Detailed explanation of Java class loading mechanism ClassLoder (picture and text)

What you see on paper is shallow, but you know you have to do it in detail --Lu You Ask the canal how clear it is so that there is a source of running water --Zhu Xi

The entire life of a class starts from being loaded into memory until it is unloaded from memory. Cycle includes: loading, verification, preparation, resolution, initialization, and using. , seven stages of unloading (Uploading). Where validation’ preparation and parsing is called linking.

1.

Loader class The loaded
architecture

1), Bootstrap

api# under java_home/lib ## Or the jar package specified by the -Xbootstrap option is included in the job. 2), Extension ClassLoader extension class loader, mainly responsible for the jar package under java_home/lib/ext 3), App CLassLoader system class loader, mainly responsible for Java -classpath/ The loading of classes and jar packages in the directory; 4), UserCustom ClassLoader user-defined class loader, during the running of the program, through the child of Java.

.Classloader Class dynamic loading class

2. Characteristics of class loading

1) Each ClassLoader will maintain its own namespace
, two identical class names cannot appear in the same namespace

2),

securitySandboxModel

3. There are three ways to load classes:

forName() method.

3), dynamic loading through the ClassLoader().loadClass() method.

static code blocks.

The following example:

public class MyHello {    static    {
        System.out.println("hello word");    }
}public class Hello {    public static void main(String[] args) {
        ClassLoader loader=Hello.class.getClassLoader();        
        try {            
        //静态代码块不运行            
        //loader.loadClass("Test.MyHello");            
        //静态代码块运行输出hello world            
        //Class.forName("Test.MyHello");            
        //静态代码块不运行            
        //Class.forName("Test.MyHello", false, loader);            
        //静态代码块运行输出hello world            
        Class.forName("Test.MyHello", true, loader);        
        } 
        catch (ClassNotFoundException e) {            
        // TODO 自动生成的 catch 块            
        e.printStackTrace();       
         }
    }
}

1. ClassLoader class loading architecture

1) Bootstrap Classloader starts the class loader, which is mainly responsible for loading the core api under java_home/lib or the jar package specified by the -Xbootstrap option.

2), Extension ClassLoader extension class loader, mainly responsible for the jar package under java_home/lib/ext

3), App CLassLoader system class loader, mainly responsible for Java -classpath/ The loading of classes and jar packages in the directory;

4), UserCustom ClassLoader user-defined class loader, during the running of the program, dynamically load classes through the subclasses of Java.lang.Classloader

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