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For maximum flexibility, Liferay supports several different types of apps. Some apps can even contain other apps. The types of apps that Liferay can run include:

  • OSGi Modules
  • Portlets
  • Web Plugins
  • Templates
  • Themes

Read on to learn about these app types.

OSGi Modules

Since Liferay runs on OSGi, apps can be implemented as OSGi modules. An OSGi module is a JAR file adapted to run on OSGi. Although it’s possible for a single module to implement a single app, an app typically consists of multiple modules that are packaged together. Also, note that apps in OSGi modules aren’t required to have a UI. For example, Liferay can run OSGi modules that expand Liferay’s built-in APIs without requiring any user interaction. This is crucial for developers that need to leverage custom APIs that Liferay doesn’t provide. By providing such an API via one or more OSGi modules, you can let developers leverage your API. To see a list of Liferay’s API modules, see the reference article Finding Liferay API Modules.

OSGi modules can also contain apps that have a UI: portlets. The next section discusses these.

Portlets

Portlets are small web applications that run in a portion of a web page. For example, Liferay’s Blogs app is a portlet. Portlet applications, like servlet applications, are a Java standard implemented by various portal server vendors. The JSR-168 standard defines the portlet 1.0 specification and the JSR-286 standard defines the portlet 2.0 specification. A Java standard portlet should be deployable on any portlet container that supports the standard. Portlets are placed on the page in a certain order by the end user and are served up dynamically by the portal server. This means certain things that apply to servlet-based projects, such as control over URLs or access to the HttpServletRequest object, don’t apply in portlet projects because the portal server generates these objects dynamically.

Portlets can be composed of OSGi modules (recommended), or contained in WAR files. For information on developing OSGi modules for Liferay, including portlets, see Liferay’s developer tutorials.

Web Plugins

Web plugins, another type of app that can run on Liferay, are regular Java EE web modules designed to work with Liferay. Liferay supports integration with various Enterprise Service Bus (ESB) implementations, as well as Single Sign-On implementations, workflow engines, and so on. These are implemented as web modules used by Liferay portlets to provide functionality.

Templates and Themes

Templates and themes are plugins that change Liferay’s appearance. Templates (layout templates) control how Liferay arranges portlets on a page. They make up a page’s body (the large area into which you can drag and drop portlets). Liferay comes with several built-in layout templates. If you have a complex page layout (especially for your home page), you may wish to create a custom layout template of your own.

Themes can completely transform Liferay’s look and feel. Most organizations have their own look and feel standards that apply to all of their web sites and applications. By using a theme plugin, an organization can apply these standards throughout their Liferay instance. There are many available theme plugins on Liferay’s web site and more are being added every day. This makes it easy for theme developers, as they can customize existing themes instead of writing a new one from scratch.

For information on developing themes and templates, see this section of tutorials.

Liferay Marketplace App Packages

Regardless of app type, each Liferay Marketplace app is distributed in an LPKG package. The LPKG package contains Marketplace metadata and the files the app needs to run. Note that it’s possible for an LPKG package to contain multiple apps. For example, a single LPKG package can contain several portlets. This is common in cases where an app requires a Control Panel portlet for administrators, and another portlet for end users.

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