Jvm-- loaded class, the connection initialization

java is a static language (first compiled, then executed), but it also has many of the characteristics of dynamic languages. java class loading, is connected, during operation of the program initialization is completed.

• Load: loads the class files compiled into memory. There are loads of ways

1. Load class files directly from the local system
2. Load class files remotely over the network
3. Load class files from the zip, jar in
4. Dynamically loaded into class files, with reference to the source file during program run java dynamic proxy design pattern

• Connection: connection divided into three stages

1. Verification: Verify that the class files that match jvm specification as class files can be artificially tampered with.
2. Preparation: static variables of the class allocates memory space, and assign default values. (Such as the default value of int is 0, the default value of the referenced object is null)
3. Analysis: Symbol reference into a direct reference.

• Initialization: static class variable assignment; performing static class code block static {}

Initialization scene class: class when active references will perform initialization, the following scenario is an active reference to the class

1. Create an instance of the class, create an object that is
2. Static variables to access or modify the class
3. Static method call class
4. reflection
5. Initializing a subclass of class (will first initialize the parent class)

/**
 * Created by IntelliJ IDEA.
 * User: chenzhubing
 * Date: 2019/6/16
 */
public class ClassInitTest {
    public static void main(String[] args) {
        System.out.println(Child.str1);
    }
}

class Parent{
    static String str1 = "hello";
    static{
        System.out.println("parent static block.....");

    }

}
class Child extends Parent{
    static  String str2 = "world";
    static{
        System.out.println("child static block.......");
    }
}

Analysis:

1. Access Child.str1 performs initialization class, static code execution classes will block content
2. For static variables, only directly define the class of the variable will be initialized. So Child class does not perform the initialization
3. Output:

parent static block.....
hello

 

      

/**
 * Created by IntelliJ IDEA.
 * User: chenzhubing
 * Date: 2019/6/16
 */
public class ClassInitTest {
    public static void main(String[] args) {
        System.out.println(Child.str2);
    }
}

class Parent{
    static String str1 = "hello";
    static{
        System.out.println("parent static block.....");

    }

}
class Child extends Parent{
    static  String str2 = "world";
    static{
        System.out.println("child static block.......");
    }
}

Analysis:

1. Access Child.str2 executes initialization Child class, because it inherits Parent, Parent and therefore will first perform initialization
2. Output:

parent static block.....
child static block.......
world