JIT compiler vs offline compilers

Compilers are essential software tools responsible for transforming human-readable code into machine-executable instructions. The terms "Ahead-of-Time" (AOT) and "Just-in-Time" (JIT) pertain to the timing of compilation, specifically in relation to runtime execution. In this context, the "time" mentioned in these terms refers to the duration when the program is actively running. A JIT compiler dynamically compiles the program during runtime, on-the-fly, as it encounters each piece of code. On the other hand, an AOT compiler performs the compilation process before the program is executed, generating machine code in advance.


In theory, a Just-in-Time (JIT) compiler holds the potential for an advantage over an Ahead-of-Time (AOT) compiler, provided it is given sufficient time and computational resources. The JIT compilation process can exhibit improved speed due to the generation of machine code taking place on the exact machine where it will subsequently execute. This proximity allows the JIT compiler to access the most accurate and relevant information, thereby enabling the emission of optimized code. By using the contextual details of the specific runtime environment, the JIT compilation approach aims to maximize performance and efficiency.


Just-in-Time (JIT) compilers possess the capability to perform optimizations that are not attainable with Ahead-of-Time (AOT) compilers. Unlike traditional compilation and optimization systems that necessitate educated guesses about critical sections of code, JIT optimization introduces a distinct advantage. It allows for initial measurement of the actively utilized code segments, enabling targeted optimization efforts. Consequently, the JIT compiler exclusively compiles the portions of code that are relevant to the user, leaving potentially untouched a significant proportion, conservatively estimated at 80%, thereby conserving valuable time and memory resources. When bytecode is pre-compiled into machine code, the compiler's optimizations are confined to the build machine, lacking the ability to tailor optimizations for the target machine(s).


JIT compilation provides a distinct advantage as it occurs on the same system where the code is executed, enabling highly precise fine-tuning tailored to the specific characteristics of that system. However, when opting for Ahead-of-Time (AOT) compilation with the intention of distributing the same package to all users, compromises must be made. In such cases, accommodating the diverse range of target systems necessitates balancing optimizations to ensure compatibility and performance across different environments.

run-time metrics

A JIT-compiler can not only look at the code and the target system , but also at how the code is used. It can instrument the running code, and make decisions about how to optimize according to, for example, what values the method parameters usually happen to have.


  1. Large applications generally benefit from being compiled ahead-of-time, but small ones generally don't

  2. Native images load faster because they don't have much startup activities, and require a static amount of fewer memory (the memory required by the JIT compiler).