title: ref vs idref attributes in spring bean declaration
body: <p>Can someone tell me the difference between</p>
  
  <pre><code>&lt;bean id="b1" class="" /&gt;
  
  &lt;bean id="" class=""&gt;
   &lt;property name="b1" ref="b1" /&gt;
  &lt;/bean&gt;
  </code></pre>
  
  <p>and</p>
  
  <pre><code>&lt;bean id="" class=""&gt;
   &lt;property name="b1" idref="b1" /&gt;
  &lt;/bean&gt;
  </code></pre>
  
  <p>and which one has to be used when?</p>