What Is the JIT Compiler in Java?

When developers talk about Java being both portable and fast, the JIT compiler is one of the biggest reasons why.

At first glance, Java appears slower than languages like C or C++ because Java code does not run directly on the operating system. Instead, it goes through the Java Virtual Machine (JVM). Yet modern Java applications power banking systems, enterprise platforms, trading systems, and large-scale backend services with impressive performance.

So how does Java achieve this?

The answer lies in the Just-In-Time (JIT) Compiler.

Understanding How Java Code Runs

Before understanding the JIT compiler, it helps to understand the Java execution flow.

When you write Java code, the Java compiler (javac) converts it into bytecode.

public class Hello {
    public static void main(String[] args) {
        System.out.println("Hello World");
    }
}

This source code becomes:

Hello.class

The .class file contains platform-independent bytecode.

That bytecode is then executed by the JVM.

Initially, the JVM interprets the bytecode line by line using an interpreter.

While interpretation provides portability, it is slower than running native machine code directly on the CPU.