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Saturday, 8 March 2014

Just In Time Compiler

           We already studied about Java Virtual Machine (JVM) in detail and observed that the diagram in the previous post was showing a part inside the JVM called JIT. So in this post we will learn about Just-In-Time(JIT) compiler , the component of Java Runtime Environment. You must be having a question in mind "Why does this JIT thing is residing inside the JVM ?"

Answer is : -
Just-In-Time is nothing but a component of JDK that improves the performance of Java applications at run time which in turn speeds up the execution time. 

JIT : -
  • Sun Microsystems suggests that it's usually faster to select the JIT compiler option, especially if the method executable is repeatedly reused.
  • According to most researches, 80% of execution time is spent in executing 20% of code. That would be great if there was a way to determine those 20% of code and to optimize them. That's exactly what JIT does - during runtime it gathers statistics, finds the "hot" code compiles it from JVM interpreted bytecode (that is stored in .class files) to a native code that is executed directly by Operating System and heavily optimizes it. 
  • The name "Hotspot" of Sun (Oracle) JVM is chosen because of the ability of this Virtual Machine to find "hot" spots in code.
  • A JIT compiler runs after the program has started and compiles the code (usually bytecode or some kind of VM instructions) on the fly (or just-in-time, as it's called) into a form that's usually faster, typically the host CPU's native instruction set.
  • It tries to predict which instructions will be executed next so that it can compile the code in advance. 
  • Compiled code resides in memory until the application is closed.
Working of JIT 
  • Just-In-Time compiler basically performs the following steps :-
      1.    Reads all the byte codes for the method that needs to be executed

2.    Converts all those byte codes to native machine instructions

3.    Executes the generated native machine instructions

          After the conversion of byte codes for a method to native machine instructions , that native code is remembered by the JVM .This enables JVM to simply run the native instructions , when next time the method has to be run.
It doesn't need to convert the bytecodes every time the method is run. This makes the program run much faster.

Note:-  The JIT compiler can be disabled, in which case the entire Java program will be interpreted. Disabling the JIT compiler is not recommended except to diagnose or work around JIT compilation problems.

So we have learnt the secret behind rapid execution of Java Programs which is Just-In-Time (JIT) compiler.

Keep Compiling !!

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