Yes — servlet containers like Tomcat can run web applications… so why do we need application servers like WildFly, GlassFish, or Payara?
Let’s break it down:
🧩 1. Servlet Containers (e.g., Tomcat, Jetty)
Only support the Web part of the Java EE (Jakarta EE) specification:
- Servlets (
HttpServlet) - JSP (JavaServer Pages)
- WebSocket
- Basic authentication, session management
✅ Good for:
- Simple web apps
- REST APIs
- Frontend + backend with just servlets and JSP
💼 2. Application Servers (e.g., WildFly, GlassFish)
Support full Java EE / Jakarta EE spec:
In addition to everything a servlet container does, they support:
| Feature | API |
|---|---|
| Dependency Injection | CDI (Contexts and Dependency Injection) |
| Transaction Management | JTA (Java Transaction API) |
| Security | JAAS, role-based access |
| Enterprise Beans | EJB (Enterprise JavaBeans) |
| Messaging | JMS (Java Message Service) |
| Persistence | JPA (Java Persistence API) |
| Batch Jobs | JSR 352 |
| Web Services | JAX-RS, JAX-WS |
✅ Good for:
- Complex enterprise-grade apps
- Apps that need distributed transactions, messaging, etc.
- Legacy systems using full Java EE stack
🔍 Example Scenario
| Task | Servlet Container (Tomcat) | Application Server (WildFly) |
|---|---|---|
| Serve HTML, handle forms | ✅ Yes | ✅ Yes |
| REST API with JSON | ✅ Yes | ✅ Yes |
| Background jobs, timers | ❌ No | ✅ Yes (via EJB Timer) |
| Messaging with JMS | ❌ No | ✅ Yes |
| Distributed transactions | ❌ No | ✅ Yes |
| Dependency injection | Limited (manual) | ✅ Built-in with CDI |
💡 Spring Boot Bonus
Spring Boot apps embed a servlet container (like Tomcat or Jetty) and provide application server-like features via Spring libraries:
@Component,@Service,@Transactional- No need for full Java EE server
So with Spring Boot, you get many of the benefits of an application server with a lightweight setup.
🧠 TL;DR
| Feature | Servlet Container | Application Server |
|---|---|---|
| Lightweight | ✅ Yes | ❌ Usually heavier |
| Fast startup | ✅ Yes | ❌ Slower |
| Servlets & JSP | ✅ Yes | ✅ Yes |
| Full Java EE support | ❌ No | ✅ Yes |
| Suitable for Microservices | ✅ Yes | ❌ Often overkill |