Memory requirements of an Object reference on a 64 bit JVM

Question

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

Answer 1

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.

Answer 2

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.