title: Spring:define bean's property(ref to other bean) as optional
body: <p>Have two bean definitions:</p>
  
  <p>file a.xml</p>
  
  <pre><code>&lt;bean id="A" class="com.A"&gt;
   &lt;property name="bClass" ref="B"/&gt;
  &lt;/bean&gt;
  </code></pre>
  
  <p>file b.xml</p>
  
  <pre><code>&lt;bean id="B" class="com.B"/&gt;
  </code></pre>
  
  <p>In some cases file b.xml does not contain definition of bean B.<br/>
  And from other side,file a.xml always contains link to B definition.</p>
  
  <p>How to define reference to B bean to be <strong>optional</strong>, in order to avoid <code>org.springframework.beans.factory.NoSuchBeanDefinitionException</code></p>