A brief introduction to the Class class and the ClassLoader class

Class in reflection mechanism

    What is inside the Class? Check out the picture below! 

Code: 
　　Class cls=Person.class;


 1. Class class:
    1. The information that the object can get after looking in the mirror: the data member name, method and constructor of a certain class, and the realization of a certain class
       which interfaces. For each class, the JRE keeps an immutable object of type Class for it. A Class object contains specific information about a class.

    2. Class objects can only be created by the system.

    3. A class has only one instance of Class in the JVM.

    4. An instance of each class will remember which Class instance it was generated by.

    5, Class is essentially a class! Is a class used to describe the internal information of the specified class itself!

2. Get the Class object [three methods]

    1. Directly pass: classname.class

            Class cls=Person.class;
    
    2. getClass() method

            Object obj=new Person();
            Class cls2=obj.getClass();

    3. Class.forName("package name.class name") [most commonly used] [most used by frameworks]
            try {
                Class cls3=Class.forName("cn.sgg.reflections.Person");
            } catch (ClassNotFoundException e) {
                e.printStackTrace ();
            }
3. Generics in reflection
    Example:
    Class<Person> cls3=(Class<Person>) Class.forName("cn.sgg.reflections.Person");
                
    Class<?> cls3=(Class<Person>) Class.forName("cn.sgg.reflections.Person");

    //? represents any object type
                
4. Methods in the Class class

    1. The method of creating an instance of a class: newInstance()
        
        Class cls3=Class.forName("cn.sgg.reflections.Person");

        Object obj=cls3.newInstance();//Get the instance (through: no parameter constructor)

        Object obj=cls3.newInstance(new Class[]{String.class,int.class});//Get the instance (through: parameter constructor)
        
        Note: Generally speaking, if a class declares a parameterless constructor, it also declares a parameterized constructor! (The parameter is reserved for reflection!)

    2、Field<------>classType.getDeclaredFields();

    3、Method<----->getDeclaredMethod(getMethodName, new Class[]{});

    4、Constructor<------>getConstructor(new Class[]{int.class,String.class});

5. Where is the reflective one used?

    In the framework: 
　　　　　　Servlet, Strust2 and other basic frameworks...
        


6. Class Loader: ClassLoader
    
    illustrate:
        ClassLoader: The class loader is used to load the class (class) into the JVM. The JVM specification defines two types of class loaders.

        Boot class loader (bootstap)

        User-defined class loader.

        The JVM generates an initialization loader hierarchy consisting of 3 class loaders at runtime,

    As shown below:
    

    Example:
        1. //Get a system class loader
            ClassLoader classLoader=ClassLoader.getSystemClassLoader();
            System.out.println(classLoader);

        2. //Get a system class loader
            ClassLoader classLoader=ClassLoader.getSystemClassLoader();
            System.out.println("System Loader: "+classLoader);
        
        3. //Get the loader of the parent class of the system class loader
            ClassLoader parentLoader=classLoader.getParent();
            System.out.println("The parent class loader of the system loader: "+classLoader);
        
        4. //Get the parent class loader of the extension class loader
             classLoader=parentLoader.getParent();
            System.out.println("Extended class loader's parent class loader: "+classLoader);
        
        5. //Detect which class loader the current class is loaded by
            try {
                ClassLoader classLoader2=Class.forName("cn.sgg.reflections.TestReflect").getClassLoader();
                System.out.println(classLoader2);
            
            } catch (ClassNotFoundException e) {
                e.printStackTrace ();
            }
            
        6. //Detect which class loader is responsible for loading the Object class provided by JDK
            ClassLoader classLoader3;
            try {
                classLoader3 = Class.forName("java.lang.Object").getClassLoader();
                System.out.println(classLoader3);
            } catch (ClassNotFoundException e) {
                e.printStackTrace ();
            }

        7. An important method about class loader:

            The system class loader can load all the corresponding class files and other files under this project! /bin/xxx. file suffix
            
            Example:
            //An important method about the class loader:
               InputStream in=TestReflect.class.getResourceAsStream("xxxx.txt");

 

 

Test code:

package cn.sgg.reflections;

/**
 * Reflection - tested class:
 */
public class Person {
    String name;    
    private int age;    
    
    public Person(){}    
    public Person(String name){
        this.name=name;
    }
    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
}

 

 

package cn.sgg.reflections;

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.InputStream;
import java.lang.reflect.Field;

/**
 * Reflection-test class:
 *
 */
public class TestReflect {

    public static void main(String[] args) {
        new TestReflect().testClassLoader();
    }
    
    /**
     * How to test reflection
     */
    static void  getProperty(){
        
        // 1. Get the Class object Class 
            cls=Person.class ;    
            
            Field[] fields = cls.getDeclaredFields();
         // 2. Get the Class object     
            Object obj= new Person();
            Class cls2 = obj.getClass();
         // 3. Get the Class object     
            try {
                Class<?> cls3=(Class<Person>) Class.forName("cn.sgg.reflections.Person");
                
                // Get the class instance 
                Object obj1= cls3.newInstance();
                
            } catch (Exception e) {
                e.printStackTrace ();
            }
    }
    
    
    static void testClassLoader(){
        
        // Get a system class loader 
        ClassLoader classLoader= ClassLoader.getSystemClassLoader();
        System.out.println( "System Loader: "+ classLoader);
        
        // Get the loader of the parent class of the system class loader 
        ClassLoader parentLoader= classLoader.getParent();
        System.out.println( "The parent class loader of the system loader: "+ classLoader);
        
        // Get the parent class loader of the extended class loader 
         classLoader= parentLoader.getParent();
        System.out.println( "Extended class loader's parent class loader: "+ classLoader);
        
        // Detect which class loader the current class is loaded by 
        try {
            ClassLoader classLoader2=Class.forName("cn.sgg.reflections.TestReflect").getClassLoader();
            System.out.println(classLoader2);
        
        } catch (ClassNotFoundException e) {
            e.printStackTrace ();
        }
        
        // Detect which class loader is responsible for loading the Object class provided by JDK 
        classLoader classLoader3;
         try {
            classLoader3 = Class.forName("java.lang.Object").getClassLoader();
            System.out.println(classLoader3);
        } catch (ClassNotFoundException e) {
            e.printStackTrace ();
        }
        
        // An important method about the class loader: 
        InputStream in= null ;
        in=TestReflect.class.getResourceAsStream("xxxx.txt");
    }
}

 

 

System Loader: sun.misc.Launcher$AppClassLoader@18d107f
The parent class loader of the system loader: sun.misc.Launcher$AppClassLoader@18d107f
The parent class loader of the extended class loader: null
sun.misc.Launcher$AppClassLoader@18d107f
null 
to get the Liu object of the picture: null