wsdl/soap/' xmlns:tns='urn:HelloWS/wsdl'
xmlns:xsd='http://www.w3.org/2001/XMLSchema'>
 <types></types>
 <message name='HelloWSSEI_greet'>
  <part name='String_1' type='xsd:string'/>
 </message>
 <message name='HelloWSSEI_greetResponse'>
  <part name='result' type='xsd:string'/>
 </message>
 <portType name='HelloWSSEI'>
  <operation name='greet' parameterOrder='String_1'>
   <input message='tns:HelloWSSEI_greet'/>
   <output message='tns:HelloWSSEI_greetResponse'/>
  </operation>
 </portType>
 <binding name='HelloWSSEIBinding' type='tns:HelloWSSEI'>
  <soap:binding style='rpc' transport='http://schemas.xmlsoap.org/soap/
http'/>
  <operation name='greet'>
   <soap:operation soapAction=''/>
   <input>
    <soap:body namespace='urn:HelloWS/wsdl' use='literal'/>
   </input>
   <output>
    <soap:body namespace='urn:HelloWS/wsdl' use='literal'/>
   </output>
  </operation>
 </binding>
 <service name='HelloWS'>
  <port binding='tns:HelloWSSEIBinding' name='HelloWSSEIPort'>
   <soap:address location='http://cxrus020:8080/HelloWebService/
HelloWS'/>
  </port>
 </service>
</definitions>