Start Tomcat in dubug mode and connect with idea.

Tomcat : 6.0.14
idea : Ultimate 9

Tomcat configurations.

copy these to catalina.sh
JAVA_OPTS="-Xdebug -Xrunjdwp:transport=dt_socket,address=52385,suspend=n,server=y $JAVA_OPTS"
JPDA_TRANSPORT=dt_socket
JPDA_ADDRESS=8000

Start tomcat : sh catalina.sh run

Idea configurations.

Go to Tools -> debug... (In server tab)
1. Select application server as tomcat 6.
2. Uncheck start browser.
3. Set host as localhost.
4. Set port as 8000

(In startup/connection tab)
1. Select debug.
2. Transport : socket
3. port : 52385

uncheck all in "berfore launch".                                                                              

Click debug.