JAXB: Access to private fields by the framework

How does the JAXB framework realize access to private fields?  I've 
tried the following example works just fine in both directions: 
marshaling and unmarshaling.

---TuBean.java
im****t java.io.StringReader;
im****t java.io.StringWriter;

im****t javax.xml.bind.JAXBContext;
im****t javax.xml.bind.Marshaller;
im****t javax.xml.bind.Unmarshaller;
im****t javax.xml.bind.annotation.XmlElement;
im****t javax.xml.bind.annotation.XmlRootElement;

@[EMAIL PROTECTED]
(name="DOC")
public class TuBean {

     @[EMAIL PROTECTED]
(name="FOO")
     private String foo;

     @[EMAIL PROTECTED]
(name="BAR")
     private String bar;

     @[EMAIL PROTECTED]
(name="BAZ")
     private String baz;

     @[EMAIL PROTECTED]
(name="TAZ")
     private String taz;

     public static void main(String[] args) throws Exception {
         TuBean data = new TuBean();
         data.foo = "123";
         data.bar = "xyz";
         data.taz = "***";

         JAXBContext bindContext =
                 JAXBContext.newInstance(TuBean.class);
         Marshaller marshaller = bindContext.createMarshaller();
         marshaller.setProperty(Marshaller.JAXB_FRAGMENT,
                                Boolean.TRUE);
         marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT,
                                Boolean.TRUE);

         StringWriter buf = new StringWriter();
         marshaller.marshal(data, buf);
         marshaller.marshal(data, System.out);
         System.out.println();

         Unmarshaller unmarshaller =
                 bindContext.createUnmarshaller();
         data = (TuBean) unmarshaller
                 .unmarshal(new StringReader(buf.toString()));
         System.out.println("foo=" + data.foo);
         System.out.println("bar=" + data.bar);
         System.out.println("baz=" + data.baz);
         System.out.println("taz=" + data.taz);
     }

}
---TuBean.java--

-- 
Stanimir