Jakarta Tomcat is an Apache project and Sun's reference implementation of the JSP and Servlet specifications. As the reference implementation, it exists as a platform to test and develop the emerging specifications and also as a battleground for the future direction of the specs.
Jetty, on the other hand, was designed to be smaller, faster, simpler and more flexible than Tomcat. Like Tomcat, it fully implements the latest JSP and Servlet specs. Unlike Tomcat, it has been refined by commercial and experimental use since 1995 and is well-known for its high-performance track record.
Weighing in at less than 300KB, Jetty can scale to thousands of simultaneous connections and consistently benchmarks as one of the fastest servlet containers on the planet. It has been successfully integrated with IBM tivolli, Sonic MQ, Cisco SESM, JXTA and can even serve JSPs off of a CDROM. Finally, Jetty conforms to Jakarta's own "watchdog" tests, so anything which runs on Tomcat will run on Jetty.
The JBoss Group Project has included the Jetty web server & servlet container for nearly three years, during which time they attempted to rebrand it as "JBossweb". High-performance servlet technologies are absolutely essential for any serious enterprise solution. Tomcat will have a very difficult time fitting into Jetty's tiny shoes and enterprise customers might face more integration hurdles in setting up JBoss to run a high-performance servlet container down the road. µ
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