How to write a dockerfile for running a java application (*.war) in Apache tomcat webserver

I have a sample hello-world maven project in github hello-world

Maven is a build tool used to compile, test and package the application developed using Java programming language.

Run the mvn clean package command to compile and package the java application.

The hello-world.war file will be created in targets folder

I have already created a file Dockerfile and kept in root folder

From tomcat:8-jre8

COPY ./target/hello-world.war /usr/local/tomcat/webapps

In the FROM command we are taking tomcat:8-jre8 as a base image.

On top of that we are copying the hello-world.war file from target folder and keeping it in /usr/local/tomcat/webapps inside a container.

we no need to explicity define the CMD here, because CMD is already written in base docker image tomcat:8-jre8 to start the tomcat webserver.

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Important Tips

Tip

War File Placement: Tomcat automatically deploys any .war file placed in the webapps directory. If you rename your file to ROOT.war, it will be deployed as the root application (accessible at http://localhost:8080/ instead of http://localhost:8080/app-name).

Note

Base Image: Using a specific version tag (e.g., tomcat:9.0-jdk11) is recommended over latest to ensure your build is reproducible and stable.

🧠 Quick Quiz — Tomcat in Docker

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Where should you copy your .war file inside the official Tomcat container for it to be auto-deployed?

/usr/local/tomcat/webapps

/var/www/html

/opt/tomcat/deploy

/tmp

The webapps directory is the standard deployment location for Tomcat.

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Why don't we need a CMD instruction in this Dockerfile?

The base image (tomcat) already defines a CMD to start the server.

Docker guesses how to start .war files.

Tomcat runs as a kernel module.

We forgot it.

Docker images inherit instructions from their base image. The official Tomcat image includes CMD ["catalina.sh", "run"] which starts the server.

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If you want your app to be available at http://localhost:8080/myapp, what should your war file be named?

myapp.war

ROOT.war

server.war

index.war

Tomcat uses the filename (minus .war) as the context path for the application. ROOT.war is a special case for the root context /.

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