# Apache Tomcat — The Java Servlet Container That Powers the Web

> Apache Tomcat is an open-source implementation of the Jakarta Servlet, JSP, and WebSocket specifications, serving as the default deployment target for millions of Java web applications worldwide.

## Install

Save the content below to `.claude/skills/` or append to your `CLAUDE.md`:

# Apache Tomcat — The Java Servlet Container That Powers the Web

## Quick Use
```bash
# Download and extract Tomcat 10
wget https://dlcdn.apache.org/tomcat/tomcat-10/v10.1.28/bin/apache-tomcat-10.1.28.tar.gz
tar xzf apache-tomcat-10.1.28.tar.gz
cd apache-tomcat-10.1.28

# Start the server
./bin/startup.sh

# Deploy a WAR file
cp myapp.war webapps/
```

## Introduction
Apache Tomcat is the reference implementation of the Jakarta Servlet and JavaServer Pages specifications. It has been the standard Java web application server for over two decades, providing a lightweight and reliable runtime for deploying servlet-based applications, REST APIs, and Spring Boot projects.

## What Apache Tomcat Does
- Serves Java web applications packaged as WAR files or exploded directories
- Implements Jakarta Servlet 6.0, JSP 3.1, EL 5.0, and WebSocket 2.1 specifications
- Manages HTTP connections with a non-blocking I/O connector (NIO/NIO2) for high concurrency
- Supports virtual hosting, session clustering, and load balancing across multiple instances
- Provides a web-based Manager application for deploying, undeploying, and monitoring apps

## Architecture Overview
Tomcat consists of a Catalina servlet container, a Coyote HTTP connector, and a Jasper JSP engine. Incoming HTTP requests arrive at the Coyote connector, which passes them through a pipeline of valves (filters) before dispatching to the appropriate servlet in the Catalina engine. Each web application runs in its own classloader for isolation. Configuration is defined in server.xml (server-level), context.xml (per-app), and web.xml (servlet mappings). The Cluster module enables session replication across nodes.

## Self-Hosting & Configuration
- Download the binary distribution from the Apache mirrors or install via package managers
- Place WAR files in the webapps/ directory for automatic deployment on startup
- Configure connectors in server.xml: set ports, thread pool sizes, and TLS certificates
- Tune JVM memory via CATALINA_OPTS (e.g., -Xmx, -XX:+UseG1GC) in setenv.sh
- Enable the Manager and Host Manager apps by adding users to tomcat-users.xml

## Key Features
- Lightweight and fast: starts in seconds with a small memory footprint compared to full Jakarta EE servers
- Auto-deploy: drop a WAR file into webapps/ and Tomcat deploys it without a restart
- Embedded mode: use Tomcat as a library inside Spring Boot or standalone Java applications
- Session clustering and replication for high availability across multiple Tomcat nodes
- Mature security: regular CVE patches, built-in CSRF protection, and security manager support

## Comparison with Similar Tools
- **Jetty** — embeddable Java server with similar features; lighter defaults, smaller community
- **WildFly** — full Jakarta EE application server; more features but heavier for simple servlet apps
- **Undertow** — high-performance web server from Red Hat; often embedded in WildFly and Quarkus
- **Nginx + app server** — reverse proxy pattern; Tomcat often runs behind Nginx for static assets and TLS
- **Spring Boot embedded** — bundles Tomcat inside the app JAR; same engine, different deployment model

## FAQ
**Q: Is Tomcat a full application server like WildFly?**
A: No. Tomcat implements the servlet specification but not the full Jakarta EE platform (EJB, JMS, etc.). For most web apps and microservices, this is sufficient.

**Q: How do I run Tomcat behind a reverse proxy?**
A: Configure Nginx or Apache HTTP Server as a reverse proxy. Set the `proxyName` and `proxyPort` attributes on the Tomcat connector so generated URLs are correct.

**Q: Can Tomcat handle production-level traffic?**
A: Yes. With proper JVM tuning and NIO connectors, Tomcat handles thousands of concurrent connections. Major organizations run Tomcat in production at scale.

**Q: How do I enable HTTPS?**
A: Add a TLS connector in server.xml pointing to a keystore file. Use Certbot or step-ca to obtain certificates and convert them to JKS or PKCS12 format.

## Sources
- https://github.com/apache/tomcat
- https://tomcat.apache.org/

---
Source: https://tokrepo.com/en/workflows/apache-tomcat-java-servlet-container-powers-web-b1c917f9
Author: AI Open Source