Resolving 404 Error in Dockerized Tomcat with Deployed Web Application

Understanding 404 Errors in Tomcat Docker Deployments

Setting up a web application on Tomcat using Docker can be a straightforward process, but errors like the 404 status are common and can disrupt deployment. The 404 error indicates that the server is unable to locate the requested resource, which can be confusing when the application appears to be correctly deployed in the webapps folder. This issue can arise from several configuration problems.

In many cases, developers who are new to Docker and containerized environments face difficulties when their application works locally but not inside the Docker container. This mismatch often relates to how Tomcat handles the deployed applications and the Docker networking setup. Ensuring the WAR file is correctly placed and the application context is accessible are crucial steps.

Deploying a Spring Boot application to Tomcat on Docker requires particular attention, especially if you've excluded Tomcat from Spring Boot. Adjustments need to be made to ensure that Tomcat serves the application correctly within the Docker container.

This article addresses the issue of receiving a 404 error on Tomcat within Docker, even when the application is deployed correctly in the webapps folder. We will explore potential causes, examine Docker and Tomcat configurations, and outline steps to resolve the problem.

Detailed Breakdown of the Tomcat Docker Setup and Error 404 Solution

The first part of the provided script uses the Dockerfile to set up a Tomcat 9.0 container. The command FROM tomcat:9.0-alpine pulls a lightweight version of Tomcat, which is crucial for minimizing the image size in production environments. The Alpine variant is commonly used for performance optimization. Next, the ADD assessmentonline.war command places the WAR file into the webapps folder, ensuring that the Spring Boot application is deployed correctly inside Tomcat. The EXPOSE command makes port 8080 available, which is where Tomcat serves web requests.

The most significant part of this setup is the CMD ["catalina.sh", "run"], which instructs Docker to run Tomcat in the foreground, enabling it to serve the application continuously. Without this, the Docker container would exit immediately after the initial startup. The build command docker build -t mywebapp1 . creates the container image tagged as "mywebapp1", which is necessary to run the container later. This section of the script handles the environment configuration, deployment, and container initialization, which are vital in containerized applications.

The second script solution involves adjusting the context path of the Spring Boot application to ensure that the web app is correctly accessible. By defining the context path using server.servlet.context-path=/assessmentonline, we ensure that requests to this path are routed to the correct resources. This setting is essential for mapping the expected URL structure to the actual application deployment within the Docker container. Incorrect context paths are a common cause of 404 errors, and fixing this ensures the app is available under the desired URL.

Another key step in debugging the 404 error is using the docker logs command. This command allows you to inspect the logs generated by the container, which provides valuable information on whether the application has been deployed correctly or if there were errors during the startup process. Additionally, the docker exec -it command opens a shell into the running container, enabling you to explore the filesystem. This is critical for verifying whether the WAR file was correctly placed inside the webapps folder and whether all resources are properly deployed. These troubleshooting methods are essential to identify configuration issues that cause 404 errors.

# Approach 1: Verify WAR Deployment and Check Docker File
FROM tomcat:9.0-alpine
LABEL maintainer="francesco"
ADD assessmentonline.war /usr/local/tomcat/webapps/
EXPOSE 8080
# Ensure Tomcat's catalina.sh is correctly invoked
CMD ["catalina.sh", "run"]
# Build and run the Docker container
docker build -t mywebapp1 .
docker run -p 80:8080 mywebapp1
# Test the URL again: curl http://localhost/assessmentonline/api/healthcheck

# Approach 2: Modify Spring Boot Application to Set Proper Context Path
# In your Spring Boot application properties, specify the context path explicitly
server.servlet.context-path=/assessmentonline
# This ensures that the application is accessible under the correct path in Tomcat
# Rebuild the WAR and redeploy to Docker
docker build -t mywebapp1 .
docker run -p 80:8080 mywebapp1
# Test the updated URL: curl http://localhost/assessmentonline/api/healthcheck
# You should now receive a valid response from your application

# Approach 3: Use Docker Logs to Diagnose 404 Issues
# Check the logs to confirm WAR deployment status
docker logs <container-id>
# Ensure no deployment errors or missing files are reported
# If WAR is not deployed correctly, consider adjusting the Dockerfile or paths
# Use docker exec to explore the running container
docker exec -it <container-id> /bin/sh
# Verify that the WAR file is in the correct directory
ls /usr/local/tomcat/webapps/assessmentonline.war

Addressing Tomcat and Spring Boot Deployment Issues in Docker

An often overlooked aspect of deploying a Spring Boot application in Tomcat is the importance of context paths and directory structure. By default, Tomcat uses the root folder for deployments, but if your WAR file is not properly configured with the correct context path, it can lead to 404 errors. This is particularly true in Docker environments where container isolation can hide issues. One effective solution is setting the Spring Boot context path explicitly to match Tomcat's directory structure.

Another critical aspect is ensuring the Docker container is correctly exposing and mapping the ports. Misconfigurations in the EXPOSE directive can cause the Tomcat server to be inaccessible externally, even if it's running fine internally. In this scenario, checking both the Docker port mapping and verifying if the application is listening on the specified port are crucial steps to troubleshoot. Always confirm the mapping using the docker run command with the correct -p flag.

Finally, the integration between Spring Boot and Tomcat can sometimes be problematic if Tomcat is excluded from Spring Boot dependencies and run as a standalone service in Docker. Ensuring all required libraries, such as JSP files and dependencies, are included in the WAR can prevent runtime issues. Debugging using docker logs and directly inspecting the running container’s filesystem can provide valuable insights, helping to identify missing resources or incorrect deployments.