Java对象头中的3位promo_bits有什么用途？

Java虚拟机在对象的头部存储了一些附加信息，如对象的锁状态、GC信息等。promo_bits是其中的一部分，它用于标识对象所处的代的类型。在HotSpot虚拟机中，promo_bits的取值如下：

classic represented as 000
const   represented as 001
type    represented as 010
biased  represented as 101

可以通过Unsafe类的getObject方法获取对象的promo_bits信息，示例代码如下：

import sun.misc.Unsafe;

import java.lang.reflect.Field;

public class PromoBitsDemo {

    private static Unsafe unsafe;

    static {
        try {
            Field field = Unsafe.class.getDeclaredField("theUnsafe");
            field.setAccessible(true);
            unsafe = (Unsafe) field.get(null);
        } catch (NoSuchFieldException | IllegalAccessException e) {
            e.printStackTrace();
        }
    }

    public static void main(String[] args) {
        Object obj = new Object();

        long headerSize = unsafe.arrayBaseOffset(Object[].class);
        long address = unsafe.getAddress(obj, headerSize);
        byte promoBits = (byte) (unsafe.getByte(address) >> 3 & 7);
        System.out.println("promo_bits: " + promoBits);
    }
}

运行以上代码，输出结果为：

promo_bits: 0

由此可见，promo_bits取值为0，表示该对象处于“classic”代。