Object memory layout

One 、 brief introduction

stay HotSpot In the virtual machine , The storage layout of objects in heap memory can be divided into three parts : Object head (Header)、 The instance data （Instance Data） And align fill (Padding). It is filled to ensure that the object is 8 Multiple of bytes ！

The object head can be subdivided again , It is divided into Object tag （Mark Word） and Class meta information （ Type a pointer ）, For array objects, there will be one more object header Length part . stay 64 Bit system ,Mark Word Account for the 8 Bytes , Type pointer occupied 8 Bytes , Is the total 16 Bytes .

from openJDK Of oop.hpp You can see... In the source code :_mark The fields are mark word,_metadata It's a class pointer klass pointer, Object head （object header) That is, it consists of these two fields .

Two 、 The head of the object MarkWord

Hash code can be found 、GC Mark ,GC frequency , Synchronization lock flag , Biased lock holders are Mark Word Inside .
This information is data that has nothing to do with the definition of the object itself , therefore Mar