Debugging a Spring Boot App in Docker

With the trending of containerizing applications, setting up your local environment is now easier than ever. Setup times can be reduced from hours to mere minutes. Saving devs time and frustration.

While setting up one of my apps to run with Docker. I realized there weren't any guides out there that suited my fancy for running apps with debug enabled through docker.

With this article I'll go over what I did to get my app setup with debug using docker-compose. You can view the complete example here.

First I created the Dockerfile:

FROM  openjdk:9-jre # Specifying the base image, I'm using Java 9
ADD target/dublin_rest_demo.jar . # Copies the compiled jar file to the root of the container -- DOUBLE CHECK THIS. 
EXPOSE 8080 8000 # Sets up ports the container listens on: 8080 (spring boots default) and 8000 (debugger port)
CMD java -jar dublin_rest_demo.jar # Specifies the command that will start up the application.

Once I had it setup I ran it to make sure everything worked.

mvn clean install && docker build -t rest_demo:latest . && docker run -p 8080:8080 -p 8000:8000

Then I setup my docker-compose yml file:

version: '3.7'
services:
  web:
    build: .
    ports:
      - "8080:8080" # Mapping the internal container ports to the external ones.
      - "8000:8000"

Then I ran that to make sure everything worked all right.

docker-compose up

Then I went about trying to get it setup to run with debug enabled. What I got finally was adding a command to the docker-compose file that started the app with debug enabled.

version: '3.7'
services:
  web:
    build: .
    ports:
      - "8080:8080"
      - "8000:8000"
    command: java -agentlib:jdwp=transport=dt_socket,server=y,suspend=n,address=*:8000 -jar dublin_rest_demo.jar

Since I'm not always running my application in debug. I'll usually create a second file and call it docker-compose-debug.yml. Then run that compose file using the '-f' flag:

docker-compose -f docker-compose-debug.yml up

And that was it, I was on my way to debugging my app!

To confirm it was actually it was setup correctly. I setup a remote config in IntelliJ:

Then I ran my docker-compose-debug file:

Finally, I set a breakpoint and hit it:

And there it is, we're running our app via docker-compose with debug enabled.