How to debug a Java application running inside a Docker container using VSCode's Java debugger

Question

I have a Java application (.tar) mounted to a container. The entrypoint of the container starts that application.

Dockerfile (the backend folder is mounted into the image as a volume)

FROM openjdk:11.0.7

ENTRYPOINT /backend/entrypoint.sh

entrypoint.sh

java -agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=*:5005 -Xmx2048M -jar backend.jar

Now I want to debug that running application using VSCode's debugger. According to the official VSCode documentation (blog: inspecting containers) this can easily be done with the command palette and the command Debugger: attach to Node.js process.

But in their example they use a Node.js server. In my container however, there is no Node.js process that I could attach the debugger to and I can't find an appropriate command for a Java Spring application. So how can I attach the Java debugger of VSCode to an Java application which is already running inside a Docker container?

At another place in their documentation (containers: debug common) they state the following:

The Docker extension currently supports debugging Node.js, Python, and .NET Core applications within Docker containers.

So no mention of Java there but then again at another place (remote: debugging in a container) they clearly talk about a Java application:

For example, adding this to .devcontainer/devcontainer.json will set the Java home path: "settings": { "java.home": "/docker-java-home" }

Answer 1

I got a Spring Boot app to run in an openjdk:11-jre-slim container and was able to successfully debug it with the following configuration.

First, set jvm args when running your container. You do this via entrypoint.sh but I decided to override my container entrypoint in docker-compose. I also expose the debug port.

ports: 
  - 5005:5005
entrypoint: ["java","-agentlib:jdwp=transport=dt_socket,address=*:5005,server=y,suspend=n","-jar","app.jar"]

Then add this configuration to your launch.json in vscode:

{
    "type": "java",
    "name": "Debug (Attach)",
    "projectName": "MyProjectName",
    "request": "attach",
    "hostName": "127.0.0.1",
    "port": 5005
}

You can now start your container and select "Debug (Attach)" under RUN AND DEBUG in VScode. This will begin your typical debug session with breakpoints, variables, etc...

Answer 2

If you set up your run command like this

java -agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=8000 -jar App.jar 

(or however you like to call it, the important bit are the options)

Then make your docker container expose that port. I usually use a docker compose file to do that, so you can easily map the port however you like at run time