Java JIT Compiler: Explained in Simple Words

Java is known for its “Write Once, Run Anywhere” promise. Behind this magic lies the Java Virtual Machine (JVM) and a powerful feature called the JIT Compiler (Just-In-Time Compiler). If you’ve ever wondered how Java balances portability with performance, the JIT Compiler is the answer.

🚀 What is a JIT Compiler?

JIT stands for “Just-In-Time”.

The JIT compiler is responsible for improving the performance of Java applications. When you run a Java program, the JVM interprets the bytecode and translates it into machine code that the operating system can understand. However, interpreting bytecode can be slow. That’s where the JIT compiler comes in.

The JIT compiler speeds up this process by converting frequently used parts of the bytecode (called hot code) into machine code at runtime. Once the bytecode is compiled into machine code, it can be executed directly by the CPU, which improves the performance of the application.

🔧 How the JIT Compiler Works

Here’s how the JIT Compiler fits into the JVM execution flow:

1. ✅ You run a Java program.
2. 🧱 Java code is compiled to bytecode (.class files).
3. 🧠 The JVM starts interpreting the bytecode line-by-line.
4. 📈 The JIT Compiler monitors frequently executed code (aka hot code).
5. ⚙️ Once a method is called many…