JVM_07 runtime data area 4-object instantiated memory layout and access location

1. The instantiation of the object

1.

• new

  • The most common way
    Variation 1: Static method of Xxx
    Variation 2: Static method of XxBuilder/XxoxFactory

• Class's newInstance(): The reflection method can only call the constructor of the null parameter, and the permission must be public

• Constructor's newInstance (Xxx): in a reflective way, you can call the constructor with empty and parameterized parameters, and there is no requirement for permissions

• Use clone(): do not call any constructor, the current class needs to implement the Cloneable interface and implement clone()

• Use deserialization: get a binary stream of an object from a file and from the network

• Third-party library Objenesis

• Determine whether the class corresponding to the object is loaded, linked, initialized

• Allocate memory for the object

  • If the memory is regular and a pointer collides

  • If the memory is irregular:

    • The virtual machine needs to maintain a list
    • Free list allocation

• Dealing with concurrency security issues

  • Use CAS with failed retry to ensure update atomicity
  • Each thread is allocated a TLAB in advance

• Initialize the allocated space-all properties are set to default values ​​to ensure that the object instance fields can be used directly when they are not assigned.
  Set the object header of the object

• Execute the init method to initialize

2. Object memory layout

1. Header: Runtime metadata Markword and KlassInstance type pointers
2. Instance data InstanceData
3. Padding alignment and padding

public class CustomerTest {
    public static void main(String[] args) {
        Customer cust = new Customer();
    }
}

3. Access location of the object

How does the JVM access its internal object instances through the object references in the stack frame? -> Positioning, accessed through reference on the stack

1. Handle access

2. Direct pointer (adopted by HotSpot)