Java 9 Modularity: Patterns and Practices for Developing Maintainable Applications by Mak Sander & Bakker Paul

Author:Mak, Sander & Bakker, Paul [Mak, Sander]
Language: eng
Format: azw3
Publisher: O'Reilly Media
Published: 2017-09-07T04:00:00+00:00

During layer creation, it’s possible to manipulate the modules that are loaded into the layer and their relations. That’s exactly what a container needs to do to get access to classes in the deployed application. Somehow the container needs to ensure that the package containing the class implementing ContainerApplication is open for deep reflection.

Cleaning up modules is done at the layer level and is surprisingly straightforward. Layers can be garbage collected just like any other Java object. If the container makes sure there are no strong references anymore to the layer, or any of its modules and classes, the layer and everything associated eventually get garbage collected.

It’s time to look at some code. The container provided as an example in this chapter is a simple command-line launcher that lists applications you can deploy and undeploy. Its main class is shown in Example 6-5. If you deploy an application, it keeps running until it is undeployed (or the container is stopped). We pass information about where the deployable apps live as command-line arguments in the format out-appa/app.a/app.a.AppA: first the directory that contains the application modules, then the root module name, and last the name of the class that implements ContainerApplication, all separated by slashes.

Usually, application containers have a deployment descriptor format that would convey such information as part of the application package. There are other ways to achieve similar results, for example, by putting annotations on modules (as shown in “Annotations”) to specify some metadata. For simplicity, we derive AppDescriptor instances containing this information from the command-line arguments.

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