What is Just-In-Time (JIT) compilation

The Just in time compiler is a facility that is invoked by the CLR to convert the IL into machine code. The .NET framework assemblies (*.dll or *.exe) files cannot be executed by the target processor unless it is converted into native code. When the Assembly is loaded the method calls are invoked for compilation into native code.

Since this is done just when the method needs to execute, it is called Just in time compilation. When the JIT is invoked for the first time there is an overhead or performance penalty, since the assembly is converted into its Native code or image using the Native Image Generator (Ngen.exe). All subsequent calls will load faster as the Native image alone is invoked thereafter from the Native image cache.

Significantly, the IL can coexist with the machine code in the application. This feature also enables the developer modify the code or add to the code even when the application is actively deployed. This functionality provided by JIT makes the .NET framework an efficient, agile and powerful means of application development.

[B]Just-In-Time compilation (JIT)[/B], also known as dynamic translation, is a technique for improving the runtime performance of a computer program. JIT builds upon two earlier ideas in run-time environments: bytecode compilation and dynamic compilation. It converts code at runtime prior to executing it natively, for example bytecode into native machine code. The performance improvement over interpreters originates from caching the results of translating blocks of code, and not simply reevaluating each line or operand each time it is met (see Interpreted language). It also has advantages over statically compiling the code at development time, as it can recompile the code if this is found to be advantageous, and may be able to enforce security guarantees. Thus JIT can combine some of the advantages of interpretation and static compilation.