element required conditioning in jaxb

I need to write a jaxb class where i need to put a condition like, if an attribute value is "abc" then an element is required otherwise it is not required. example :

<element attr1="abc">
  <property attr2="xyz"/> (required as attr1="abc")
</element>

<element attr1="def">
 </element>

Can anyone suggest me the possible syntax? thanks in advance.