what is the difference between a portlet and a servlet?

Enhanced from Source: Servlets Vs Portlets

Similarities

1. Servlets and Portlets are web based components which use Java fortheir implementation.

2. Portlets are managed by a portlet container just like servlet ismanaged by servlet container.

3. Both static and dynamic content can be generated by Portlets andServlets.

4. The life cycle of portlets and servlets is controlled by the container

5. The client/server model is used for both servlets and portlets

6. The packaging and deployment are essentially the same, WAR/EARs.

7. Application Session exists in both Servlet and Portlet containers. It is one of the ways of of sharing data (crude Inter-Portlet Communication) from the render phase to the action phase (or any lower phases) in the portlet containers.

8. Both Servlets and Portlets use similar server / VM environments that support it. Although, some additional configurations might needed in case of portlets to make it tick

9. The build/DI tools are similar for both - Ant, Maven, Gradle, etc are all supported. Mostly :) - This has changed a bit with Liferay 7.

Dissimilarities

1. Servlets can render complete web pages, whereas portlets renders htmlfragments. These fragments are aggregated by the portal into acomplete web page.

2. The content type of JSR 168 portlets can be only cHTML, XHTML, WML. Itdoes not support other content types.

3. Portlets are not allowed to generate HTML code that contains tags suchas body, frame, frameset, head, html, or title.

4. A Portlet unlike a servlet doesn’t have URL attached to it so itcannot be accessed directly. Access is only through the portal pagewhich holds the portlet.

5. Portlets can be provided with controls to manipulate its window statesor portlet modes.

6. Multiple instances of a single portlet can be placed onto the samepage.

7. Portlets support persistent configuration and customization, profileinformation.

8. Portlets can have two types of request viz. render request and actionrequest.

9. Portlets have two scopes within session; application scope forcommunication across portlets and portlet scope for intra portletcommunication.

10. Portlet cannot set the character set encoding of the response nor canit set the HTTP response headers.

11. Portlets doesn’t have access to request URL. So it cannot access thequery parameters appended to the URL. Portlets cannot set cookies.

12. Typical methods of Portlet API are doView(), doEdit(), doHelp() andprocessAction() while those of servlet are service(), doPost(),doGet().

13. Servlet Specifications - JSR 369(Servlet 4.0), JSR 340(Servlet 3.1), JSR 315(Servlet 3.0), JSR 154(Servlet 2.5 & 2.4).Portlet Specifications - JSR 168(Portlet Spec v1.0), JSR 286(Portlet Spec v2.0), JSR 362(Portlet Spec v3.0)

14. Deployment of Portlets involves different approach than a Servlet application. Some Providers (Liferay/Alfresco/WebSphere) support hot-deploying of portlets without the need to restart the server which is not possible in case of servlets without modularizing the application using special libraries such as OSGi.

Edit _{(From comments)}

A Portlet container is built on a Servlet container. So ultimately it can be said that the portlet runs on a Servlet Container. But while developing apps, we view a portlet container separately from the Servlet/Java EE container.

Portlets are part of JSR-168 standard that regulates portal containers and components. This is different standard from standards for web containers (and servlets). Though there are definitely strong parallels between these two standards they differ in containers, APIs, life cycle, configuration, deployment, etc.

The main difference between portlet vs. servlet could be that while servlet always responds to single type of action - request, portlet (due to nature of its life cycle and stronger container bindings) has to respond to two types of actions: render and request. There are of course more to it but I found this as the core difference between the two when I studied portal development.

Both portlets and servlets receive an http request and return a response, which is usally some HTML that can be rendered by a browser. A portlet is used in the context of a "Portal", the idea being that a single page seen by the user has lots of parts, think tiles, each coming from a different portlet.

Now, you can get that "tiled" effect from normal servets (See Struts + Tiles for an example of how) the extra bit from the portlets is that the portlets are in a richer environment provided by the Portal, extra APIs are provided so that what is displayed by any portlet can be configured by individual users to their preferences, and the porlets can communicate with each other - press a button in one, something happens in a another.