JEP 450 : Compact Object Headers i

What is JEP 450?

• In Java, every object has a header (used for synchronization, identity hashcode, GC, etc.)

• Historically, it was 96 bits (12 bytes) on 64-bit platforms

• With JEP 450, object headers are now optional and compact — down to 64 bits (8 bytes) when possible

Why does it matter?

• More efficient object layout

• Better cache locality

• Up to 5–10% memory savings in some apps

DEMO

Try to see class layout - attempt 1 - org.openjdk.jol

manual run :

mvn dependency:copy-dependencies

to copy org.openjdk.jol to traget to have it on classpath later

then run :

java \
  -XX:+UnlockExperimentalVMOptions \
  -XX:+UseCompactObjectHeaders \
  -cp "target/classes:target/dependency/*" \
  com.wlodar.jeeps.jep450CompactHeaders.CompactHeadersDemo
  
  com.wlodar.jeeps.jep450CompactHeaders.CompactHeadersDemo$SmallObject object internals:
OFF  SZ   TYPE DESCRIPTION               VALUE
  0   8        (object header: mark)     N/A
  8   4    int SmallObject.x             N/A
 12   4        (object alignment gap)    
Instance size: 16 bytes
Space losses: 0 bytes internal + 4 bytes external = 4 bytes total

SCRIPT RUN :

This presentation works on openjdk but on azul the same amount of memory is allocated

./scripts/jep450/run-compact.sh compact
./scripts/jep450/run-compact.sh classic

If the same output

JOL cannot tell if the JVM has used compact 
headers or not at runtime — it only prints memory layout 
assumptions based on class fields, alignment, and architecture.

Try to see class layout - attempt 2 - mass allocation

with method :

private static void allocateMemory(){
        WorkshopPrinter.subtitle("Allocating memory");
        List<Object> list = new ArrayList<>();
        for (int i = 0; i < 10_000_000; i++) {
            list.add(new SmallObject());
        }
        System.gc();
        WorkshopPrinter.print("Used memory: " , (Runtime.getRuntime().totalMemory() - Runtime.getRuntime().freeMemory()) / (1024 * 1024) + " MB");
    }

We have two different results :

./scripts/jep450/run-compact.sh compact
-----  Allocating memory
Used memory:  : 3 MB

-----  Allocating memory
Used memory:  : 4 MB

Education :

https://openjdk.org/jeps/450