The ThreadMonitor is distributed as part of the jetty-monitor module.
In order to start ThreadMonitor when server starts up, the following command line should be used.
java -jar start.jar OPTIONS=monitor jetty-monitor.xml
To run ThreadMonitor on a Jetty installation that doesn't include jetty-monitor module, the jetty-monitor-[version].jar file needs to be copied into ${jetty.home}/lib/ext directory, and jetty-monitor.xml configuration file needs to be copied into ${jetty.home}/etc directory. Subsequently, the following command line should be used.
java -jar start.jar etc/jetty-monitor.xml
If running Jetty on Java VM version 1.5, the -Dcom.sun.management.jmxremote option should be added to the command lines above in order to enable the JMX agent.