Runtimes

In computing, runtime denotes the period during which a program is executed and the software environment that services that execution. It contrasts with compile time, when code is translated, and with link-time, load time.

A runtime environment provides services such as memory management, type checking, exception handling, I/O, and access to APIs. Some runtimes implement managed execution with garbage collection and Just-In-Time compilation; others are simple interpreters.

Examples include the Java Virtual Machine (JVM) and the .NET Common Language Runtime (CLR), class-based managed runtimes; CPython, JRuby, and V8 are other forms of runtimes; Node.js offers a JavaScript runtime built on V8; browsers also provide runtime environments for JavaScript.

Components often include runtime libraries, interoperation with native code, threading and scheduling facilities, garbage collection, exception handling, Just-In-Time or Ahead-Of-Time compilation, dynamic module loading, and security sandboxes.

The term also distinguishes between the actual execution phase and the software stack that supports it. Some runtimes emphasize cross-platform portability, while others are tightly bound to a particular operating system or hardware. Specialized runtimes exist for mobile, embedded, or high-performance computing contexts.