How
Does the Just in Time Compiler for Java Works?
The
working of the Just-In-Time compiler is more or less the same for all the JIT
compilers. The source code is compiled to the bytecode by the Java Virtual Machine
(JVM).
The JVM also comes with the JIT compiler which is used when the
application is run. Upon executing the application the bytecode is converted to
the machine code or the native code that is run by the machine.
The job
of the JIT compiler is to convert this bytecode to the machine code. All the JIT
compilers work in the same manner. The JIT compiler uses a V-table which is a
pointer to the methods in the class. This internal table is used to compile the
methods to native code.
The address of the JIT compiler itself is placed in the V-table and this address
is called during execution and the JIT compiler executes the native code and stores
that address to the V-table. And from now on this address is called whenever that
method is called and the native code is executed.
Only
during the first call to a method it is compiled and for the subsequent calls
the native code for that particular method is called. The V-table maintains the
addresses of the native code for all the methods that are compiled. V-table also
maintains another table which has the addresses of the bytecode itself in case
there is a need to compile it for the first time.
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