Java 和 Docker 的缓慢开发工作流程/反馈循环：我是否需要在每次更改后重建 JAR 和图像？

Question

I am trying to dockerize a Java app, but currently my development feedback loop is much slower when working with Docker compared to when I am working without Docker. With Docker, I have to rebuild my jar and the image after every change (see more detailed workflow below), causing a slow feedback loop. What do Java devs in the Docker world do to avoid this?

Dockerfile (for reference):

FROM openjdk:8    
WORKDIR /usr/src/app  
COPY /target/my-project.jar .  
CMD ["java",  "-jar",  "my-project.jar"]

Java development workflow with Docker (feedback takes 3-5 minutes after each change):

1. Make a change locally to my Java codebase.
2. However small the change in #1 is, I need to rebuild my jar (using maven package); this takes 3-4 minutes for my codebase.
3. Rebuild image.
4. Run container and see if change went through.

Java development workflow without Docker (feedback takes seconds after each change):

1. Make a change locally to my Java codebase.
2. Click run on my Intellij run configuration, which runs my main method immediately, and see if changes went through.

From what I can tell, my local development feedback loop (without Docker) is so much faster because Intellij is able to cache all bytecode (ie, the class files in the "target/" directory), rebuilds the class file(s) for only the class(es) that changed, keeps track of my classpath, and runs my main method. It doesn't need to rebuild the entire jar, and therefore is much, much faster. I'm not sure how to replicate this in Docker.

UPDATE/ANSWER

I solved this problem with the following approach:

1. Bind-mounted my project's target folder to /usr/src/app/target in the container and now just call the main class in my Dockerfile CMD, not ever bringing the JAR into the container. Thanks to @kutschkem.

2. Added maven-dependency-plugin to my pom.xml, so that all of my external jar dependencies could be saved to one folder (/usr/src/app/target/dependency-jars/), which I could then add to my classpath in one fell swoop in my Dockerfile CMD below. Got the idea from this article: https://medium.com/holisticon-consultants/dont-build-fat-jars-for-docker-applications-6252a5571248 .

New Dockerfile:

FROM openjdk:8    
WORKDIR /usr/src/app/target
CMD ["java",  "-cp", "/usr/src/app/target/classes:/usr/src/app/target/dependency-jars/*",  "com.me.MainClass"]

Answer 1

There are two things that can be improved here:

1) Don't copy the jar file to the image, mount the folder in the container as a volume with -v

2) If building the jar is an issue, then don't, Prepare a java invocation that includes the necessary classpath. and calls the main class. No need for a jar.

Of course this is for development, but functionally there shouldn't be much difference. Also, is there a good reason to test the classes inside the docker container? If I were to do it, I would seperate the development of the Java classes and the Docker image, and use the workflow you described WITHOUT docker to test my Java code, and only once I am satisfied turn to the image development.