JVM architecture, classloader, parental delegation mechanism

1.JVM Architecture of

2. Class loader

1. The loader of the virtual machine

2. Start class （ root ） loader

3. Extend the classloader

4. Application Loader

public class Car {
    
    public static void main(String[] args) {
    
        // Class is a template , Object is specific , Store in a pile 
        Car car1 = new Car();
        Car car2 = new Car();
        Car car3 = new Car();

        System.out.println(car1.hashCode());
        System.out.println(car2.hashCode());
        System.out.println(car3.hashCode());

        Class<? extends Car> aClass1=car1.getClass();
        Class<? extends Car> aClass2=car2.getClass();
        Class<? extends Car> aClass3=car3.getClass();

        System.out.println(aClass1.hashCode());
        System.out.println(aClass2.hashCode());
        System.out.println(aClass3.hashCode());
        
        
        ClassLoader classLoader = aClass1.getClassLoader();
        System.out.println(classLoader);//AppClassLoader
        System.out.println(classLoader.getParent());//ExtClassLoader
        System.out.println(classLoader.getParent().getParent());//null 1. non-existent  2.java The program cannot get （c file ）rt.jar
    }
}

 The operation effect is as follows ：
356573597
1735600054
21685669
1956725890
1956725890
1956725890
sun.misc.Launcher$AppClassLoader@18b4aac2
sun.misc.Launcher$ExtClassLoader@7f31245a
null

3. Parent delegate mechanism

effect ：（1） Prevent malicious code from interfering with well meaning code ;

​ （2） Guard the trusted class library boundary ;

/* * 1. Class loader received a class load request  * 2. Delegate this request up to the parent loader to complete , All the way up , Until you start the class loader  * 3. Start the class loader to check whether the current class can be loaded , If you can load it, it's over , Use current loader , otherwise , Throw an exception , Notify the subclass loader to load  * 4. Repeat step 3 * */

expand ：

Sandbox security mechanism ： Put the code in a protected domain , Determine what the code can do .

native： I've brought native Keywords , explain java It doesn't reach the scope of , Will call the underlying layer c The library of languages ！（ It goes to the local method stack , Call the local method interface JNI,（JNI effect ： Expand java Use , Integrate different programming languages into Java used ！））

Method area ： Static variables static, Constant final, Class information （ Construction method 、 Interface definition ）Class、 The constant pool of runtime exists in the method area , But instance variables exist in heap memory , It has nothing to do with the method area