Only on 32 bit IIRC, headers are 12 byte on 64 bit with CompressedOops (the default for heaps <32gb) and 16 bytes without. The bit layout of the markoop is here

Only on 32 bit IIRC, headers are 12 byte on 64 bit with CompressedOops (the default for heaps <32gb) and 16 bytes without. The bit layout of the markoop is here: http://hg.openjdk.java.net/jdk8/jdk8/hotspot/file/87ee5ee27509/src/share/vm/oops/markOop.hpp http://hg.openjdk.java.net/jdk8/jdk8/hotspot/file/87ee5ee275...