What are the differences between application servers and servlet containers, and what is the role of each in web application architecture?

The application server provides a complete runtime environment for enterprise applications (Java EE/Jakarta EE), including transaction management, connection pools, security, and other services. It may include a servlet container.

The servlet container focuses solely on managing the lifecycle of servlets and JSPs, handling incoming HTTP requests, and sending responses. It is part of the application server but can also be used independently.

Role:

• Application Server: Provides a comprehensive platform for deploying and executing complex, distributed web applications using the full spectrum of Java EE/Jakarta EE technologies.
• Servlet Container: Provides basic functionality for hosting and executing web components (servlets, JSPs) that handle web requests. Used for simpler web applications or as an embedded component within an application server.

Main differences:

Feature	Servlet Container	Application Server
Functionality	Servlets, JSPs, Static content	Everything from the container + JPA, EJB, JMS, JTA, Security, etc.
Complexity	Simpler	More complex
Resources	Fewer	More
Cost (sometimes)	Free/Open Source	Often commercial
Examples	Tomcat, Jetty	WildFly, GlassFish, WebLogic, WebSphere

For example, Tomcat is a servlet container. WildFly is an application server that includes an integrated servlet container.