Just-In-Time (JIT) Compiler in Java

Overview

• The Just-In-Time (JIT) Compiler is a core component of the Java Virtual Machine (JVM) responsible for improving runtime performance.
• Instead of interpreting bytecode repeatedly, JIT compiles frequently executed bytecode into native machine code, enabling faster execution.

Why JIT Exists?

• Java source code is compiled into platform-independent bytecode
• Bytecode interpretation is slower than native execution
• JIT bridges this gap by providing dynamic, runtime compilation
• This allows Java to retain portability without sacrificing performance

How JIT Works (Execution Flow)

• javac compiles source code → bytecode
• JVM initially executes bytecode using an interpreter
• JIT monitors execution and identifies hot methods (frequently executed code)
• Hot methods are compiled into native machine code
• Native code is cached and reused, bypassing interpretation

Runtime Optimizations Performed by JIT

• Method inlining
• Dead code elimination
• Loop unrolling
• Adaptive optimization based on runtime behavior
• These optimizations are dynamic, meaning they are applied based on how the application actually runs, not assumptions made at compile time.

Real-World Analogy

First execution → interpreter (learning phase)
Repeated execution → JIT compilation (optimized phase)

Similar to memorizing a frequently used route instead of checking a map every time