Why are only some of my attributes shown in the response xml of jaxws?

I created a jaxws webservice, but it returns only some of the attributes of my objects in the response xml.

E.g.

public class MyObject {
   private String attribute1;
   private String attribute2;

   //getter and setter
}

But the returned XML only contains

<soap:Envelope xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
   <soap:Body>
      <ns2:mynamespaceResponse xmlns:ns2="myn开发者_JAVA技巧amespace">
         <myObject>
            <attribute1>stringcontent</attribute1>
         </myObject>
             ....

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Attributes which are null are not shown in the XML. In the example, attribute 2 is null, and therefore it's not shown.