Hey there! Let's walk through fixing this frustrating Docker error—since you're new to Docker, I'll break down the most likely causes and simple fixes to get your app running smoothly.

Common Causes & Fixes #

1. Your Docker image doesn't have Java installed #

This is the most frequent culprit. If you're using a minimal base image (like alpine or debian:-slim) that doesn't include a JRE/JDK, the java command simply doesn't exist in the container.

Fix:
Either use an official OpenJDK base image that comes with Java pre-installed, or install Java explicitly in your Dockerfile:

Option 1: Use an OpenJDK base image

# Start with an official OpenJDK image (match your app's Java version)
FROM openjdk:17-jdk-alpine
# Copy your built jar into the container
COPY target/hello-world-rest-api.jar /tmp/
# Run the app with correct array syntax
CMD ["java", "-jar", "/tmp/hello-world-rest-api.jar"]

Option 2: Install Java on a minimal base image (e.g., Alpine)

FROM alpine:latest
# Install OpenJRE (no-cache keeps the image small)
RUN apk add --no-cache openjdk17-jre
COPY target/hello-world-rest-api.jar /tmp/
CMD ["java", "-jar", "/tmp/hello-world-rest-api.jar"]

2. Incorrect CMD/ENTRYPOINT syntax #

If your CMD or ENTRYPOINT is formatted wrong, Docker will try to run the entire string as a single command (instead of recognizing java as the executable and the rest as arguments).

What to avoid:

• Don't wrap the entire command in quotes as a single string: CMD "[java,-jar,/tmp/hello-world-rest-api.jar]" (this tells Docker to look for a command named [java,-jar,/tmp/hello-world-rest-api.jar], which doesn't exist)
• Don't omit spaces between arguments in the array: CMD ["java,-jar,/tmp/hello-world-rest-api.jar"] (same issue as above)

Correct syntax:
Use the array format with separate strings for each argument:

CMD ["java", "-jar", "/tmp/hello-world-rest-api.jar"]

Or use the simpler shell format (less flexible in some cases, but works for basic setups):

CMD java -jar /tmp/hello-world-rest-api.jar

3. The JAR file isn't actually in the container #

Sometimes the COPY command in your Dockerfile isn't copying the JAR to the right path, or the local JAR file doesn't exist where you expect.

How to verify:

1. Build your image: docker build -t my-api .
2. Run an interactive shell in the image to check the files:

   docker run --rm -it my-api sh
   

3. Inside the shell, list the /tmp directory: ls /tmp
   • If you don't see hello-world-rest-api.jar, fix your COPY command to point to the correct local path (e.g., if your JAR is in build/libs instead of target, update the command to COPY build/libs/hello-world-rest-api.jar /tmp/)

4. Java's absolute path is missing (rare but possible) #

In some minimal images, the java executable might not be in the default PATH. To fix this, use the absolute path to java in your CMD.

How to find the path:
Run the interactive shell as above, then run which java to get the full path (e.g., /usr/bin/java). Update your CMD to use that path:

CMD ["/usr/bin/java", "-jar", "/tmp/hello-world-rest-api.jar"]

Next Steps to Test #

1. Fix your Dockerfile based on the issue you identified
2. Delete your old image: docker rmi my-api (replace my-api with your image name)
3. Rebuild the image: docker build -t my-api .
4. Run the container: docker run -p 8080:8080 my-api

内容的提问来源于stack exchange，提问作者Manoj Prabhakar Munagasetty