Skip to content

Latest commit

 

History

History
95 lines (60 loc) · 3.38 KB

activity_1.md

File metadata and controls

95 lines (60 loc) · 3.38 KB
Raw

Activity 1

🗺 Exploration

🔎 Exercise 1.1 - Exploring the Code

If you have previously created a Java Spring application then most of the files should look familiar.

However, there is one new file to point out and that is the Dockerfile.

The Dockerfile looks like this:

FROM amazoncorretto:17-alpine3.17-jdk
RUN addgroup -S spring && adduser -S spring -G spring
USER spring:spring
ARG JAR_FILE=target/*.jar
COPY ${JAR_FILE} app.jar
EXPOSE 8080
ENTRYPOINT ["java","-jar","/app.jar"]
  • What do you think might be happening in the first line of that Dockerfile?
Click here to see the answer 
Think of Docker images like layers of an onion.


We can create Docker images that build upon previous layers.


The first line of this particular Dockerfile tells Docker build
on a Java image provided on the DockerHub by Amazon.


That particular image makes sure the version 17 of the
Java Development Kit is present.

Wait....hold up....in the answer you mentioned the Docker Hub. What is the Docker Hub?!

The Docker hub is a public site where you can publish Docker images. You can make them publicly available for the community to use.

Very similar to the Java dependencies you've got in your code. Those Java dependencies get pulled from the Maven Central servers.

Well in Docker that image (amazoncorretto:17-alpine3.17-jdk) we've "based" our Dockerfile from gets "pulled" from the Docker Hub.

In container talk, the Docker Hub is known as a Container Registry but more on that later...

Moving on to the following two lines:

RUN addgroup -S spring && adduser -S spring -G spring
USER spring:spring

Well those two lines run instructions within the container image. The RUN instruction tells the image to add a user and a group called "spring" to the operating system.

Much like you would when adding a new user to your own computer. Then the USER instruction tells Docker to use the spring user for all following commands.

This is a bit of security, making sure we're not running our Java application using the root user.

The next two lines look complicated but are just a fancy way of saying copy a file from my local computer (your computer) into the Docker image

ARG JAR_FILE=target/*.jar
COPY ${JAR_FILE} app.jar

The file it copies is a JAR file located in the target directory. The JAR file is what is produced when you build your Spring boot application.

It stands for Java ARchive.

Then we encounter the port exposure.

EXPOSE 8080

This line tells Docker that our application will be listening for connections on port 8080. Thats the default port for a Java spring boot application.

Then the final line is the powerful one. An ENTRYPOINT instruction tells Docker what command should be executed when starting the container.

ENTRYPOINT ["java","-jar","/app.jar"]

Essentially this says, when the docker container runs please run the command java -jar /app.jar. Just like you might do on your local computer.

Now we've had a bit of an explore, I expect you're excited to get building a docker image and getting it running.

===

Head over to Activity 2 and we can get things built and running.