JVM memory and garbage collection-3-object instantiation and memory layout

Object instantiation

Mind mapping

• processon Mind mapping

Example

Code

package xcrj;

/* *  Object instantiation process  * */
public class VMObjCreation {
    
    public static void main(String[] args) {
    
        Object obj = new Object();
    }
}

command

#  compile 
javac -d D:\workspace\idea\JVMDemo\blog\target\classes\ -encoding UTF-8 VMObjCreation.java
#  Decompile 
javap -classpath D:\workspace\idea\JVMDemo\blog\target\classes\ -v xcrj.VMObjCreation

result - Decompile

Introduce

• new #2：#2 Symbol reference ; Judge whether the type is loaded , In the method area Class Object instances ;new Open up storage space in the heap ;new Member properties are initialized by default
• dup：duplicate OS Top 1 individual byte, Re press into stack ;OS Store object references （int value ）,OS The object reference in refers to the object entity in the heap
• invokespecial #1： Call the object constructor <init>
• astore_1： take OS The object reference at the top of the stack （int value ） Put it in LV Medium slot 1 The location of

Object memory layout

Object content

• Runtime metadata
• Type a pointer
• The instance data （ Parent instance data + Own instance data ）
• Alignment filling

Mind mapping

• processon Mind mapping

Example

Code

package xcrj;

public class VMObjLayout {
    
    private int age = 20;
    private String name;
    private Xcrj xcrj;

    {
    
        name = "xcrj";
    }

    public VMObjLayout() {
    
        xcrj = new Xcrj();
    }
}

class Xcrj {
    }

command

#  compile 
javac -d D:\workspace\idea\JVMDemo\blog\target\classes\ -encoding UTF-8 VMObjLayout.java
#  Decompile 
javap -classpath D:\workspace\idea\JVMDemo\blog\target\classes\ -v xcrj.VMObjLayout

result - Decompile

Memory layout

• jdk7=+ String constant table and static variables are put in the heap