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.
JIT Examples:
In Java JIT is JVM(java virtual machine)
In C# it is in dot net framework
In Android DVM(Dalvik virtual machine)