javademos/object-headers/src/HeaderDemo.java

import java.util.ArrayList;
import java.util.List;
import org.openjdk.jol.info.ClassLayout;
import org.openjdk.jol.info.GraphLayout;
import org.openjdk.jol.vm.VM;

/**
 * JEP 534 seen from the inside: how many bytes does each object spend on its header, and what
 * does that add up to across a million objects?
 *
 * <p>Uses JOL 0.17 ({@code org.openjdk.jol:jol-core}). Explained in docs/03-compact-headers.md.
 * Run by scripts/object-headers.sh, once per JDK.
 */
public class HeaderDemo {

    /** Two ints. The smallest object that is not empty. */
    static class Point {
        int x;
        int y;
    }

    /** A more typical mix: a long, an int, a boolean and a reference. */
    static class Order {
        long id;
        int quantity;
        boolean paid;
        String customer;
    }

    public static void main(String[] args) {
        System.out.println("java.version = " + System.getProperty("java.version"));
        System.out.println(VM.current().details());

        section("java.lang.Object");
        System.out.println(ClassLayout.parseClass(Object.class).toPrintable());

        section("Point { int x; int y; }");
        System.out.println(ClassLayout.parseClass(Point.class).toPrintable());

        section("Order { long id; int quantity; boolean paid; String customer; }");
        System.out.println(ClassLayout.parseClass(Order.class).toPrintable());

        section("java.lang.Long and java.lang.Integer (boxed values)");
        System.out.println(ClassLayout.parseInstance(Long.valueOf(123_456_789L)).toPrintable());
        System.out.println(ClassLayout.parseInstance(Integer.valueOf(123_456)).toPrintable());

        section("int[3]");
        System.out.println(ClassLayout.parseInstance(new int[3]).toPrintable());

        section("What it adds up to: one million objects");
        List<Integer> boxed = new ArrayList<>();
        List<Long> longs = new ArrayList<>();
        List<Point> points = new ArrayList<>();
        for (int i = 0; i < 1_000_000; i++) {
            boxed.add(1_000 + i);          // above the Integer cache, so every one is a real object
            longs.add(10_000_000_000L + i);
            Point p = new Point();
            p.x = i;
            p.y = -i;
            points.add(p);
        }
        System.out.printf("1,000,000 boxed Integer values in an ArrayList : %,d bytes%n",
                GraphLayout.parseInstance(boxed).totalSize());
        System.out.printf("1,000,000 boxed Long values in an ArrayList    : %,d bytes%n",
                GraphLayout.parseInstance(longs).totalSize());
        System.out.printf("1,000,000 Point objects in an ArrayList        : %,d bytes%n",
                GraphLayout.parseInstance(points).totalSize());
    }

    static void section(String title) {
        System.out.println();
        System.out.println("=== " + title);
    }
}