Java runtime (JDK) support is a paid service that gives a company a tested, tuned and supported version of Java, the software needed to run programs written in the Java language. JDK stands for Java Development Kit. Its core is the Java virtual machine, or JVM, the program that actually runs Java code. A large share of enterprise software, from core banking systems to insurance and telecom billing, is written in Java.
How does Java run? A programmer writes Java code and compiles it into bytecode, a set of instructions that no real processor understands directly. The JVM reads the bytecode and runs it on whatever computer and operating system it is installed on, whether Linux, Windows or another. So the same program runs everywhere without changes. While the program runs, the JVM turns its most-used parts into fast machine code, a step called just-in-time compilation. It also frees memory that the program no longer needs, a step called garbage collection.
Despite its name, the JVM is not a virtual machine of the kind that a hypervisor creates. It does not pretend to be a whole computer. It only runs Java programs.
Why would a company pay for Java? The code is open source, in a project called OpenJDK, and free builds exist. Companies pay for three things. The first is long-term support: security fixes for older versions of Java, such as Java 8 or Java 11, which much business software still uses years after free updates for them have stopped. The second is a stable promise about licences, because companies have faced changes in how some Java builds are licensed. The third is performance. Some sellers offer a tuned JVM that starts faster, needs fewer servers, or avoids long pauses for garbage collection.
Consider an IT services company in Thiruvananthapuram that runs a health insurer's claims system for it. The system is written in Java and handles about 15,000 claims a day from hospitals. Every few minutes, the JVM pauses the program for up to 2 seconds while it collects garbage. During each pause, about 30 requests from hospitals time out, and the hospitals' staff must submit those claims again. The team moves the system to a supported JVM whose garbage collector does most of its work while the program keeps running. The pauses drop to a few thousandths of a second, and the timeouts stop. The same support contract also covers security fixes for the Java 11 version that the system uses.
The JVM runs on an operating system, usually from an enterprise Linux subscription, or on a virtual private server for a smaller app. In a modern setup, each Java service is packed with its JVM into a container, and run on a Kubernetes platform. The page on the open source business model explains why support for free software is sold.