What is a Virtual Machine?

A virtual machine (virtual PC) is technically defined as an isolated replica of a real machine. A virtual machine is essentially an environment, usually a program or operating system, which does not physically exist. Virtual machines are created within another environment.

The virtual machine is referred to as the guest, while the environment is referred to as the host. Virtual machines are used to execute an instruction set different than that of the host system.

Java Runtime Environment (JRE)

A notable example of a virtual machine is the Java Runtime Environment. A program coded in Java will receive services from the Java Runtime Environment, or JRE. The Java program will act the virtual machine because it takes the place of the operating system for which the program would be used.

Types Of Virtual Machines

There are two major types of virtual machines: system virtual machines and process virtual machines. A system virtual machine acts as a complete system platform that supports the execution of an entire operating system. However, a process virtual machine is designed to run as a single program that supports a single process. This is how the Java Runtime Environment works.

The JRE acts as a platform for Java programs to run on. Java-specific commands are interpreted by the Java Runtime Environment. The Java programming language does not contain platform-specific codes that correspond to a single operating system that will display output and access resources. To accomplish this task, the JRE will simulate virtualized resources. These tasks are then transferred to the machine's actual resources.

One major component of virtual machines is the fact that the program running on the virtual machine is limited to the resources provided by the virtual machine. Both of these types of virtual machines act as an environment that does not physically exist, but which is created in another environment, such an operating system.

Process virtual machines like the Java Runtime Environment uses application virtualization in order to allow the binary codes for applications to be run on several different types of operating systems. The program is usually run on the computer with the use of an interpreter or just-in-time compilation.

An interpreter will allow the program made with virtual instructions to be loaded and run without compilation. A just-in-time compilation, or JIT, is a compilation method used by virtual machines that interprets the machine's native instructions at the latest possible time, which is usually the time right before the program runs.