the runtime illusion: what Java really runs

Wed, 18 Feb 2026 07:00:02 +0000

Introduction

Java’s bytecode is made abstract to be portable and neutral from specific hardware. Yet in production, it performs with the precision of native code. How? The secret lies in the JIT compiler, a near magical blend of static analysis and real-time insights. It reinterprets, restructures and often reinvents the code based on the runtime behaviour. This article traces that silent transformation using the Semeru (OpenJ9) JIT, following a method from source to bytecode to intermediate representation and ultimately to highly optimised machine code. You will be amazed to see how your logic is optimized beyond recognition. What you write is not always what runs, and what runs is often much better version of what you imagined.

Harshitha Popuri on JAVAPRO International

the runtime illusion: what Java really runs

Introduction