Modules as an Improvement over Traditional Plugins

In Liferay DXP 7.1, you can develop applications using OSGi modules or using traditional Liferay plugins (WAR-style portlets, hooks, EXT, and web applications). Liferay’s Plugin Compatibility Layer (explained later) makes it possible to deploy traditional plugins to the OSGi runtime framework. To benefit from all Liferay DXP and OSGi offer, however, you should use OSGi modules.

Modules offer these benefits:

  • Better Encapsulation - The only classes a module exposes publicly are those in packages it exports explicitly. This lets you define internal public classes transparent to external clients.

  • Dependencies by Package - Dependencies are specified by Java package, not by JAR file. In traditional plugins, you had to add all of a JAR file’s classes to the classpath to use any of its classes. With OSGi, you need only import packages containing the classes you need. Only the classes in those packages are added to the module’s classpath.

  • Lightweight - A module can be as small as you want it to be. In contrast to a traditional plugin, which may require several descriptor files, a module requires only a single descriptor file–a standard JAR manifest. Also, traditional plugins are typically larger than modules and deployed on app server startup, which can slow down that process considerably. Modules deploy more quickly and require minimal overhead cost.

  • Easy Reuse - Modules lend themselves well to developing small, highly cohesive chunks of code. They can be combined to create applications that are easier to test and maintain. Modules can be distributed publicly (e.g., on Maven Central) or privately. And since modules are versioned, you can specify precisely the modules you want to use.

  • In-Context Descriptors - Where plugins use descriptor files (e.g., web.xml, portlet.xml, etc.) to describe classes, module classes use OSGi annotations to describe themselves. For example, a module portlet class can use OSGi Service annotation properties to specify its name, display name, resource bundle, public render parameters, and much more. Instead of specifying that information in descriptor files separate from the code, you specify them in context in the code.

These are just a few ways modules outshine traditional plugins. Note, however, that developers experienced with Liferay plugins have the best of both worlds. Liferay DXP 7.1 supports traditional plugins and modules. Existing Liferay developers can find comfort in the simplicity of modules and their similarities with plugins.

Here are some fundamental characteristics modules share with plugins:

  • Developers use them to create applications (portlets for Liferay)

  • They’re zipped up packages of classes and resources

  • They’re packaged as a standard Java JARs

Now that you’ve compared and contrasted modules with plugins, it’s time to tour the module anatomy.

Module Structure: A JAR File with a Manifest

A module’s structure is extremely simple. It has one mandatory file: META-INF/MANIFEST.MF. You add code and resources to the module and organize them as desired.

Here’s the essential structure of a module JAR file:

  • [Project root]
    • [Module's files]
    • META-INF

The MANIFEST.MF file describes the module to the system. The manifest’s OSGi headers identify the module and its relationship to other modules.

Here are some of the most commonly used headers:

  • Bundle-Name: User friendly name of the module.

  • Bundle-SymbolicName: Globally unique identifier for the module. Java package conventions (e.g., com.liferay.journal.api) are commonly used.

  • Bundle-Version: Version of the module.

  • Export-Package: Packages from this module to make accessible to other modules.

  • Import-Package: Packages this module requires that other modules provide.

Other headers can be used to specify more characteristics, such as how the module was built, development tools used, etc.

For example, here are some headers from the Liferay Journal Web module manifest:

Manifest-Version: 1.0
Bundle-ManifestVersion: 2
Bundle-Name: Liferay Journal Web
Bundle-SymbolicName: com.liferay.journal.web
Bundle-Vendor: Liferay, Inc.
Bundle-Version: 1.1.2
    com.liferay.journal.service,com.liferay.journal.util, [..]
    com.liferay.asset.kernel.exception, [..]

Note: to remove unnecessary “noise” from this example, some headers have been abbreviated ([..]) and some have been removed.

You can organize and build a module’s Java code and resources however you like. You’re free to use any folder structure conventions, such as those used in Maven or by your development team. And you can use any build tool, such as Gradle or Maven, to manage dependencies.

Liferay Workspace is an environment for managing module projects (and theme projects). A default Workspace provides Gradle build scripts and a Workspace created from the Liferay Project Templates Workspace archetype provides Maven build scripts for developing on Liferay. Workspace can be used from the command line or from within Liferay Dev Studio DXP. Note also that Liferay Dev Studio DXP provides plugins for Gradle, Maven, and BndTools. Tooling details are covered later in this series.

Now that you’re familiar with the module structure and manifest, it’s time to explore how to build modules.

Building Modules with bnd

The most common way to build modules is with a little tool called bnd. It’s an engine that, among other things, simplifies generating manifest metadata. Instead of manually creating a MANIFEST.MF file, developers use bnd to generate it. bnd can be used on its own or along with other build tools, such as Gradle or Maven. Liferay Workspace uses bnd together with Gradle or Maven.

One of bnd’s best features is that it automatically transverses a module’s code to identify external classes the module uses and adds them to the manifest’s list of packages to import. bnd also provides several OSGi-specific operations that simplify module development.

bnd generates the manifest based on a file called bnd.bnd in the project root. This file’s header list is similar to (but shorter than) that of the MANIFEST.MF. Compare the Liferay Journal Web module’s bnd.bnd file content (simplified a bit) below to its MANIFEST.MF file content that was listed earlier:

Bundle-Name: Liferay Journal Web
Bundle-SymbolicName: com.liferay.journal.web
Bundle-Version: 1.1.2

The main difference is that the bnd.bnd file doesn’t specify an Import-Package header. It’s unnecessary because bnd generates it in the MANIFEST.MF file automatically. It’s metadata made easy!

bnd plugins are available to use with Gradle and Maven. And since Liferay Workspace includes bnd, you can use bnd from the command line and from Liferay Dev Studio DXP.

It’s time to get hands-on experience creating and deploying an OSGi module. That’s next.

« OSGi and Modularity for Liferay Portal 6 DevelopersExample: Building an OSGi Module »
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