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Could you give me some information on what is exactly stored in object header? I know, that it's probably JVM dependent, but maybe for HotSpot at least? I'm looking for exact description specifically for a first row.
I've read several information that I can't verify positively with information I find. Maybe you have a link to OpenJDK wiki that says it all?
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A stack and a heap are used for memory allocation in Java. However, the stack is used for primitive data types, temporary variables, object addresses etc. The heap is used for storing objects in memory.
Object memory is a type of working memory that is not motivated by food rewards or aversive environments but by the natural tendency of mammals to investigate novel objects or familiar objects in novel locations (Ennaceur et al., 1997).
A memory layout can be used to describe the contents of a memory segment in a language neutral fashion.
Heap space is used for the dynamic memory allocation of Java objects and JRE classes at runtime. New objects are always created in heap space, and the references to these objects are stored in stack memory. These objects have global access and we can access them from anywhere in the application.
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.
Biased Locking 2 through which HotSpot can implement efficient locking.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.
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You can find the object layout from HotSpot sources.
The header consists of markOop followed by a pointer (or compressed pointer) to instanceKlass.
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