Do you know JCP and it's JSRs?
A JSR is a part of the Java Community Process(JCP), which has a process in place to allow specification to be proposed, reviewed and finalized by a potentially larg number of community esperts . but before a JSR can be made public, it must also provide example code that not only validate the design, but also serves as a starting point for others who may wish to provide their own implementation . this example code, called Reference Implementation (RI), is a very important aspect of a JSR - many times, the actual implementation of a specification illustarates weakness in the design .

for example JSR 053, specifies both servlet and jsp and its RI is Apache Tomcat . RIs are always free for test purpose; but Integrating RI's to deploy a real applications is usually difficult for you and me !