Difference between JIT and JVM in Java

1. Introduction

In this blog post, we discuss the difference between Java Virtual Machine (JVM) and the Just-In-Time (JIT) compiler. While both play critical roles in executing Java applications, they serve different purposes and operate in distinct ways. 

2. Key Points

Java Virtual Machine (JVM): The JVM is an abstract computing machine that enables a computer to run Java programs. It is platform-independent, meaning Java programs can run on any device or operating system with the JVM installed. 

Just-In-Time (JIT) Compiler: The JIT compiler is a component of the JVM that improves the performance of Java applications by compiling bytecode into native machine code at runtime. This means that instead of interpreting bytecode line by line, the JIT compiles entire blocks of bytecode into native code, which can be directly executed by the computer's CPU.

3. Differences

JVM (Java Virtual Machine)	JIT Compiler (Just-In-Time Compiler)
A specification that provides a runtime environment in which Java bytecode can be executed.	A part of the JVM that dynamically compiles bytecode into native machine code to improve performance.
Responsible for loading, verifying, and executing bytecode.	Optimizes bytecode execution by compiling it directly to machine code, bypassing the interpreter.
Provides system independence to Java applications (write once, run anywhere - WORA).	Works within the JVM to speed up the execution time of applications by compiling hot spots of bytecode.
Uses various components like the classloader, bytecode verifier, garbage collector, and interpreter.	Uses profiling information collected during runtime to decide which parts of the code to compile.
The execution engine within the JVM can run in interpretation, compiling, or both.	Enhances the performance of Java applications by reducing the interpretation overhead.
Does not directly improve performance but provides an environment conducive to secure and efficient execution.	Directly impacts performance by compiling frequently executed code paths into faster, optimized machine code.

4. Example

Consider a Java application that calculates Fibonacci numbers. The JVM is responsible for loading the .class files containing bytecode for this application, verifying the bytecode, and executing it. Initially, the JVM interprets the bytecode, which allows for platform-independent execution but isn't the fastest method of execution. 

As the application runs, the JIT compiler identifies the frequently called methods (hot spots) involved in calculating Fibonacci numbers. It compiles these hot spots into native machine code, which the CPU can execute directly, significantly speeding up the calculation process. This compilation happens dynamically at runtime and is transparent to the user.

5. Conclusion 

While the JVM provides a robust and secure platform for executing Java bytecode, the JIT compiler enhances performance by compiling bytecode to native machine code on the fly. Understanding the roles and differences between the JVM and JIT compiler is essential for Java developers looking to optimize their applications effectively. Both are integral to the Java runtime environment, working together to ensure Java applications run efficiently across different platforms.