Sun's specification for or implementation of a software execution engine that safely and compatibly executes the byte codes in Java class files on a microprocessor (whether in a computer or in another electronic device).

- Java HotSpot performance engine - Sun's ultra-high-performance engine for implementing the Java runtime environment which features an adaptive compiler that dynamically optimizes the performance of running applications.

- KJava virtual machine - Sun's small-footprint, highly optimized foundation of a runtime environment within the Java 2 Platform, Micro Edition. Derived from the Java virtual machine, it is targeted at small connected devices and can scale from 30KB to approximately 128KB, depending on the target device's functionality.

- Java Card virtual machine - Sun's ultra-small-footprint, highly-optimized foundation of a runtime environment within the Java 2 Platform, Micro Edition. Derived from the Java virtual machine, it is targeted at smart cards and other severely memory-constrained devices and can run in devices with memory as small as 24K of ROM, 16K of EEPROM, and 512 bytes of RAM.