[JVM] detailed description of the process of creating objects

Preface

Development process , We must often create an object , Don't say you don't know , To create an object, you should first think of new, and you? new It's cool , But you know you are so new once , How did you create this object ？ Do you know the principle ？ It doesn't matter if I don't know , Next, let's explain it in combination with the figure

Detailed steps

 

 1、JVM Received our new Instructions

First, I will check the quilt new Whether this parameter of can locate the symbolic reference of this class in the constant pool , And also check whether the class referenced by this symbol has been loaded 、 analysis 、 initialization

2、 Check whether the current class has been initialized

If this class has been initialized , Then proceed to the following serial number 3 Steps for , Otherwise, you need to load the class first

3、 Allocate memory

After the class load verification passes , Then the memory allocation of new objects will be carried out

4、 Confirm the memory size

The memory size required for object generation can be determined when the class is loaded , Allocating space for an object is equivalent to transferring a certain amount of memory from Java Divide it up in the heap

5、 Allocated memory size

There are two main situations

(1)JVM The memory is regular , All memory used is put aside , The free memory is on the other side , Put a pointer in the middle as an indicator of the dividing point , In this case , It is easier to allocate memory , At that time, you just need to put the pointer 2 Point to the free memory and move a distance equal to the size of the object , This is what is often said “ Pointer collision ”

(2)JVM Memory of is irregular , It is equivalent to that the used memory and unused memory are staggered with each other , In this case, there is no way to achieve pointer collision , So what should we do ？ Only one table needs to be maintained , To record the information available in memory , When allocating, find enough space in the list to allocate to the object instance and update the memory state in the record table

6、 initialization

After the above memory space is allocated ,JVM You need to initialize the memory space to 0, There are also steps in the picture TLAB, If you use TLAB, You can go to TLAB The initialization can be carried out during allocation

7、 Basic information settings

JVM Make the necessary Settings for the object , For example, hash code 、 Memory address and other information

8、 Generate executable objects

Here we need to execute init Method , Initialize the object according to the initialization operation set in the program , At this time, an object is really initialized , Created a virtual machine executable java type