JVM advanced features-2. JVM allocation, layout, and access process of objects in the heap

After introducing the jvm runtime data area before, let’s explain other details of the data in the memory to see how they are created, laid out and accessed

　 1. Objects Creation

　1. Allocation of objects

Type:

Pointer collision and free list

　

Pointer collision

：　 Assuming that the heap memory is absolutely regular, then , when allocating memory space for a new object, you only need to move the pointer to the free space direction of the new object by a section of the required size.

Usually when using a collector with compact (compacting) process, pointer collision is used

　

Free list

:

If the memory is not regular, then you need to maintain a list to record which memory is free. When allocating space, find a piece of from the list that is large enough and divide it into Object instance and update list record

When using a collector based on the mark-sweep algorithm, use

free list

　

2. Initialization and setting of objects

　　　Initialization: After the memory allocation is completed, the object is initialized, and the virtual machine will initialize the memory space are all initialized to a value of 0,

　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　##　　　　This is why the instance field of the object does not need to be initialized in the java code and can also be used

　　　　

SET

: After that, the virtual machine performs operations on the object Some necessary settings are mainly to store the metadata information of the class, the hash value of the object, the generational age, etc. in the object header.

　　　　　　　　init

: After the above two steps, the object creation is completed, but all fields are still 0 values. The last step is to execute the init method and initialize the object according to the programmer's intention

After completing the above three steps, an object is created.

　

2. Memory layout of objects

The layout of objects stored in memory is mainly divided into three parts. Object Header, instance data, alignment padding

　　　　　　

Object header

：

　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　　Class metadata information, hash value, generation age, etc.

The object is mainly divided into two parts: runtime data and type pointer

　　　

Runtime data

: It mainly stores the hash code, generation information, lock status identification, locks held by threads, etc. stored in the object setting stage above.

　　　　　Type pointer

: A pointer that mainly stores the class metadata of the object, that is, which class the object is an instance of. In addition, if the object type is an array,

　　　Object header A data representing the data length will also be stored in it

　　　　　　

Alignment padding

: It does not necessarily exist. In Hotspot, the starting address of the object must be a multiple of 8 bytes,

　　　　　When the object instance part is not aligned, use alignment padding to complete it.

　

3. Object access positioning

The Java program operates the object instance through the reference data on the stack, so it is just a reference , the specific positioning method jvm has different implementations

There are two mainstream implementation methods: using handles and direct pointers

　　　Principle: An area will be set aside in the Java heap as a handle pool, and reference stores the handle address, which contains pointers to object instance data and object type pointers

Advantages: If the object is moved, you only need to change the address stored in the handle pool

Direct pointer

Principle: If you are using a direct pointer, then reference What is stored in is the address of the object

Advantages: It saves the time of positioning and is more efficient

Hotspot uses the direct pointer method

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