• Docker/Java - How to debug a container using Java application

    Gorhan Hudebine , 2022-04-29, 1 min and 45 sec read

    Docker let you run applications or webapps under containers.

    When working with automating deployment like Kubernetes , some bug or undesired behaviour can happen on a java application inside a container, but not reproducible outside the container.

    Debug a container in IDE

    It is quite easy to pass on debug mode when launching a container, by adding to the "docker run" command : 

    docker run -e \ (1)
"_JAVA_OPTIONS= -agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=0.0.0.0:5005" \ (2)
-p 5005:5005 \ (3)
image-name:image-version (4)
    1. Launches a container, using environment variable (`-e)
    2. Permits remote java debugging, on port 5005 inside the created container, and waits for listener on this port before starting.
    3. Binds port 5005 inside the container on port 5005 on the host.
    4. Container will use the image image-name , and version image-version .

    After executing the command, the container will be running, waiting for a listener on port 5005 to start. Take your favourite IDE and listen or attach process on port 5005, you're on debug inside of your container.

    Basic Example

    Create a TestDebugMain.java in package "test", and put a breakpoint on the "System.out.println()" line : 

    package test;

public class TestDebugMain {

    public static void main(String[] args) {
        System.out.println("HelloWorld from Java Application running in Docker.");
    }
}

    Create a folder "debug-test", put the TestDebugMain.java under "debug-test/test/". Create a Dockerfile under "debug-test" : 

    FROM java:8
COPY test/TestDebugMain.java /test/TestDebugMain.java
RUN javac test/TestDebugMain.java
CMD ["java", "test/TestDebugMain"]

    Here is the final tree : 

    test-debug
├── Dockerfile
└── test
    └── TestDebugMain.java

    With the terminal, go under "test-debug" folder, and build the docker image "test-debug", : 

    docker build -t test-debug .

    finally, let's launch a container with that image, and the debug mode, as explained at the begin of this topic (don't forget to add the breakpoint in your IDE) : 

    docker run \
-e "_JAVA_OPTIONS= -agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=0.0.0.0:5005" \
-p 5005:5005 test-debug:latest

    Go to your IDE, and attach to process at port 5005, and that's it !

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