Compact object headers

Code Comparison

// Default: 128-bit object header
// = 16 bytes overhead per object
// A boolean field object = 32 bytes!
// Mark word (64) + Klass pointer (64)

// -XX:+UseCompactObjectHeaders
// 64-bit object header
// = 8 bytes overhead per object
// 50% less header memory
// More objects fit in cache

Why the modern way wins

📦

50% smaller headers

8 bytes instead of 16 per object.

⚡

Better cache usage

More objects fit in CPU cache lines.

📊

Higher density

Fit more objects in the same heap size.

Old Approach

128-bit Headers

Modern Approach

64-bit Headers

JDK Support

Compact object headers

Available

Finalized in JDK 25 LTS (JEP 519, Sept 2025).

How it works

Compact object headers reduce the per-object overhead from 128 bits to 64 bits on 64-bit platforms. This saves memory and improves cache utilization, especially for applications with many small objects.