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The Java Portlet Specification defines a contract between the portlet container and portlets and provides a convenient programming model for Java portlet developers.
Portlets are pluggable user interface software components that are managed and displayed in a web portal, for example an enterprise portal or a web CMS. A portlet can aggregate and personalize content from different sources within a web page. A portlet responds to requests from a web client with and generates dynamic content.
Portlets produce fragments of markup that are aggregated into a portal. Typically, following the desktop metaphor, a portal page is displayed as a collection of non-overlapping portlet windows, where each portlet window displays a portlet. Hence a portlet resembles a web-based application that is hosted in a portal.
Some examples of portlet applications are e-mail, weather reports, discussion forums, and news.