How to Create a User-Defined Javap Tool?

At times, we need information related to a class file. In such a case, we can use the javap tool provided by the Java Development Kit (JDK). We can get more information related to the methods, constructors, and fields present in the class. The purpose of the javap tool is to disassemble one or more class files. It is also known as Java Class File Disassembler. Using the javap tool, we can get more information about the bytecode information about that particular class. The output may vary depending on the options used.

Syntax

The syntax of javap is as follows 

javap [options] [classes]

Explanation

The javap keyword is used at the start followed by the options command. Options are an optional parameter. Options can be one or more than one and are separated by space. These are a few widely used options in Java.

• -help: It is used to display help for javap menu.

• -version: It is used to print the release information

• -l: It is used to display line numbers and local variables

• -public: It is used to show the public classes and public members

• -protected: It is used to show the protected classes and private members

• -c: It is used to disassemble the code

• -sysinfo: It is used to show the system information

Example

In order to understand how the javap tool works, let us execute the command on the command prompt.

javap java.util.Scanner

Here, we have used the javap command to disassemble the Scanner class in the java.util package. The following image shows the output of the above code. It shows the class modifiers, public methods, and detailed information about the same. One pre-requisite is to have JDK installed for javap tool to work on command prompt.

Output

User-Defined Javap Tool

Now that we know we can use the javap tool on pre-built classes, it is also possible to use the javap tool on user-defined classes.

Steps

• Create a user-defined class. Add a constructor and methods to it.

• In the Main class, obtain the class instance associated with the custom class. We use the Class.forName()method.

• Then the class fields of the custom class are displayed. The getDeclaredFields() method is used to return an array of Field objects. All the public, private, protected, and default fields are included.

• Class constructors are shown using the getDeclaredConstructors() method we get an array of constructors defined in the custom class. All the public, private, protected, and default constructors are included.

• All the class methods are obtained using the getDeclaredMethods() method that returns an array of methods.

Example

Following is an example to create a user-defined javap tool

import java.lang.reflect.*;

// class to use custom javap tool 
class Customclass{

   int x;
   float y;
   String z;

   // class constructor
   Customclass()
   {
      x = 19;
      y=89.78894532f;
      z="javap";
   }

   //class methods
   void printData()
   {
      System.out.println(x);
      System.out.println(y);
      System.out.println(z);
   }
}

//custom javap tool
public class Main {

   public static void main(String[] args) throws Exception
   {
      Class cn = Class.forName("Customclass");

      //display the fields
      System.out.println("Class fields");
      Field[] field= cn.getDeclaredFields();
      for (int i=0;i<field.length;i++) {
         System.out.println(field[i]);
      }
   System.out.println("\n");
      //display the constructor
      System.out.println("Class constructors");
      Constructor[] cs= cn.getDeclaredConstructors();
      for (int i=0;i<cs.length;i++) {
         System.out.println(cs[i]);
      }
   System.out.println("\n");
      //display the methods
      System.out.println("Class methods");
      Method[] m= cn.getDeclaredMethods();
      for (int i=0;i<m.length;i++)
         System.out.println(m[i]);
   }
}

Output

Following is an output of the above code

Class fields
int Customclass.x
float Customclass.y
java.lang.String Customclass.z

Class constructors
Customclass()

Class methods
void Customclass.printData()

Explanation

We have obtained the class fields, constructors, and methods using the methods mentioned before. Since our class had 3 fields- int, float and String variables, they got printed. There is a single constructor and a single method.