jar中的Java反射和清单文件

I would like to (and I don't know if it's possible) do something if jarA is in my classpath and do something else if jarB is in my classpath. I am NOT going to be specifying these jars in the Netbeans project library references because I don't know which of the jars will be used.I would like to (and I don't know if it's possi