Memory requirements of an Object reference on a 64

问题:

A reference to an Object on a 32 bit JVM (at least on Hotspot) takes up 4 bytes.

Does the 64 bit Hotspot JVM need 8 bytes? Or is some clever compression going on? If not, every Object[] would require twice as much heap memory, which I somehow think (hope, expect) is not the case.

Update/extra question: Does this really matter, or is this a negligible increase, because most references point to objects that are much larger than a few bytes (whereas one might argue that those objects are in turn mostly comprised of references to other objects)?

回答1:

In a 64-bit system, object references are typically 8-byte long. But in recent JVMs from Sun/Oracle you can enable Compressed Oops, which reduce reference size to 4 bytes at the cost of a smaller limit on heap size.

回答2:

According to Java Platform Performance it is not strictly defined, but typically 8 bytes on a 64-bit system:

The size of a reference isn't well defined, but it is typically 4 bytes on a 32-bit system and 8 bytes on a 64-bit system.