title: why setDisallowedFields for id? -- Spring petclinic example
body: <p>From the Spring API, i understood that <code>@InitBinder</code> is used to initialize some binding rules..</p>
  
  <p>In the petclinic example why we have <code>setdisallowed("id")</code>? when the id is not displayed on the form?</p>
  
  <pre><code>@InitBinder
  public void setAllowedFields(WebDataBinder dataBinder) {
      dataBinder.setDisallowedFields("id");
  }
  </code></pre>
  
  <p>The id field is not displayed on the web page then why we are using the above code?</p>
  
  <p>can we have some thing like this:</p>
  
  <pre><code>@InitBinder
  public void setAllowedFields(WebDataBinder dataBinder) {
      dataBinder.setDisallowedFields("FirstName");
  }
  </code></pre>
  
  <p>as per the above code the first name field of the owner object will not be set though user enters on the form? Is that correct?</p>
  
  <p><a href="https://src.springframework.org/svn/spring-samples/petclinic/trunk/src/main/java/org/springframework/samples/petclinic/web/AddOwnerForm.java" rel="noreferrer">link for the source</a></p>