What Is Just-in Time Compiler? Difference Between Compiler And Interpreter

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The concept of Just-in Time Compiler is well known not only in Java but also in other languages like Ruby, .Net, C# etc., While JVM is a part of JRE; JIT is a part of JVM that is used to speed up the execution time of a program. JVM is the just-in-time compiler for Java that allows you to run programs written in Java as well as in many other high-level languages, such as Scala, JRuby, Python etc. The set of languages supported by JVM is combinedly known as JVM languages.

We all know the basic definition of compiler as “a program that translates the source code to a machine executable code.” So how Just in Time compiler differs? When compiling a java program, the static compiler that is run using the command javac converts the source code to byte code which are in the form of .class files. These .class files or the byte code needs to be interpreted to convert it into a machine executable code.

Byte code of a program may usually consist of methods, variables, threads and other instructions.  So, here instead of invoking a method every time the code is interpreted an optimized code is generated by the JIT for a particular machine.

JIT compiles the code when it is needed but not before runtime. Whenever a program is executed this compiled object code is invoked instead of interpreting the entire byte code and is quite efficient. This increases the performance of the program as well. Just in time compiler coverts the byte code to a platform specific executable code that can be executed immediately. However the use of JIT is optional, but Sun Microsystems suggest that it is quite efficient to use JIT especially if there exists a repeated code in the program.

Overview of How JIT Compiler helps in Compilation of Code:

  • Using a compiler, Java source code is converted to Java byte code (.class files).
  • Once this is done, JVM loads the .class files at run time and converts them to a machine understandable code using an interpreter.
  • JIT compiler is a feature of JVM which when enabled makes the JVM analyze the method calls in byte code and compiles them to more native and efficient code. JIT optimizes the prioritized method calls at this point of time.
  • Once these method calls are compiled, the JVM then executes this optimized code instead of interpreting it which is likely to increase the performance of the execution.

Differences between Compiler and Interpreter:

Though both compiler and an interpreter do the same job of converting a high level language to a machine executable code, there are few differences in the way they do it.

A compiler is a program that converts the entire code into a equivalent machine code at once. That is a compiler scans the entire program first, translate it to a machine executable code which will then be executed by the process and the corresponding output is generated. Compiler generates errors if any at the end of execution of the program.

On the other side, interpreter takes the single instruction of the code, translates it into an intermediate code and then into a machine code, executes it and takes another instruction. Interpreter generates errors as soon as it finds error in any of the sequence of instructions.

Though the time for analyzing the program and converting high level language to a machine executable code is high in compiler, when it comes to overall performance compiler is fast. This is because when an error occurs during the interpretation of the code, it has to reinterpret the previously interpreted code once again which is a tedious process.

About Aditi Malhotra

Aditi Malhotra is the Content Marketing Manager at Whizlabs. Having a Master in Journalism and Mass Communication, she helps businesses stop playing around with Content Marketing and start seeing tangible ROI. A writer by day and a reader by night, she is a fine blend of both reality and fantasy. Apart from her professional commitments, she is also endearing to publish a book authored by her very soon.

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