A Simple Plugin System for Web Applications

We need to make multiple web-based projects with a lot of shared functionality. For that, some sort of a plugin system would be a good option (as an alternative to copy-pasting stuff). Some frameworks (like grails) have the option to make web plugins, but most don’t, so something custom-made is to be implemented.

First, let’s define what is the required functionality. The “plugin”:

• should be included simply by importing via maven/ivy
• should register all classes (either automatically, or via a one-line configuration) in a dependency injection container, if one is used
• should be vertical – i.e. contain all files, from javascript, css and templates, through controllers, to service layer classes
• should not require complex configuration that needs to be copy-pasted from project to project
• should allow easy development and debugging without redeployment

The java classes are put into a jar file and added to the lib directory, therefore to the classpath, so that’s the easy part. But we need to get the web resources extracted to the respective locations, where they can be used by the rest of the code. There are three general approaches to that: build-time extraction, runtime extraction and runtime loading from the classpath.

The last approach would require a controller (or servlet) that loads the resources from the classpath (the respective jar), cache them, and serve them. That has a couple of significant drawbacks, one of which is that being in a jar, they can’t be easily replaced during development. Working with classpath resources is also tricky, as you don’t know the names of the files in advance.

The other two approaches are very similar. Grails, for example, uses the build-time extraction – the plugin is a zip file, containing all the needed resources, and they are extracted to the respective locations while the project is built. This is fine, but it would require a little more configuration (maven, in our case), which would also probably have to be copied over from project to project.

So we picked the runtime extraction approach. It happens on startup – when the application is loaded, a startup listener of some sort (a spring components with @PostConstruct in our case) iterates through all jar files in the lib folder, and extracts the files from a specific folder (e.g. “web”). So, the structure of the jar file looks like this:

com
   company
      pkg
         Foo.class
         Bar.class
web
   plugin-name
       css
           main.css
       js
          foo.js
          bar.js
       images
          logo.png
       views
          foo.jsp
          bar.jsp

The end-result is that on after the application is started, you get all the needed web resources accessible from the application, so you can include them in the pages (views) of your main application.

And the code that does the extraction is rather simple (using zip4j for the zip part). This can be a servlet context listener, rather than a spring bean – it doesn’t make any difference.

/**
 * Component that locates modules (in the form of jar files) and extracts their web elements, if any, on startup
 *
 * @author Bozhidar
 */
@Component
public class ModuleExtractor {

	private static final Logger logger = LoggerFactory.getLogger(ModuleExtractor.class);

	@Inject
	private ServletContext ctx;

	@SuppressWarnings("unchecked")
	@PostConstruct
	public void init() {
		File lib = new File(ctx.getRealPath("/WEB-INF/lib"));
		File[] jars = lib.listFiles();
		String targetPath = ctx.getRealPath("/");
		String viewPath = "/WEB-INF/views"; //that can be made configurable
		for (File jar : jars) {
			try {
				ZipFile file = new ZipFile(jar);
				for (FileHeader header : (List<FileHeader>) file.getFileHeaders()) {
					if (header.getFileName().startsWith("web/") && !fileExists(header)) {
						// extract views in WEB-INF (inaccessible to the outside world)
						// all other files are extracted in the root of the application
						if (header.getFileName().contains("/views/")) {
							file.extractFile(header, targetPath + viewPath);
						} else {
							file.extractFile(header, targetPath);
						}
					}
				}
			} catch (ZipException ex) {
				logger.warn("Error opening jar file and looking for a web-module in: " + jar, ex);
			}
		}
	}

	private boolean fileExists(FileHeader header) {
		return new File(ctx.getRealPath(header.getFileName())).exists();
	}
}

So, in order to make a plugin, you just make a maven project with jar packaging, and add it as dependency to your main project, everything else is taken care of. You might need to register the ModuleExtractor if classpath scanning for beans is not enabled (or you choose to make it a listener), but that’s it.

Note: this solution doesn’t aim to be a full-featured plugin system that solves all problems. It doesn’t support versioning, submodules, etc. That’s why the title is “simple”. But you can do many things with it, and it’s has a very low complexity.