Annotation and reflection

Get the reflection object

package TestReflected;

public class Test02 {
    
    public static void main(String[] args) throws ClassNotFoundException {
    

        Person person = new Student();
        System.out.println(" This man is "+person.name);

        // Mode one 
        Class C1=person.getClass();
        System.out.println(C1.hashCode());

        // Mode two 
        Class C2 = Class.forName("TestReflected.Test02");
        System.out.println(C2.hashCode());

        // Mode three : By class name .class get 
        Class C3=Test02.class;
        System.out.println(C3.hashCode());

        // Mode 4 
        Class C4 = Integer.TYPE;
        System.out.println(C4);

        // Get the parent type 
        Class C6 = C1.getSuperclass();
        System.out.println(C6);
    }
    }
    class Person{
    
        String name;
        public Person() {
    
        }

        public Person(String name) {
    
            this.name = name;
        }

        @Override
        public String toString() {
    
            return "Person{" +
                    "name='" + name + '\'' +
                    '}';
        }
    }
    class Student extends Person{
    
        public Student() {
    
            this.name=" Student ";
        }
    }
    class Teacher extends Person{
    
        public Teacher() {
    
            this.name=" teacher ";
        }
    }

All types of Class object

package TestReflected;

import java.lang.annotation.Annotation;
import java.lang.annotation.ElementType;

public class Test03 {
    
    public static void main(String[] args) {
    
        Class C1=Object.class;  // class 
        Class C2=Comparable.class;  // Interface 
        Class C3=String[].class;
        Class C4=int[][].class;
        Class C5= Annotation.class;
        Class C6= ElementType.class;  // Enumeration type 
        Class C7=Integer.class;  // Basic data type 
        Class C8 =void.class;  //void
        Class C9=Class.class;

        System.out.println(C1);
        System.out.println(C2);
        System.out.println(C3);
        System.out.println(C4);
        System.out.println(C5);
        System.out.println(C6);
        System.out.println(C7);
        System.out.println(C8);
        System.out.println(C9);

        // As long as the element type is the same as the dimension , It's the same Class
        int[] a =new int[10];
        int[]b=new int[100];
        System.out.println(a.hashCode());
        System.out.println(b.hashCode());
    }
}

Class initialization

package TestReflected;
// When does the test class initialize 
public class Test04 {
    
    static {
    
        System.out.println("main Class loaded ");
    }
    public static void main(String[] args) throws ClassNotFoundException {
    
        //1, Active citation 
       // Son son =new Son();
        // main Class loaded 
        // The parent class is loaded 
        // Subclasses are loaded 

        // Reflection also produces active references 
     // Class.forName("TestReflected.Son");
        // main Class loaded 
        // The parent class is loaded 
        // Subclasses are loaded 

        // Methods that do not produce a reference to a class 
     // System.out.println(Son.b);
        //main Class loaded 
        // The parent class is loaded 
        //2
        
        // Array will not load class 
        Son[] array=new Son[5];
    }
}

class Father{
    
    static int b=2;
    static {
    
        System.out.println(" The parent class is loaded ");
    }
}
class Son extends Father{
    
    static {
    
        System.out.println(" Subclasses are loaded ");
        m=300;
    }
    static int m=100;
    static final int M=1;
}

Class loader

Class loading function ： take class File bytecode content loaded into memory , And convert these static data into the runtime data structure of the method area , Then generate a... Representing this class in the heap java.lang.Class object , As an access to the class data in the method area ;
Class cache ： The standard JavaSE The class loader can find classes as required , But once a class is loaded into the classloader , It will keep loading （ cache ） A span , however jvm The garbage collection mechanism can recycle these Class object ;

package TestReflected;

public class Test05 {
    
    public static void main(String[] args) throws Exception {
    

        // Get the loader of the system class 
        ClassLoader systemClassLoader = ClassLoader.getSystemClassLoader();
        System.out.println(systemClassLoader);

        // Get the system classloader's parent classloader --> Extend the classloader 
        ClassLoader parent=systemClassLoader.getParent();
        System.out.println(parent);

        // Get the parent loader of the extension class loader --> Root loader 
        ClassLoader parent1= parent.getParent();
        System.out.println(parent1);

        // Test which loader is loading the current class 
        ClassLoader classLoader= Class.forName("TestReflected.Test05").getClassLoader();
        System.out.println(classLoader);

        // Test which class the current class is loaded 
        ClassLoader classLoader2 =Class.forName("java.lang.Object").getClassLoader();
        System.out.println(classLoader2);

        // How to get the path that the system class loader can load 
        System.out.println(System.getProperty("java.class.path"));
    }
}

Get the runtime structure of the class