Reflection

It is a process of examining or modifying the run time behavior of a class at run time. Packages used for reflection in java is java.lang and java.lang.reflect

Methods of Reflection,

• getName( )
• Class.forName( )
• isInterface( )
• isArray()
• isPrimitive()
• getSuperclass()
• getDeclaredFields()
• getDeclaredMethods()
• getDeclaredConstructors()
• newInstance()

Sample Program

package viki;

class Simple{

public void getmsg()

{

System.out.println("Hi this class is called by newInstance() method!!!!!");

}

 }

interface My{ }  

class Reflection{  

 public static void main(String args[]) throws ClassNotFoundException,  InstantiationException, IllegalAccessException{ 

 

 

  @SuppressWarnings("rawtypes")

  Class c=Class.forName("viki.Simple");

  Simple s=(Simple)c.newInstance();  

  s.getmsg();  

  System.out.println(c.getName());

  

  Class c3=int.class;

  System.out.println(c3.isPrimitive());

  System.out.println(c3.isArray());

  

  Simple s1=new Simple();

  System.out.println(s1.getClass());

  

  

  Class c2=Class.forName("viki.My");  

  System.out.println(c2.isInterface());

  

  Class c1=Reflection.class;

  System.out.println(c1.getName());

  

 }  

 }   

Output:

Hi this class is called by newInstance() method!!!!!

viki.Simple

true

false

class viki.Simple

true

viki.Reflection

Javap using Reflection:

Javap command displays information about methods,fields and constructors for the given class.This can be acheived in reflection through, 

• getDeclaredFields()
• getDeclaredMethods()
• getDeclaredConstructors()

these methods. The sample code follows,

   package viki;

   import java.lang.reflect.*;  

   public class Javaptool{  

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

    Class c=Class.forName("viki.Simple");

      

    System.out.println("Fields........");  

    Field f[]=c.getDeclaredFields(); 

    if(f.length>0)

    {

    for(int i=0;i<f.length;i++)  

        System.out.println(f[i]);  }

    else

    {

    System.out.println("There is no fields");  

    }

      

    System.out.println("Constructors........");  

    Constructor con[]=c.getDeclaredConstructors();  

    if(con.length>0)

    {

    for(int i=0;i<con.length;i++)  

        System.out.println(con[i]);  

    }

    else

    {

    System.out.println("There is no constructors");  

    }

        System.out.println("Methods........");  

    Method m[]=c.getDeclaredMethods();  

    if(m.length>0)

    {

    for(int i=0;i<m.length;i++)  

        System.out.println(m[i]);  

    

   }

   else{

  System.out.println("There is no methods");  

   }

   }  

   }

Output:

Fields........

There is no fields

Constructors........

viki.Simple()

Methods........

public void viki.Simple.getmsg()

AppletViewer using Reflection:

Using reflection it is possible to create our own applet frame.The sample code as follows,

    package viki;

    import java.applet.Applet;  

    import java.awt.Frame;  

    import java.awt.Graphics;  

  

    public class MyViewer extends Frame{  

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

    Class c=Class.forName("viki.MyViewer");  

          

    MyViewer v=new MyViewer();  

    v.setSize(400,400);  

    v.setLayout(null);  

    v.setVisible(true);  

          

    Applet a=(Applet)c.newInstance();  

    a.start();  

    Graphics g=v.getGraphics();  

    a.paint(g);  

    a.stop();  

          

    }  

  

    }  

Calling private method using reflection:

Normally ,It is not possible to call a java class which is declared as private from outside the class .But it can be achieved through reflection and example below,

package viki;

import java.lang.reflect.*;  

class Test{  

private void cube(int n){System.out.println(n*n*n);} 

private void sayHello(){System.out.println("Hi.......Hello.....");}

}

class CallingPrivateMethod{  

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

Class c=Test.class;  

Object obj=c.newInstance();  

  

Method m=c.getDeclaredMethod("cube",new Class[]{int.class});  

m.setAccessible(true);  

m.invoke(obj,4);  

Method m1=c.getDeclaredMethod("sayHello");

m1.setAccessible(true);

m1.invoke(obj, null);

}}  

Output:

64

Hi.......Hello.....