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Tooling Advanced

Compact object headers

Cut object header size in half for better memory density and cache usage.

Code Comparison

✕ Java 8

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

✓ Java 25

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

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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

Since JDK

Difficulty

Advanced

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.

JShell for prototyping

Java 8

// 1. Create Test.java
// 2. javac Test.java
// 3. java Test
// Just to test one expression!

Java 9+

$ jshell
jshell> "hello".chars().count()
$1 ==> 5
jshell> List.of(1,2,3).reversed()
$2 ==> [3, 2, 1]

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JDK 9+ →

Tooling Beginner

Single-file execution

Java 8

$ javac HelloWorld.java
$ java HelloWorld
// Two steps every time

Java 11+

$ java HelloWorld.java
// Compiles and runs in one step
// Also works with shebangs:
#!/usr/bin/java --source 25

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JDK 11+ →

Tooling Intermediate

Multi-file source launcher

Java 8

$ javac *.java
$ java Main
// Must compile all files first
// Need a build tool for dependencies

Java 22+

$ java Main.java
// Automatically finds and compiles
// other source files referenced
// by Main.java

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JDK 22+ →