Hey there! Let's work through this environment variable issue you're hitting with your Java app in Docker. I've stumbled through similar problems before, so here's what to check and fix:

Chances are, you didn't pass the variable to the container at all when you first ran it. Docker doesn't magically inject environment variables—you have to either:

• Define them in your Dockerfile with the ENV directive, or
• Pass them at runtime using the -e flag with docker run

If you did neither, System.getenv() obviously won't find anything. That's the first easy fix.

This is where the common pitfalls hide. Let's go through the most likely causes:

a) You didn't override the Dockerfile's ENV at runtime #

If you added ENV DB_HOST localhost to your Dockerfile, that sets a default value. But to use somehost instead, you need to explicitly pass it when starting the container. For example:

docker run -e DB_HOST=somehost your-java-image-name

Docker runtime arguments (-e) always override the ENV values defined in the Dockerfile. If you skip this step, it'll stick with the localhost default.

b) You have conflicting ENV directives in your Dockerfile #

Double-check your Dockerfile to make sure you didn't accidentally redefine the variable later with localhost. For example:

# Oops, this gets overwritten later!
ENV DB_HOST somehost
# ... other commands ...
ENV DB_HOST localhost

The last ENV directive in the file wins, so if you have a duplicate, it'll overwrite your desired value.

c) Your Java code is reading the wrong variable name #

It sounds silly, but typos happen! Make sure the variable name in your code exactly matches what you're setting in Docker. Environment variables are case-sensitive—so DB_HOST is not the same as db_host or Db_Host.

d) You're using Docker Compose and it's overriding the value #

If you're using Compose to run your container, check your docker-compose.yml file. The environment section will override both Dockerfile ENV values and runtime -e flags. For example:

services:
  app:
    image: your-java-image-name
    environment:
      - DB_HOST=localhost  # This will override everything else!

If that's present, change it to somehost or remove it to let the runtime value take over.

To confirm what environment variables are actually present in your container, run this command while the container is running:

docker exec <your-container-id> env

This will list all variables in the container—you can check if DB_HOST is set to the value you expect.

Here's a simple, correct Dockerfile with a default value, plus the runtime command to override it:

FROM openjdk:17-jdk-slim
# Set a default for local testing
ENV DB_HOST localhost
COPY target/your-app.jar /app.jar
CMD ["java", "-jar", "/app.jar"]

Then start the container with your desired database host:

docker run -e DB_HOST=somehost your-java-image-name

Your Java app should now pick up somehost via System.getenv("DB_HOST").

内容的提问来源于stack exchange，提问作者li-raz