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To deploy a Java web application in Docker, package the application as a WAR, choose a Tomcat image whose release and Java runtime satisfy that application, copy the WAR into the image’s documented deployment directory, build the image, and publish Tomcat’s listening port when you start the container. A Dockerfile’s EXPOSE line documents a port; it does not make that port reachable from the host.
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The exact Tomcat image tag, Java level, build command, WAR filename, and deployment directory must be checked for your project. Tomcat’s default Host appBase is $CATALINA_BASE/webapps, but an image can change its configuration. The procedure below separates what Tomcat documents from values you must verify.
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What the deployment looks like
The finished path has four artifacts and one network mapping:
- Your existing Java web project produces a Web Application Archive, normally ending in
.war. - A Tomcat base image supplies the application server and its Java runtime environment.
- A Dockerfile copies the WAR into the image’s configured Tomcat deployment directory.
docker buildcreates an image containing Tomcat and the application.docker run -p hostPort:containerPortstarts a container and maps a host port to Tomcat’s container port.
Tomcat accepts either an unpacked web-application directory or a packed WAR. Its documented default Host appBase is $CATALINA_BASE/webapps; applications placed there can be deployed at startup. See the Apache Tomcat Application Developer’s Guide deployment documentation for the appBase and startup-deployment behavior.
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Docker’s Java guide distinguishes an executable JAR from an application that depends on an application server. For the latter category, it directs you to use the relevant server as the base image and adjust the package filename as required by the project. That is the model used here: Tomcat remains the server, while your WAR is the deployable application.
Before you write the Dockerfile
Identify the application contract
Record the Servlet or Jakarta Servlet API level expected by the application, the Java runtime it requires, the artifact filename produced by the build, and whether the application expects a particular Tomcat configuration. Those values are project-specific and are not inferable from the application title alone. A WAR compiled for one API family is not automatically compatible with every Tomcat generation.
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Choose a compatible Tomcat release
Do not select the newest Tomcat release solely because it is newest. Compare the APIs your application targets, its Java requirement, and the maintenance policy you want for the deployment. The Apache Tomcat 11.0.27 documentation index, dated October 6, 2026, identifies Servlet 6.1 and Pages 4.0 as the APIs implemented by that release. Those numbers describe Tomcat 11.0.27; they do not prove that an unspecified application is compatible with it. The release documentation is at https://tomcat.apache.org/tomcat-11.0-doc/index.html.
Verify the selected image’s layout
Image maintainers can set environment variables, change the active CATALINA_BASE, or use a deployment directory different from the default. Check the documentation for the exact image tag before hard-coding a destination in COPY. Tomcat’s default appBase is a useful reference, not permission to assume that every image has the same filesystem layout.
Keep the build context intentional
Docker’s build context is the set of files available to COPY. Put the Dockerfile and the WAR in a context that contains only what the image needs, and make sure the WAR is produced before the image build starts. The Dockerfile reference explains that FROM selects a base image and COPY transfers files into the image; the Dockerfile overview describes how docker build uses the Dockerfile and context. Consult Docker’s Dockerfile reference and the Dockerfile overview.
Step 1: Build the WAR
Use the build system already defined by the project to produce the deployable WAR. The exact Maven, Gradle, or other command is project-specific, so use the command in that project’s build instructions rather than copying a command that may not match its plugins or profiles.
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After the build completes, locate the resulting file and note its exact name. For the examples below, the file is called myapp.war; replace that name with the artifact your build actually creates. If the build creates an unpacked directory instead of a WAR, either configure the build to package a WAR or use the directory form supported by Tomcat. A packed WAR is usually the clearest artifact to copy into an image because it gives the image build one explicit input.
- Confirm that the file ends in
.war. - Confirm that it contains the web application’s classes and web resources produced by the build.
- Confirm that its API and Java requirements match the Tomcat image you intend to use.
- Keep the artifact in the Docker build context, or change the build context so Docker can read it.
Step 2: Create a Dockerfile that copies the WAR
The following pattern leaves the image tag as a build argument so you can supply the verified Tomcat image rather than silently assuming a tag:
ARG TOMCAT_IMAGE
FROM ${TOMCAT_IMAGE}
COPY myapp.war $CATALINA_BASE/webapps/
EXPOSE 8080Set TOMCAT_IMAGE to the image and tag you have checked for your application’s API and Java requirements. The COPY source must match the WAR filename in the build context. The destination uses Tomcat’s documented default appBase. If the selected image documents another path, use that path instead.
What each instruction does
ARG TOMCAT_IMAGEandFROM: select the Tomcat base image. The image supplies the server; your application is added in a later layer.COPY: transfers the WAR from the build context into the image. A typo in the source filename or destination path prevents the expected deployment.EXPOSE 8080: records that the image intends to use container port 8080. Docker’s reference states, “The EXPOSE instruction doesn’t actually publish the port.” Publishing happens only when the container is run with a port mapping.
If your Tomcat configuration listens on a different container port, document that port in EXPOSE and use the same container-port value in docker run. Do not treat EXPOSE as a firewall rule or host binding.
When the root context matters
With Tomcat’s default naming convention, a WAR named myapp.war is commonly available at /myapp. The root application is a special case; its context is not the same as an ordinary filename-derived context. The Tomcat Manager documentation covers application deployment and context handling at https://tomcat.apache.org/tomcat-9.0-doc/manager-howto.html. Decide the URL you expect before testing, and make the artifact name or context configuration agree with it.
Step 3: Build the Docker image
Run the build from the directory containing the Dockerfile and WAR. The image name below is an example chosen for readability:
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docker build --build-arg TOMCAT_IMAGE="$TOMCAT_IMAGE" -t my-tomcat-app .Set the shell variable to the verified image reference before running the command. For example, in a shell you could assign the value supplied by your image documentation, then execute the build command. The important points are that the argument resolves to a real, compatible image and that the final dot supplies a context containing both the Dockerfile and myapp.war.
Docker reads the Dockerfile, pulls the FROM image if necessary, copies the WAR, and produces the tag my-tomcat-app. A successful image build does not prove that the application itself starts correctly; it proves that Docker could assemble the filesystem described by the Dockerfile.
Build failures to resolve immediately
- Source file not found: the WAR is outside the build context or its filename does not match the
COPYinstruction. Move the artifact into the context or correct the source name. - Base image cannot be resolved: the value supplied for
TOMCAT_IMAGEis not a valid image reference available to your Docker engine. Recheck the selected image documentation and tag. - Application and image mismatch: the image builds successfully, but compatibility problems appear only when Tomcat loads the application. Revisit the Servlet/Jakarta API and Java requirements before changing deployment paths.
Step 4: Run the container and publish Tomcat’s port
If Tomcat listens on container port 8080, start the image with a host-to-container mapping:
docker run --rm -p 8080:8080 my-tomcat-appThe first number is the host port; the second is the container port. This command makes the service reachable through port 8080 on the Docker host while Tomcat continues listening on 8080 inside the container. You may choose another host port, such as 9090:8080, when 8080 is already occupied. If the selected Tomcat image listens on a different container port, replace the second number accordingly.
Docker’s port-publishing documentation describes the -p hostPort:containerPort form at https://docs.docker.com/engine/network/port-publishing/. The Dockerfile reference at https://docs.docker.com/reference/dockerfile explains why an EXPOSE declaration alone is insufficient.
Test the URL that matches the context path
Open the host address and the context path derived from the WAR. With the illustrative mapping and filename, the first URL to try is http://localhost:8080/myapp. If the application is configured as the root context, test http://localhost:8080/ instead. A 404 at the wrong context path can look like a deployment failure even when Tomcat has started and deployed the WAR correctly.
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Use the container’s startup output and the Tomcat configuration selected by the image to determine whether deployment completed. A browser request by itself cannot distinguish a wrong URL from a failed deployment, so check both the path and the server startup result.
Static deployment versus Manager deployment
Copying a WAR into the image is static deployment: the artifact is present before Tomcat starts. Tomcat also supports dynamic deployment through its Manager application while the server is running. The two approaches solve different operational problems.
| Deployment mode | When the artifact is introduced | Restart behavior | Operational model | Access concern |
|---|---|---|---|---|
| Static image deployment | WAR or expanded application is placed in the appBase before startup | Tomcat discovers it during startup | Build a new image when the application artifact changes | Protect the Docker host and image pipeline |
| Manager deployment | WAR is submitted to a running Tomcat Manager application | Deployment can occur without restarting Tomcat | Runtime administration through Manager operations | Restrict Manager to authorized administrators |
For a newly built container intended to run one application, Manager is not required. Static deployment keeps the application artifact and server image together, making the container’s contents explicit. Manager is useful when your operating process requires deployment to a running server, but it introduces an administrative interface that must be protected and governed. Tomcat’s Manager guide documents its deployment and management operations at https://tomcat.apache.org/tomcat-9.0-doc/manager-howto.html.
Choosing the Tomcat and Java combination
Compatibility is a decision, not a default. Start with the APIs and Java level declared by the application, then choose a Tomcat release and image that satisfy them. Check all of the following before building:
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- Pages/JSP requirements: applications using server-side pages may require a particular Pages API level.
- Java runtime: match the image’s runtime to the version required by the application and its dependencies.
- Maintenance policy: decide whether you need a maintained release line, and record the exact image tag so later builds are reproducible.
- Image configuration: verify the active
CATALINA_BASE, appBase, listening port, and any startup options documented for that image.
Tomcat 11.0.27 is documented as implementing Servlet 6.1 and Pages 4.0 in the October 6, 2026 index. That is a concrete version fact, not a universal recommendation. If your application targets another API level, select the Tomcat release that matches the application’s contract and verify the corresponding image documentation.
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The container starts, but the application returns 404
- Confirm that the WAR was copied to the image path used by the selected Tomcat configuration. The default documented appBase is
$CATALINA_BASE/webapps. - Confirm that Tomcat started without a deployment error.
- Check the URL’s context path. A WAR named
myapp.warnormally maps to/myappunder the default naming convention; it is not automatically the root application. - Check whether the application uses an explicitly configured context that overrides the filename-derived path.
A 404 at / does not prove that /myapp failed, and a 404 at /myapp does not prove that the container port is unpublished. Separate path checks from server and network checks.
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The application cannot be reached from the host
- Identify the port on which Tomcat listens inside the container.
- Ensure the
docker runcommand maps a host port to that exact container port, for example-p 8080:8080. - Do not rely on
EXPOSE; it records intent but does not publish the port. - If the host port is already in use, select a different host-side number while keeping the container-side number unchanged.
Deployment fails or classes and APIs are missing
Recheck the application target against the Tomcat and Java combination. A newer Tomcat generation can expose a different Servlet or Pages API than the one for which the WAR was built. Verify the selected release documentation, image tag, and Java runtime instead of trying random destination paths.
The WAR is not available during the build
Docker can copy only files in its build context. Run the build from a context that includes the generated WAR, or place the Dockerfile where the intended context can see the artifact. Ensure the case-sensitive filename in COPY exactly matches the generated file.
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The Manager application performs runtime deployment and administration. Keep it restricted to authorized administrators and limit its exposure according to your environment. The exact access policy depends on where and how Tomcat runs; the Manager documentation describes the operations, while your deployment environment determines the controls.
What this workflow does not decide for production
The WAR-in-image pattern answers how to package and reach the application. It does not, by itself, define TLS termination, health checks, resource limits, persistent storage, secrets management, backups, logging retention, or orchestration. Those choices depend on the environment in which the container will run. Add them deliberately rather than assuming that a successful local port mapping provides production hardening.
Likewise, a successful image build is not an application acceptance test. Validate the application with the checks appropriate to its own endpoints, dependencies, and data stores. Keep the Tomcat image tag, WAR build inputs, and runtime configuration recorded so a later rebuild uses the same compatibility decisions.
Quick Recap
End-to-end checklist
- The project builds a WAR with the expected filename.
- The WAR’s Servlet/Jakarta API and Java requirements are known.
- The selected Tomcat release and image tag satisfy those requirements.
- The image documentation confirms the deployment directory and listening port.
- The Dockerfile’s
COPYsource matches the WAR in the build context. - The destination matches the selected image’s active Tomcat appBase.
EXPOSEdocuments the intended container port but is not treated as host publication.docker buildcompletes with the intended image tag.docker run -pmaps an available host port to Tomcat’s actual container port.- The test URL includes the context path derived from the WAR name or configured explicitly.
- Any Manager deployment interface is limited to authorized administration.
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