In Java just-in-time (JIT) compiler is a program that turns Java bytecode (a
program that contains instructions that must be interpreted) into instructions
that can be sent directly to the processor. you have to write and compile a
program only once. The Java on any platform will interpret the compiled bytecode
into instructions understandable by the particular processor. However java
virtual machine handles only one bytecode instruction at a time that makes
execution slow. But Using the Java just-in-time compiler at the particular
system platform compiles the bytecode into the particular system code. After the
code has been (re-)compiled by the JIT compiler, it will usually run more
quickly on the computer.
The just-in-time compiler comes with JVM and is used optionally. It compiles the bytecode into platform-specific executable code that is immediately executed. JIT compiler option should be used specially if the method executable is repeatedly reused in the code.
In Java JIT is JVM(java virtual machine)
In C# it is in dot net framework
In Android DVM(Dalvik virtual machine)