Debugging Tomcat in Docker container

Question

I have a CoreOS running in Vagrant. Vagrant private network IP is 192.168.111.1. Inside a CoreOS is a docker container with Tomcat 8.0.32. Pretty much everything works ok (app deployment etc.) just debugging does not. Tomcat is mapped to 8080 port and the JPDA port should be 8000.

Facts

Tomcat JPDA is configured with:

JDPA_OPTS -agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=8000

It starts with catalina.sh jpda start command. The output in the console when running it with docker-compose is:

tomcat | Listening for transport dt_socket at address: 8000

From the container info I assume that ports are mapped as they should:

CONTAINER ID        IMAGE       COMMAND      CREATED             STATUS              PORTS                                            NAMES
dcae1e0148f8        tomcat      "/run.sh"    8 minutes ago       Up 8 minutes        0.0.0.0:8000->8000/tcp, 0.0.0.0:8080->8080/tcp   tomcat

My docker image is based on this Dockerfile.

Problem

When trying to run Remote debug configuration (screenshot below) I get the error Error running Debug: Unable to open debugger port (192.168.111.1:8000): java.net.ConnectException "Connection refused". I've tried everything from changing various configuration but no luck. Am I missing something?

Answer 1

This is the command I use for this:

docker run -it --rm \
  -e JPDA_ADDRESS=8000 \
  -e JPDA_TRANSPORT=dt_socket \
  -p 8888:8080 \
  -p 9000:8000 \
  -v D:/tc/conf/tomcat-users.xml:/usr/local/tomcat/conf/tomcat-users.xml \
  tomcat:8.0 \
  /usr/local/tomcat/bin/catalina.sh jpda run

Explanation

• -e JPDA_ADDRESS=8000
  debugging port in container, passed as environment variable
• -e JPDA_TRANSPORT=dt_socket
  transport type for debugging as socket, passed as environment variable
• -p 8888:8080
  expose tomcat port 8080 on host as port 8888
• -p 9000:8000
  expose java debugging port 8000 on host as port 9000
• -v {host-file}:{container-file}
  overwrite tomcat-user.xml with my local on, since I need access to the manager api
  omit this line if this isn't necessary for your use case
• tomcat:8.0
  see https://hub.docker.com/_/tomcat/
• /usr/local/tomcat/bin/catalina.sh jpda run
  command to run in the container

Answer 2

I resolved a similar, if not the same, issue when using docker-compose.

It involved the environment variables not being passed properly from the docker-compose.yml file.

See my stack overflow issue:

Answer 3

The accepted answer didn't work for me, apparently because I was using Java 11. It seems that if you're using Java 9 or newer, you need to specify the JPDA address like this:

JPDA_ADDRESS=*:8100

Answer 4

You need to make sure that port 8080 is exposed to IntelliJ for connection. That is while running docker you shall require something like docker run -p 8080:8080

For example, I am able to achieve the similar requirement like this by doing below mentioned steps/checks.

This is what my docker run command looks like:

sudo docker run --privileged=true -d -p 63375:63375 -p 63372:8080 -v /tmp/:/usr/local/tomcat/webapps/config <container name>:<tag>

NOTE: I am exposing an extra port 63375 on container and on my host both. The same port I am using in CATALINA_OPTS below.

This is what my entry point (for the image that I am building) looks like. NOTE: I am using CATALINA_OPTS. Also, I am using maven to create image so below is excrept from pom.xml.

<entryPoint>
    <shell>cd /usr/local/tomcat/bin; CATALINA_OPTS="-agentlib:jdwp=transport=dt_socket,address=63375,server=y,suspend=n" catalina.sh run</shell>
</entryPoint>

Answer 5

You can always update the Dockerfile to something like the following: -

FROM tomcat:8-jre8
MAINTAINER me

ADD target/app.war /usr/local/tomcat/webapps/app.war

ENV JPDA_ADDRESS="8000"
ENV JPDA_TRANSPORT="dt_socket"

EXPOSE 8080 8000
ENTRYPOINT ["catalina.sh", "jpda", "run"]

This does mean though that your docker file has debug on by default which is probably not suited to a production environment.

Answer 6

Try add to your Dockerfile

ENV JPDA_ADDRESS=8000
ENV JPDA_TRANSPORT=dt_socket

It works for me

Answer 7

I have similar setup in my local environment. I included JPDA_ADDRESS as environment variable in the Dockerfile and recreated the containers.

ENV JPDA_ADDRESS 8000

#Expose port 8080, JMX port 13333 & Debug port 8000
EXPOSE 8080 13333 8000

CMD ["tail", "-f", "/dev/null"]  

Answer 8

For me, it's cleaner this way:

docker run -e JAVA_TOOL_OPTIONS="-agentlib:jdwp=transport=dt_socket,address=8000,server=y,suspend=n" -p 8000:8000 tomcat:8.5-jdk8

This way, you don't have to modify your container Dockerfile.

Explanation: all java versions check the JAVA_TOOL_OPTIONS environment variable: https://docs.oracle.com/javase/8/docs/technotes/guides/troubleshoot/envvars002.html