Reflection : Javap Command Working With Example Java Program Code

Javap Command gives overview of the class file . It is also called java class disassembler.
It gives the information depending upon the options used by the user .

1. Programmer NOT providing any helper option with javap command

Results in javap command shows all the package,public and protected fields and methods of the classes passed . We can also make the similar program with the help of reflection . Reflection is one of the core features of java allowing access to the Class class object .

javap command example without using options  :

Compile the following class JavapExample

public class JavapExample {

  String firstname ;
  String lastname ;
  
  public  void printName(){
   System.out.println("Full name of the candidate is " + firstname + lastname);
  }
 
  public void printNameUSAformat(){
   System.out.println("Full name of the candidate in reverse is " + lastname + firstname);
  }
}

  
The output from javap  JavapExample.class  will be :

Compiled from "JavapExample.java"

public class JavapExample extends java.lang.Object {

  java.lang.String firstname;

  java.lang.String lastname;

  public JavapExample();

  public void printName();

  public void printNameUSAformat();

}

2. Other options Programmer has for the javap command are as follows :

 -help : it will print the help messages to the user

 -l :  it print out all the local and line variables of the table

 -public : it only prints the public members and classes

 -protected : it only prints the protected and public members and classes .

 -c : it print out the disassembled code

 -j : flag is directly pass to the runtime

 -s : outputs the internal type signature


javap command example using options  :

Compile the following class JavapExample

public class JavapExample {

  String firstname ;
  String lastname ;
  
  public  void printName(){
   System.out.println("Full name of the candidate is " + firstname + lastname);
  }
 
  public void printNameUSAformat(){
   System.out.println("Full name of the candidate in reverse is " + lastname + firstname);
  }
}

  
The output from javap -public JavapExample.class will  be :

Compiled from "JavapExample.java"

public class JavapExample extends java.lang.Object   {

  public JavapExample();

  public void printName();

  public void printNameUSAformat();

}

Here in below code we use forName() and other reflection methods ,

But the most important question arises

What is the difference between forName() method , Class.getClass() method . Why there are two different names as  both have almost same functionality .

Actually there is very minute difference , forName() loads and initializes the class while object.getClass() returns the class object loaded into the permgen space .

You can find the javap.exe  in the bin directory  of the jdk not in jre.
For java 1.6 the path is
  jdk1.6.0_20\bin\javap.exe

So in short we use  javap command to read the bytecodes .

For more info about javap :  http://docs.oracle.com/javase/1.5.0/docs/tooldocs/windows/javap.html



Demo for Java inbuilt functionality of  javap command when you type on console

After running this program the output will be look like this :

Code :

import java.lang.reflect.*;
import java.awt.*;
import java.io.*;

public class Javap
{
    public static void printStart(String s)
    {
        System.out.println("Compiled from  "+s );
        try
        {
            Class c=Class.forName(s);
            int m=c.getModifiers();
            printModifier(m);
            System.out.print(" class "+s+"  ");
        }
        catch(Exception e){}
    }
    
    public static void printModifier(int m)
    {
        if(Modifier.isPublic(m))
        System.out.print("public ");
        if(Modifier.isStatic(m))
        System.out.print("static ");
        if(Modifier.isAbstract(m))
        System.out.print("abstract ");
        if(Modifier.isFinal(m))
        System.out.print("final ");
    }
    
    public static void printSuperClass(String s)
    {
        try
        {
            Class subclass=Class.forName(s);
            Class superclass=subclass.getSuperclass();
            System.out.print("extends "+superclass.getName()+" ");
        }
        catch(Exception e){}
    }
    
    
    public static void printInterface(String s)
    {
        try
        {
            Class c=Class.forName(s);
            Class inter[]=c.getInterfaces();
            System.out.print("implements ");
            for(int i=0;i<inter.length;i++)
            {
                System.out.print(inter[i].getName());
                System.out.print(", ");
            }
            System.out.println(" ");
            System.out.println(" { ");
            }
            catch(Exception e)  {}
        }
        
        public static void printField(String s)
        {
            try
            {
                Class c=Class.forName(s);
                Field f[]=c.getFields();
                for(int i=0;i<f.length;i++)
                {
                    int m=f[i].getModifiers();
                    printModifier(m);
                    System.out.print(" ");
                    Class type=f[i].getType();
                    System.out.print(type.getName());
                    System.out.println(" "+f[i].getName());
                }
            }
            catch(Exception e)  {}
        }
        
        
        public static void printConstructor(String s)
        {
            try
            {
                Class c=Class.forName(s);
                Constructor cs[]=c.getConstructors();
                for(int i=0;i<cs.length;i++)
                {
                    int m=cs[i].getModifiers();
                    printModifier(m);
                    System.out.print (c.getName()+"(");
                    Class type[]=cs[i].getParameterTypes();
                    for(int k=0;k<type.length;k++)
                    {
                        System.out.print(type[k].getName()+",");
                    }
                    System.out.print(")");
                    System.out.println(" ");
                }
            }
            catch(Exception e){}
        }
        
        
        
        
        public static void printMethods (String s)
        {
            try
            {
                Class c=Class.forName(s);
                Method m[]=c.getMethods();
                for(int i=0;i<m.length;i++)
                {
                    int m1=m[i].getModifiers();
                    printModifier(m1);
                    System.out.print(m[i].getReturnType().getName());
                    System.out.print(" "+m[i].getName());
                    System.out.print("(");
                    Class type[]=m[i].getParameterTypes();
                    for(int k=0;k<type.length;k++)
                    {
                        System.out.print(type[k].getName()+",");
                    }
                    System.out.print(")");
                    System.out.println(" ");
                }
            }
            catch(Exception e){}
            
        }
        
        
        
        
        public static void main(String s[])
        {
            
            printStart(s[0]);
            printSuperClass(s[0]);
            printInterface(s[0]);
            printField(s[0]);
            printConstructor(s[0]);
            printMethods(s[0]);
        System.out.print("} ");
    }
    
}