Unveiling Java Object Headers: Inside the JVM Memory Layout

This article explains the three-part structure of a Java object in the HotSpot JVM—object header, instance data, and alignment padding—detailing the Mark Word and Klass Pointer fields, their bit‑level layout, and how they affect memory size, GC age, and locking behavior.

JVMJavaMark Word

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Unveiling Java Object Headers: Inside the JVM Memory Layout

Code Ape Tech Column

Sep 1, 2021 · Fundamentals

Understanding Java Object Memory Layout with JOL

This article explains how Java objects are stored in memory, covering the object header, instance data, alignment padding, lock states, pointer compression, and field reordering, and demonstrates how to inspect these structures using the OpenJDK JOL tool with practical code examples.

JOLJVMJava

0 likes · 20 min read

Understanding Java Object Memory Layout with JOL