Talk about a monolith! Java has come a long way since its first release in 1996. The first major release of the JDK had a little over 500 public classes. A far cry indeed from JDK 8 released in 2014, which counts upwards of 4,200 public classes and over 20,000 total files.

The following commands extract the rt.jar file, a library JAR file bundled in JDK 8, and count the number of classes in it. With the Java 8 version I have installed on my machine, the count is 20651:

The JDK and the runtime, the JRE, have continued to grow over the years. There are a lot of features that have been added to the language, so this growth is understandable. However, the Java language is also notorious for going to great lengths to maintain backward compatibility and for its reluctance to deprecate features unless it is absolutely necessary. So, in a way, the current size of the runtime is a little over what it could have ideally been.

Normally, most application developers wouldn't need to worry about the JDK code base. They just focus on their application code. However, the contents of the runtime does matter for application execution because of the way it is bundled. Traditionally, every JRE has had all the classes necessary for runtime bundled into a single JAR that resides in the lib directory called rt.jar. The name rt, as you might have guessed, stands for runtime.

Not only is this huge monolith of classes unnecessarily bulky in size, it also adds performance overheads for the Java Virtual Machine to manage. And that's a price the execution environment of all your applications have to pay, irrespective of whether all of those classes are being used or not.