Execution order of free blocks in Java

Author：Eve Cole Update Time：2024-11-19 12:54:01

Free blocks in Java are divided into static free blocks and non-static free blocks.
The execution time of non-static free blocks is: before executing the constructor.
The execution time of static free blocks is: executed when the class file is loaded.
Non-static free blocks can be executed multiple times and will be executed as long as an object is initialized, but static free blocks will only be executed once when the class is loaded and are generally used to initialize the values of static variables of the class.
Each time an object is initialized, a non-static block is executed.
If inheritance is involved, it is: first execute the non-static block of the parent class, then the constructor of the parent class, then its own free block, and finally its own constructor.

The execution timing of the static block is when the class file is loaded. The class file will only be loaded once, so the static block will only be executed once. When this class is used later, the static block will not be executed again.
The execution time of static blocks is during the initialization phase after class loading. If you use ClassLoader's loadclass to only load the class without initializing it, the execution of the static block will not be triggered. Using Class's forname (String) uses the default initialize as true, which is initialization. If you use forname(String name, boolean initialize, ClassLoader loader) and set initialize to false, the static block will not be executed.
The initialization phase after executing class loading includes: running the <clinit> method, which contains the initialization statement of the class variable and the static free block statement. This method is generated by the Java compiler after collecting information and cannot be called explicitly.

The following is illustrated by examples:

parent class

Copy the code code as follows:

father.java

public class father {

static{//static block
System.out.println("father'sSTATIC free block running");
}

{//Non-static block
System.out.println("father'sfree block running");
}

public father(){

System.out.println("father's constructor running");

}
}

Subclass

Copy the code code as follows:

son.java
public class son extends father{
static{//static block
System.out.println("son'sSTATIC free block running");
}

{//Non-static block
System.out.println("son's freeblock running");
}

public son() {
// TODO Auto-generated constructor stub
System.out.println("son's constructor running");
}
}

The class where the main function is located

Copy the code code as follows:

test.java

public class test{

public static void main(String[] args) {

Class f;

try {

System.out.println("--------beforeload father--------");

f=Class.forName("freeblock.father");

System.out.println("--------afterload father---------");

System.out.println("--------beforeinitial father object--------");

f.newInstance();

System.out.println("--------afterinitial father object--------");

} catch (ClassNotFoundException e) {

e.printStackTrace();

} catch (InstantiationException e) {
e.printStackTrace();

} catch (IllegalAccessException e) {

e.printStackTrace();
}
Class s;

try {

System.out.println("-------beforeload son--------");

s=Class.forName("freeblock.son");

System.out.println("--------afterload son--------");

System.out.println("--------beforeinitial son object----------");

s.newInstance();

System.out.println("--------afterinitial son object-----------");

} catch (ClassNotFoundException e) {
e.printStackTrace();
} catch (InstantiationException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
}
}
}

Execution result:

--------before loadfather--------

father's STATIC free blockrunning

--------after loadfather---------

--------before initial fatherobject--------

father's free block running

father's constructor running

--------after initial fatherobject--------

-------before load son--------

son's STATIC free block running

--------after load son-------

--------before initial sonobject---------

father's free block running

father's constructor running

son's free block running

son's constructor running

--------after initial son object----------