Just-In-Time (JIT) Compilation

A JIT compiler operates within a runtime environment or virtual machine. When a méthode is called for the first time, its platform-independent bytecode is compiled into native machine code and cached. Subsequent calls to that method execute the fast, native code directly, bypassing the overhead of interpretation. This introduces a small startup or warm-up delay but can result in significant long-term performance gains for long-running applications.

The key advantage of JIT compilation is the ability to perform profile-guided or adaptive optimization. The runtime can monitor the code’s execution, gathering statistics on which code paths are ‘hot’ (frequently executed) and the types of data being processed. This runtime information, unavailable to a static AOT compiler, allows the JIT to make more informed optimization decisions. For example, it can perform speculative inlining of virtual method calls if it observes that a particular concrete type is always used. If this assumption later proves false, the JIT can perform ‘deoptimization’ and fall back to a safer execution path. This dynamic nature makes JIT compilation extremely effective for object-oriented and dynamic languages.