Remote Debugging a Java Application Inside a Docker Container

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One problem when trying to debug a Java application running inside a Docker container is:

You can't expose an additional port when re(starting) a Docker container.

Here are the steps I used to create a remote debugging session for Planets:

In our scenario we can't simply override the Dockerfile CMD or pass an environment variable JAVA_OPTS to enable remote debugging.

So we have to "enter" the container with exec and add the JAVA_OPTS to the start script there:

$ docker exec -it planets-staging /bin/bash
# vi eclipse.ini
<append JAVA_OPTS="-agentlib:jdwp=transport=dt_socket,address=8000,server=y,suspend=n">
Next we create a snapshot of the container and stop it afterwards.

$ docker commit planets-staging debug/staging-snapshot
$ docker stop planets-staging
Now we can expose an additional port for remote debugging when we run the snapshot:

$ docker run -i -p 80:80 -p 8000:8000 debug/staging-snapshot
Just for completeness in case you have a scenario where you can pass JAVA_OPTS as environment to your start script.

$ docker run -i -p 80:80 \
  -e "JAVA_OPTS=\"-agentlib:jdwp=transport=dt_socket,address=8000,server=y,suspend=n\"" \
  -p 8000:8000 debug/staging-snapshot
Happy remote debugging!

Pd: not tried :-S
Credits to: https://devops.datenkollektiv.de/remote-debugging-a-java-application-inside-a-docker-container.html

didn't work

This works

That only works if your container is running Tomcat because JAVA_OPTS is a Tomcat specific environment variable that it's launcher scripts pass on to the JVM.

In Java 8 the JDK supports a JAVA_TOOL_OPTIONS environment variable so to enable the debugger for any Java application you need to use:
-e "JAVA_TOOL_OPTIONS=\"-agentlib:jdwp=transport=dt_socket,address=8000,server=y,suspend=n\""

In Java 10, the JDK supports a JDK_JAVA_OPTIONS (JDK-8170832) environment variable for memory settings and such.

I have found that I explicitly need to bind to all interfaces to make it work, either address=0.0.0.0:8000 or address=*:8000

I have found that I explicitly need to bind to all interfaces to make it work, either address=0.0.0.0:8000 or address=*:8000

This works!!!

I have found that I explicitly need to bind to all interfaces to make it work, either address=0.0.0.0:8000 or address=*:8000

This works!!!

I'll give a big 'ol +1 on this one: added *:8000 was the only thing that worked for me

@telekosmos and @pfrandsen thank you! this gist and *:8000 save my day! "big 'ol +1" indeed!!