JVM Java Virtual Machine
Runtime that executes Java bytecode. Stack-based VM with JIT compilation, garbage collection, class loading. Powers Java, Kotlin, Scala, Clojure, Groovy.
The JVM spec is open → multiple implementations: OpenJDK HotSpot (default), GraalVM (multi-language, AOT), Eclipse OpenJ9 (IBM, smaller footprint), Azul Zulu/Prime (commercial). Recent versions have dramatically improved GCs (G1 → ZGC, Shenandoah) with <1ms pauses. Project Loom (Java 21, 2023) added virtual threads. Project Valhalla (upcoming) brings value types.