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What is JIT in Java? – Understanding Java Fundamentals

Last updated on Jul 10,2019 15.1K Views

12 / 72 Blog from Java Core Concepts

Every programming language uses a compiler to convert the high-level language code into machine level binary code, because the system understands only the binary code. Based on the type of the programming language, compiler differs. Now talking about the Java Programming language, it uses this amazing compiler called the JIT (Just-in-Time) in Java. This blog will tell you all about JIT Java Compiler.

Below are the topics covered in this article:

So, let’s get started!

Java JIT Compiler – Overview

The Just-In-Time compiler is one of the integral parts of the Java Runtime Environment. It is mainly responsible for performance optimization of Java-based applications at run time or execution time. In general, the main motto of the compiler is increasing the performance of an application for the end user and the application developer.

Deep Dive into JIT in Java

  • Byte code is the chief potential of Java’s WORA (Write once, run anywhere) environment. Speed of the Java application depends on the way the byte code gets converted to the native machine code. The bytecode can either be interpreted or compiled to native code or directly executed on the processor. But, if the bytecode is interpreted, it directly affects the speed of the application.

  • In order to speed up the performance, JIT compiler communicates with JVM at the execution time in order to compile byte code sequences into native machine code. Basically, when using JIT Compiler, the native code is easily executed by the hardware when compared to JVM Interpreter. By doing so, there will be a huge gain in execution speed.

  • When the JIT Compiler compiles the series of byte code, it also performs certain optimizations such as data analysis, translation from stack operations to register operations, eliminating subexpressions, etc. This makes Java very efficient when it comes to execution and performance.

Now that you know the fundamentals of JIT Compiler, let’s move further and understand its working.

Working of JIT Compiler in Java

The JIT Compiler speeds up the performance of the Java applications at run time. As Java is an object-oriented approach, it comprises of classes and objects. Basically, it constitutes a byte code that is platform independent and executed by the JVM across diversified architectures.

Work Flow: 

Below diagram depicts how the actual working of compilation takes place in Java Runtime Environment.

JIT Compiler - JIT in Java - Edureka

  1. When you code the Java Program, JRE uses javac compiler to compile the high-level Source code to byte code. After this, JVM loads the byte code at run time and converts into machine level binary code for further execution using Interpreter.

  2. As I have already mentioned above, interpretation of Java byte code reduces the performance when compared to a native application. That’s where JIT Compiler aids to boost up the performance by compiling the byte code into native machine code “just-in-time” to run.

  3. The JIT Compiler is activated and enabled by default when a method is invoked in Java. When a method is compiled, Java Virtual Machine invokes the compiled code of the method directly without interpreting it. Hence, it does not require much memory usage and processor time. That basically speeds up the performance of the Java Native Application.

So, that’s how it works. Now let’s dive deeper into this article and understand the security aspects of JIT Compiler in Java.

Security Aspects of JIT in Java

The compilation of byte code into machine code by JIT Compiler is done directly in memory. i.e. the compiler feeds the machine code directly into the memory and executes it. In this case, it doesn’t store the machine code into the disk before invoking the class file and executing it. Basically, the memory should be marked as executable. For the sake of security issues, this should be completed after the code is written into the memory. It also should be marked as read-only as executable memory is a security hole. If you wish to know more about this, you can check out this article on JIT Compiler Security aspects.

Now, let’s now move further and know the pros and cons of the Just-In-Time Compiler in Java.

Pros and Cons of JIT in Java


  1. The Java code that you have written years before will run faster even today and that improves the performance of Java Programs.

  2. Native images also execute faster as they do not possess start-up activities and requires less memory.


  1. Increases the complexity of Java Programs.

  2. Programs with less code do not benefit with Just-In-Time Compilation.

This brings us to the end of this article on JIT in Java. I hope you found it informative.

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Got a question for us? Please mention it in the comments section of this “JIT in Java” article and we will get back to you as soon as possible.

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What is JIT in Java? – Understanding Java Fundamentals