JustInTimekääntäjä

JustInTimekääntäjä, often abbreviated as JIT compiler, is a software component that compiles code during the execution of a program. Instead of translating source code into machine code entirely before the program runs, a JIT compiler translates it on the fly, just before it is needed. This approach bridges the gap between interpretation, which is slower but more flexible, and ahead-of-time compilation, which is faster but less adaptable.

The primary benefit of a JIT compiler is improved performance. By compiling code segments as they are executed, the JIT compiler can optimize the machine code based on the actual runtime conditions and program behavior. This allows for dynamic optimizations that are not possible with static compilation. For example, a JIT compiler might notice that a particular loop is executed frequently and spend extra time optimizing that specific loop for maximum speed.

JIT compilation is widely used in virtual machines for various programming languages, such as Java (JVM) and .NET (CLR). These environments typically start by interpreting the intermediate bytecode. As parts of the code are executed repeatedly, the JIT compiler identifies these "hot spots" and compiles them into native machine code for the underlying hardware. This results in a significant performance boost as the program continues to run.

While JIT compilation offers performance advantages, it introduces an initial overhead. The act of compiling code during runtime can temporarily slow down the program's startup phase. However, for applications that run for an extended period or have computationally intensive sections, the long-term performance gains typically outweigh this initial cost. The decision to use JIT compilation involves a trade-off between startup time and overall execution speed.