JVMs

A Java Virtual Machine (JVM) is an abstract computing machine that enables Java programs and other languages compiled to Java bytecode to run on any device or operating system that provides a compatible JVM. It provides a runtime environment, including memory management, loading of class files, verification, and execution of bytecode in a secure sandbox.

Key components include the class loader subsystem, which reads and links class files; the runtime data areas (heap for objects, metaspace or method area for class metadata, stacks for frames, program counter, native method stack); and the execution engine, which may interpret bytecodes and/or compile frequently executed paths to native code via a Just-In-Time (JIT) compiler. The garbage collector reclaims unused objects.

Before execution, class files are verified, prepared, resolved, and initialized to ensure type safety. During run, code is executed by interpreter or JIT-compiled code; garbage collection runs as needed; native interfaces allow calls to platform APIs.

JVMS (Java Virtual Machine Specification) defines the platform- and language-neutral behavior. Major JVM implementations include Oracle/OpenJDK HotSpot, Eclipse OpenJ9, and GraalVM. Android uses a different runtime (ART) rather than the standard JVM.

Languages such as Java, Kotlin, Scala, Groovy compile to bytecode and run on the JVM, enabling interoperation. The JVM provides platform independence; the same bytecode runs on any device with a compliant JVM. Performance is aided by JIT and adaptive optimization, and memory management by garbage collection.