In depth study of JVM bottom layer (II): hotspot virtual machine object

HotSpot Virtual machine objects

Object creation

Main process ：

1. Determine whether the class corresponding to the object is loaded 、 link 、 initialization ： The virtual machine received a new When the command , First, I will check whether the specified parameter can be in Constant pool To locate a symbolic reference to a class , And check the class represented by this symbol reference Whether it has been loaded by the class loader 、 Linked and initialized . If not, execute the corresponding class loading process first .

   

2. Allocate memory for objects ： Choose which allocation method is by Java Pile up Whether it is regular or not Of , and Java Whether the heap is regular or not depends on whether the garbage collector is equipped with compression and sorting function

   • Pointer collision ： If Java The memory of the heap is regular , That is, all the used memory is put aside , And the free ones are on the other side . When allocating memory, move the pointer indicator in the middle to the free memory by a distance equal to the size of the object , This completes the allocation of memory .
   • Free list ： If Java Heap memory is not regular , The virtual machine needs to maintain a list to record what memory is available , In this way, when allocating, you can query the list to allocate enough memory to the object , And update the list record after allocation .

   

3. Dealing with concurrency security issues

   • Synchronize actions to allocate memory space （ use CAS + Failed retries to guarantee atomicity of update operation ）;

   • The action of memory allocation is divided into different spaces according to threads , That is, each thread Java A small chunk of memory is pre-allocated in the heap , Called local thread allocation buffer （Thread Local Allocation Buffer, TLAB）. Which thread will allocate memory , On which thread TLAB The distribution of . Only TLAB Use up and assign new TLAB when , Need synchronization lock . adopt -XX:+/-UserTLAB Parameter to set whether the virtual machine uses TLAB.

   

4. Initialize the allocated memory space ： After memory allocation , The virtual machine should initialize the allocated memory space to zero （ Object headers are not included ）. If used TLAB, This step will arrive ahead of time TLAB On assignment . This step ensures that the instance fields of the object are in Java Code can be used directly without initial value .

5. Set the object header of the object ： Set object header （Object Header） Information , Include the class of the object 、 Object's HashCode And objects GC Data such as generation age is stored in the object header of the object .

6. perform init Method to initialize ： perform init Method , Initialize the member variables of the object 、 Call the constructor of the class , So an object is created

Memory layout of objects

HotSpot In the virtual machine , The layout of objects stored in memory can be divided into three areas ： Object head （Header）、 The instance data （Instance Data） and Alignment filling （Padding）.

Object head

stay HotSpot In the virtual machine , The object header consists of two parts of information ： Runtime data and Type a pointer , If it's an array object , There is also a space to hold the length of the array .

• Mark Word（ Runtime data ）： Used to store the data of the object's own runtime , Such as hash code （hashCode）、GC Sub band age 、 A lock held by a thread 、 To the thread ID Etc . stay 32 Bit system occupies 4 byte , stay 64 In the bit system 8 byte ;
• Class Pointer（ Type a pointer ）： Used to point to the corresponding object Class object （ Its corresponding metadata object ） Memory address of . stay 32 Bit system occupies 4 byte , stay 64 In the bit system 8 byte ;
• Length： If it's an array object , There is also a space to hold the length of the array , Occupy 4 Bytes

The instance data

The instance data It's the object that actually stores Effective information , Whether from Parent inheritance It's still this kind of thing Self , It needs to be recorded , And this part of Storage order By the virtual machine Allocation policy and The order of definition Influence .

The allocation policy is always in the order of byte size from large to small , Put the same byte size together .

Alignment filling

No special meaning , It's not necessary , Placeholder only .

HotSpot The virtual machine requires that each object be Initial address Must be 8 Integer multiples of bytes , It has to be the size of the object 8 Bytes of An integral multiple . and Object header Is precisely 8 Multiple of bytes （32 Position as 1 times ,64 Position as 2 times ）, therefore , When the object instance data part is not aligned , It needs to pass Alignment filling To make up for .

Object access location

Java The program needs to go through JVM References on the stack access specific objects in the heap . How objects are accessed depends on JVM The realization of virtual machine . At present, the mainstream access methods are Handle and Direct Pointers Two ways

Handle access

Java The heap is divided into a block of memory as Handle to the pool , Of the object stored in the reference Handle address , And the handle contains Object instance data And Object type data Respective Specific address Information , The specific structure is shown in the figure below ：

advantage ： What is stored in the reference is Stable Handle address of , When the object is moved （ Moving objects during garbage collection is a very common behavior ） It just changes In handle Of Instance data pointer , and quote There is no need to modify itself .

Direct Pointers

If you use Direct Pointers visit , quote The direct storage in is Object address , that Java The layout inside the heap object must consider how to place access Type data Information about .

advantage ： Speed faster , Saved Once the pointer is positioned Time cost of . Because the access of the object Java Very often , As a result, this kind of overhead can add up to a considerable execution cost .HotSpot That's how it works .