What does a just-in-time (JIT) compiler do?

Question

What does a JIT compiler specifically do as opposed to a non-JIT compiler? Can someone give a succinct and easy to understand description?

Answer 1

Just In Time Compiler (JIT) :
It compiles the java bytecodes into machine instructions of that specific CPU.

For example, if we have a loop statement in our java code :

while(i<10){
    // ...
    a=a+i;
    // ...
 }

The above loop code runs for 10 times if the value of i is 0.

It is not necessary to compile the bytecode for 10 times again and again as the same instruction is going to execute for 10 times. In that case, it is necessary to compile that code only once and the value can be changed for the required number of times. So, Just In Time (JIT) Compiler keeps track of such statements and methods (as said above before) and compiles such pieces of byte code into machine code for better performance.

Another similar example , is that a search for a pattern using "Regular Expression" in a list of strings/sentences.

JIT Compiler doesn't compile all the code to machine code. It compiles code that have a similar pattern at run time.

See this Oracle documentation on Understand JIT to read more.