[[container-images.dockerfiles]]
== Dockerfiles
While it is possible to convert a Spring Boot fat jar into a docker image with just a few lines in the Dockerfile, we will use the <<container-images#container-images.efficient-images.layering,layering feature>> to create an optimized docker image.
When you create a jar containing the layers index file, the `spring-boot-jarmode-layertools` jar will be added as a dependency to your jar.
With this jar on the classpath, you can launch your application in a special mode which allows the bootstrap code to run something entirely different from your application, for example, something that extracts the layers.

CAUTION: The `layertools` mode can not be used with a <<deployment#deployment.installing, fully executable Spring Boot archive>> that includes a launch script.
Disable launch script configuration when building a jar file that is intended to be used with `layertools`.

Here’s how you can launch your jar with a `layertools` jar mode:

[source,shell,indent=0,subs="verbatim"]
----
$ java -Djarmode=layertools -jar my-app.jar
----

This will provide the following output:

[subs="verbatim"]
----
Usage:
  java -Djarmode=layertools -jar my-app.jar

Available commands:
  list     List layers from the jar that can be extracted
  extract  Extracts layers from the jar for image creation
  help     Help about any command
----

The `extract` command can be used to easily split the application into layers to be added to the dockerfile.
Here is an example of a Dockerfile using `jarmode`.

[source,dockerfile,indent=0,subs="verbatim"]
----
FROM eclipse-temurin:17-jre as builder
WORKDIR application
ARG JAR_FILE=target/*.jar
COPY ${JAR_FILE} application.jar
RUN java -Djarmode=layertools -jar application.jar extract

FROM eclipse-temurin:17-jre
WORKDIR application
COPY --from=builder application/dependencies/ ./
COPY --from=builder application/spring-boot-loader/ ./
COPY --from=builder application/snapshot-dependencies/ ./
COPY --from=builder application/application/ ./
ENTRYPOINT ["java", "org.springframework.boot.loader.JarLauncher"]
----

Assuming the above `Dockerfile` is in the current directory, your docker image can be built with `docker build .`, or optionally specifying the path to your application jar, as shown in the following example:

[source,shell,indent=0,subs="verbatim"]
----
	$ docker build --build-arg JAR_FILE=path/to/myapp.jar .
----

This is a multi-stage dockerfile.
The builder stage extracts the directories that are needed later.
Each of the `COPY` commands relates to the layers extracted by the jarmode.

Of course, a Dockerfile can be written without using the jarmode.
You can use some combination of `unzip` and `mv` to move things to the right layer but jarmode simplifies that.

