Week 9 - Object Layout, Headers, and Cache Effects¶

Conceptual Core¶

A Java object is not free. Every object has a header (mark word + class pointer), every reference is a pointer, and the layout interacts directly with CPU cache lines. "Compact" data structures in Java require understanding what HotSpot does to your fields.

Mechanical Detail¶

Object header: mark word (8 bytes, holds lock state, identity hash, GC age, biased-locking bits in older JVMs) + klass pointer (typically 4 bytes with compressed oops). JEP 450 (Compact Object Headers), stable in 24+: reduces header to 8 bytes total. Material for any heap-sizing math.
Field reordering: HotSpot reorders fields for alignment. Use JOL (Java Object Layout) - java -jar jol-cli.jar internals com.foo.MyClass - to see the actual layout.
Compressed oops (-XX:+UseCompressedOops, default ≤32GB heap). What happens above 32GB and how -XX:ObjectAlignmentInBytes extends the range.
Cache lines (typically 64 bytes). False sharing between adjacent fields written by different threads. @Contended (JDK-internal in jdk.internal.vm.annotation, requires -XX:-RestrictContended).
Primitive arrays are contiguous and dense; object arrays are arrays of pointers. Why int[] is 4× smaller than Integer[].

Lab¶

Use JOL to measure the size of: an empty object, a String, a HashMap with 0/1/10/100 entries, an ArrayList vs LinkedList of 1000 Integers. Predict each before measuring.

Idiomatic Drill¶

Read Aleksey Shipilëv's "Java Object Header" series.

Production Hardening Slice¶

Enable compact object headers (-XX:+UseCompactObjectHeaders if on 24+). Measure heap usage delta on a real app.