Talk about the parental delegation mechanism in detail (often asked in interviews) [easy to understand]

Hello everyone , I meet you again , I'm your friend, Quan Jun .

Preface

Java Virtual machines are right class The document uses Load on demand The way , That is to say, the class will be used only when it is needed class File loading into memory generation class object , and , Load the of a class class When you file ,Java The virtual machine uses Parent delegate mechanism , namely Leave the request to the parent class , It's a task delegation model

working principle

（1） If a class loader receives a class load request , It doesn't load itself first , Instead, the request is delegated to the loader of the parent class

（2） If the parent loader still has its parent loader , Then further entrust , Recursion in turn , The request will eventually reach the top-level boot class loader ;

（3） If the parent loader can complete the class loading task , You're back , If the parent loader cannot complete the loading task , Sub loader will try to load by itself , This is the parent delegation mechanism

（4） The parent loader allocates tasks one layer at a time , If the subclass loader can load , Then load this class , If the load task is assigned to the system class, the loader cannot load this class , Throw an exception

Code example

give an example 1 ：

I built one myself java.lang.String class , write static Code block

package java.lang;

public class String { 
   
    static{ 
   
        System.out.println(" I'm custom String Static code block of class ");
    }
}

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Load... In another program String class , Look at the loaded String Class is JDK Self contained String class , We wrote it ourselves String class

public class StringTest { 
   

    public static void main(String[] args) { 
   
        java.lang.String str = new java.lang.String();
        System.out.println("hello,atguigu.com");

        StringTest test = new StringTest();
        System.out.println(test.getClass().getClassLoader());
    }
}

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The program doesn't output the contents of our static code block , So what's still loaded is JDK Self contained String class

Why? ? Because we define String Class loader of this application system , But it doesn't load itself first , Instead, the request is delegated to the loader of the parent class , To the extension class loader discovery String Class is not under your own control , Then delegate to the parent class loader （ Boot class loader ）, It turns out that java.lang package , This belongs to the boot loader , So what's loaded is JDK Self contained String class

give an example 2 ：

In our own String Entire in class main() Method

package java.lang;

public class String { 
   
    static{ 
   
        System.out.println(" I'm custom String Static code block of class ");
    }
    // error :  In the class  java.lang.String  I can't find it in China.  main  Method 
    public static void main(String[] args) { 
   
        System.out.println("hello,String");
    }
}

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Because the parent delegation mechanism finds JDK Self contained String class , But in the core class library of the boot class loader API Inside String There is no main() Method

give an example 3：

stay java.lang Wrap the whole ShkStart class ( Custom class name )

package java.lang;

public class ShkStart {
    public static void main(String[] args) {
        System.out.println("hello!");
    }
}

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Out of the protection mechanism ,java.lang We are not allowed to customize classes under the package

The advantages of parental appointment mechanism

Through the example above , We can know , The parental mechanism can

• Avoid duplicate loading of classes
• Protect program security , Prevention core API Be tampered with at will
  • Custom class ：java.lang.String ( useless )
  • Custom class ：java.lang.ShkStart（ Report errors ： Block creation java.lang Initial class ）

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