Debugging a Java/Tomcat App


If you test your Java/Tomcat web App by running it in a UBOS container, the following setup has proven to be useful:

  1. In the container, have systemd start Tomcat with the debug flags on. To do so, say:

    % sudo systemctl edit tomcat8

    and enter the following content:

    Environment='CATALINA_OPTS=-Xdebug -Xrunjdwp:transport=dt_socket,address=8888,server=y,suspend=n'

    Note the quotes.

    Then invoke systemctl restart tomcat8. This will restart Tomcat and your App, but instead of running normally, it will wait for your IDE’s debugger to connect on port 8888 before proceeding.

  2. In the container, open port 8888 in the firewall so the debugger running on the host can connect to Tomcat:

    % sudo vi /etc/iptables/iptables.rules

    Add the following line where similar lines are:

    -A OPEN-PORTS -p tcp --dport 8888 -j ACCEPT

    Restart the firewall: systemctl restart iptables. Note that this setting will be overridden as soon as you invoke ubos-admin setnetconfig, but that should not be an issue in a debug scenario.

    Alternatively you can create a file named, say /etc/ubos/open-ports.d/java-debug with content tcp/8080 and ubos-admin setnetconfig will always keep that port open.

  3. On your host, attach your debugger to the container’s port 8888. In NetBeans, for example, select “Debug / Attach Debugger”, select “JDPA”, “SocketAttach”, “dt_socket”, enter the IP address of your container and port 8888. For good measure, increase the timeout to 60000msec.