What is in Java object header?

The following presentation gives you a general idea of the object contents and the object header: http://www.slideshare.net/cnbailey/memory-efficient-java

The actual header for any object is JVM vendor, version and object type specific.

For HotSpot:

The object header consists of a mark word and a klass pointer.

The mark word has word size (4 byte on 32 bit architectures, 8 byte on 64 bit architectures) and

the klass pointer has word size on 32 bit architectures. On 64 bit architectures the klass pointer either has word size, but can also have 4 byte if the heap addresses can be encoded in these 4 bytes.

This optimization is called "compressed oops" and you can also control it with the option UseCompressedOops.

You can also find a wiki entry about this 1.

The mark word is actually used for many things.

1. One is Biased Locking 2 through which HotSpot can implement efficient locking.
2. It is also used during GC to set forward pointers, and to store the age of the objects. The identity hash code of an object can be stored inside the mark (the System.identityHashCode/Object.hashCode one).

There is a comment in the source code of markOop.hpp that describes the layout depending on the architecture:

//  32 bits:
//  --------
//             hash:25 ------------>| age:4    biased_lock:1 lock:2 (normal object)
//             JavaThread*:23 epoch:2 age:4    biased_lock:1 lock:2 (biased object)
//             size:32 ------------------------------------------>| (CMS free block)
//             PromotedObject*:29 ---------->| promo_bits:3 ----->| (CMS promoted object)
//
//  64 bits:
//  --------
//  unused:25 hash:31 -->| unused:1   age:4    biased_lock:1 lock:2 (normal object)
//  JavaThread*:54 epoch:2 unused:1   age:4    biased_lock:1 lock:2 (biased object)
//  PromotedObject*:61 --------------------->| promo_bits:3 ----->| (CMS promoted object)
//  size:64 ----------------------------------------------------->| (CMS free block)
//
//  unused:25 hash:31 -->| cms_free:1 age:4    biased_lock:1 lock:2 (COOPs && normal object)
//  JavaThread*:54 epoch:2 cms_free:1 age:4    biased_lock:1 lock:2 (COOPs && biased object)
//  narrowOop:32 unused:24 cms_free:1 unused:4 promo_bits:3 ----->| (COOPs && CMS promoted object)
//  unused:21 size:35 -->| cms_free:1 unused:7 ------------------>| (COOPs && CMS free block)

You can also find the oop header file here.

• 1 https://wiki.openjdk.java.net/display/HotSpot/CompressedOops
• 2 https://wiki.openjdk.java.net/display/HotSpot/Synchronization

You can find the object layout from HotSpot sources.

The header consists of markOop followed by a pointer (or compressed pointer) to instanceKlass.