title: Specify default property value as NULL in Spring
body: <p>I want to define default property value in Spring XML configuration file. I want this default value to be <code>null</code>.</p>
  
  <p>Something like this:</p>
  
  <pre><code>...
  &lt;ctx:property-placeholder location="file://${configuration.location}" 
                            ignore-unresolvable="true" order="2" 
                            properties-ref="defaultConfiguration"/&gt;
  
  &lt;util:properties id="defaultConfiguration"&gt;
      &lt;prop key="email.username" &gt;
          &lt;null /&gt;
      &lt;/prop&gt;  
      &lt;prop key="email.password"&gt;
          &lt;null /&gt;
      &lt;/prop&gt;  
  &lt;/util:properties&gt;
  ...
  </code></pre>
  
  <p>This doesn't work. Is it even possible to define <code>null</code> default values for properties in Spring XML configuration?</p>