Web apps in Liferay Portal are called portlets. Like many web apps, portlets process requests and generate responses. In the response, the portlet returns content (e.g., HTML, XHTML) for display in browsers. One key difference between portlets and other web apps is that portlets run in a portion of the web page. When you’re writing a portlet application, you only need to worry about that application: the rest of the page–the navigation, the top banner, and any other global components of the interface–is handled by other components. Another difference is that portlets run only in a portal server. Portlets can therefore use the portal’s existing support for user management, authentication, permissions, page management, and more. This frees you to focus on developing the portlet’s core functionality. In many ways, writing your application as a portlet is easier than writing a standalone application.
Portlets can be placed on pages by users (if they have permission) or portal administrators, who can place several different portlets on a single page. For example, a page in a community site could have a calendar portlet for community events, an announcements portlet for important announcements, and a bookmarks portlet for links of interest to the community. And because the portal controls page layout, you can reposition and resize one or more portlets on a page without altering any portlet code. Doing all this in other types of web apps would require manual re-coding. Alternatively, a single portlet can take up an entire page if it’s the only app you need on that page. For example, a message boards or wiki portlet is best suited on its own page. In short, portlets alleviate many of the traditional pain points associated with developing web apps.
What’s more, portals and portlets are standards-based. In 2003, Java Portlet Specification 1.0 (JSR-168) first defined portal and portlet behavior. In 2008, Java Portlet Specification 2.0 (JSR-286) refined and built on JSR-168, while maintaining backwards compatibility, to define features like inter-portlet communication (IPC) and more. The recently released Java Portlet Specification 3.0 (JSR-362) continues portal and portlet evolution. Liferay leads in this space by having a member in the Expert Group.
So what do these specifications define? The links above show the complete definition; here we’ll briefly summarize how portlets differ from other types of servlet-based web apps.
Portlets handle requests in multiple phases. This makes portlets much more flexible than servlets. Each portlet phase executes different operations:
- Render: Generates the portlet’s contents based on the portlet’s current state. When this phase runs on one portlet, it also runs on all other portlets on the page. The Render phase runs when any portlets on the page complete the Action or Event phases.
- Action: In response to a user action, the Action phase performs some operations that change the portlet’s state. The Action phase can also trigger events that are processed by the Event phase. Following the Action phase and optional Event phase, the Render phase then regenerates the portlet’s contents.
- Event: Processes events triggered in the Action phase. Events are used for inter-process communication (IPC). Once the portlet processes all events, the portal calls the Render phase on all portlets on the page.
- Resource-serving: Serves a resource independent from the rest of the lifecycle. This lets a portlet serve dynamic content without running the Render phase on all portlets on a page. The Resource-serving phase handles AJAX requests.
Compared to servlets, portlets also have some other key differences. Since portlets only render a portion of a page, tags like
<body> aren’t allowed. And because you don’t know the portlet’s page ahead of time, you can’t create portlet URLs directly. Instead, the portlet API gives you methods to create portlet URLs programmatically. Also, because portlets don’t have direct access to the
javax.servlet.ServletRequest, they can’t read query parameters directly from a URL. Portlets instead access a
javax.portlet.PortletRequest object. The portlet specification only provides a mechanism for a portlet to read its own URL parameters or those declared as public render parameters. Liferay Portal does, however, provide utility methods that can access the
ServletRequest and query parameters. Portlets also have a portlet filter available for each phase in the portlet lifecycle. Portlet filters are similar to servlet filters in that they allow request and response modification on the fly.
Portlets also differ from servlets by having distinct modes and window states. Modes distinguish the portlet’s current function:
- View mode: The portlet’s standard mode. Use this mode to access the portlet’s main functionality.
- Edit mode: The portlet’s configuration mode. Use this mode to configure a custom view or behavior. For example, the Edit mode of a weather portlet could let you choose a location to retrieve weather data from.
- Help mode: A mode that displays the portlet’s help information.
Most modern applications use View Mode only.
Portlet window states control the amount of space a portlet takes up on a page. Window states mimic window behavior in a traditional desktop environment:
- Normal: The portlet can be on a page that contains other portlets. This is the default window state.
- Maximized: The portlet takes up an entire page.
- Minimized: Only the portlet’s title bar shows.
When you develop portlets, you can leverage all the features defined by the portlet specification. Depending on how you develop and package your portlet, however, it may not be able to run on other portal containers. You may now be saying, “Hold on a minute! I thought Liferay Portal was standards-compliant? What gives?” Liferay Portal is standards-compliant, but it contains some sweeteners in the form of APIs designed to make developers’ lives easier. For example, Liferay Portal contains an MVC framework that makes it simpler to implement MVC in your portlet. This framework, however, is only available in Liferay Portal. Without modification, a portlet that uses this framework won’t run if deployed to a non-Liferay portal container. Note, though, that we don’t force you to use our MVC framework or any of its other unique APIs. For example, you can develop your portlet with strictly standards-compliant frameworks and APIs, package it in a WAR file, and then deploy it on any standards-compliant portal container.
Liferay Portal also contains an OSGi runtime. This means that you don’t have to develop and deploy your portlet as a traditional WAR file; you can do so as OSGi modules instead. We recommend the latter, so you can take advantage of the modularity features inherent in OSGi. For a detailed description of these features, see the tutorial OSGi and Modularity. Note, however, that portlets you develop as OSGi modules won’t run on other portlet containers that lack an OSGi runtime. Even so, the advantages of modularity are so great that we still recommend you develop your portlets as OSGi modules.
So what’s the benefit to adopting Liferay’s frameworks and APIs? There are several:
- They follow Liferay’s design patterns. The better you understand them, the better you understand Liferay Portal.
- They are the result of nearly 15 years of portlet development.
- They provide many conveniences that make development easier and faster.
- They make your applications fit more naturally with the rest of the system.
- If necessary, they’re easy to migrate from, because they’re built on top of the standards.
With that said, you can use a variety of technologies to develop portlets. This section shows you how to develop portlets using the following frameworks and techniques: