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Java 9 Modularity by Sander Mak, Paul Bakker

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Chapter 2. Modules and the Modular JDK

Java is over 20 years old. As a language, it’s still popular, proving that Java has held up well. The platform’s long evolution becomes especially apparent when looking at the standard libraries. Prior to the Java module system, the runtime library of the JDK consisted of a hefty rt.jar (as shown previously in Figure 1-1), weighing in at more than 60 megabytes. It contains most of the runtime classes for Java: the ultimate monolith of the Java platform. In order to regain a flexible and future-proof platform, the JDK team set out to modularize the JDK—an ambitious goal, given the size and structure of the JDK. Over the course of the past 20 years, many APIs have been added. Virtually none have been removed.

Take CORBA—once considered the future of enterprise computing, and now a mostly forgotten technology. (To those who are still using it: we feel for you.) The classes supporting CORBA in the JDK are still present in rt.jar to this day. Each and every distribution of Java, regardless of the applications it runs, includes those CORBA classes. No matter whether you use CORBA or not, the classes are there. Carrying this legacy in the JDK results in unnecessary use of disk space, memory, and CPU time. In the context of using resource-constrained devices, or creating small containers for the cloud, these resources are in short supply. Not to mention the cognitive overhead of obsolete classes showing up in IDE autocompletions and documentation ...

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