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

First 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
  
  om.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

now without Compact object headers

now try

./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 :

PreviousJEP 404: Generational Shenandoah NextJEP 472 : Prepare to Restrict the Use of JNI

Last updated 13 days ago

java \ -XX:+UnlockExperimentalVMOptions \ -XX:+UseCompactObjectHeaders \ -cp "target/classes:target/dependency/*" \ com.wlodar.jeeps.jep450CompactHeaders.CompactHeadersDemo om.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

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"); }