1) Modify catalina.bat file to select the right JPDA debug protocol and port, say,
set JPDA_TRANSPORT=dt_socket
set JPDA_ADDRESS=8000
Then, start Tomcat in the debug mode.
catalina.bat jpda start
2) Under Eclipse, select “Run->Debug Configurations…”. Click on “Remote Java Applications”, then click “New”. Type in the title and all. Save and run. Eclipse will connect to the VM that Tomcat is running under.