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Java - JIT Compiler

📌 What You Will Learn
  • What the JVM does step by step
  • The Class Loader Subsystem
  • The memory areas: stack, heap, method area, PC registers
  • The Execution Engine (Interpreter + JIT)
  • Stack vs Heap — the classic interview question

JIT Compiler is a core concept of the Java language. This lesson explains ecution Engine — Interpreter and JIT with complete, runnable code examples, clear step-by-step explanations, and common mistakes to avoid with exam-style MCQs at the end.

Execution Engine — Interpreter and JIT

The execution engine actually runs the bytecode. It uses two machines together:

  • Interpreter — reads bytecode one instruction at a time and runs it. Simple but slow.
  • JIT (Just-In-Time) Compiler — watches which methods run again and again, and compiles those hot methods into fast machine code. That is why Java becomes faster the longer it runs.

In short: the interpreter starts quickly, and the JIT speeds up the frequently used code. Together they give Java both quick startup and high performance.

In simple words: The interpreter starts the program fast, and the JIT makes the hot methods fast. The JIT watches which code runs again and again and compiles it into machine code — that is why Java speeds up the longer it runs.
📝 Key Takeaways
  • JVM = Load (Class Loader) + Store (Runtime Areas) + Execute (Execution Engine).
  • Five memory areas: Method Area, Heap, Stack, PC Registers, Native Stack.
  • Stack holds local variables and references; Heap holds actual objects.
  • Objects in the heap are freed by the Garbage Collector.
  • Execution Engine = Interpreter (starts fast) + JIT (speeds up hot code).
  • The JVM being OS-specific is what makes bytecode platform independent.

🧠 Test Your Knowledge

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