How to create a user defined javap tool?

Last Updated : 06 May, 2019

What is a javap tool?

The javap tool is used to get the information of any class or interface. The javap command (also known as the Java Disassembler) disassembles one or more class files. Its output depends on the options used (“-c” or “-verbose” for byte code and byte code along with innards info, respectively). If no options are used, javap prints out the package, protected, and public fields and methods of the classes passed to it.

Syntax:

javap [classname] [option]

To understand it clearly, see the below example on Command prompt which prints the details of String class
The command used is:

javap java.lang.String

Output:

How to create a user defined javap tool?

To create a user defined javap tool, we will use following methods of java.lang.class

Method[] getDeclaredMethods(): This method returns a Method object that reflects the specified declared method of the class or interface represented by this Class object.
Field[] getDeclaredFields(): This method returns an array of Field objects reflecting all the fields declared by the class or interface represented by this Class object. This includes public, protected, default (package) access, and private fields, but excludes inherited fields.
Constructor[] getDeclaredConstructors(): This method returns an array of Constructor objects reflecting all the constructors declared by the class represented by this Class object. These are public, protected, default (package) access, and private constructors.

We will use Reflection API of Java also. Reflection API is used to examine or modify the behavior of methods, classes, interfaces at runtime.

Below programs implement user defined javap tool:
Example 1: In this example we use this program on user defined class.

// The program implements 
// user defined javap tool 
  
// Import Reflection 
import java.lang.reflect.*; 
  
// Test class on which 
// custom javap tool will be used 
class test_class { 
  
    // Variables 
    int a; 
    double d; 
    char c; 
    String s; 
  
    // Constructors 
    test_class() 
    { 
        a = 0; 
        d = 0.0; 
        c = 'a'; 
        s = "Hello"; 
    } 
  
    test_class(int a, double d, char c, String s) 
    { 
        this.a = a; 
        this.d = d; 
        this.c = c; 
        this.s = s; 
    } 
  
    // Some class Methods 
    void printData() 
    { 
        System.out.println(a + d + c + s); 
    } 
  
    void setData() 
    { 
        a = 1; 
        d = 0.0; 
        c = 'A'; 
        s = "Hello Geeks"; 
    } 
} 
  
// Custom javap tool 
public class javapcustom { 
  
    public static void main(String[] args) 
        throws Exception 
    { 
        Class class_name = Class.forName("test_class"); 
  
        // Print field of class 
        System.out.println("Fields of class"); 
  
        Field f[] = class_name.getDeclaredFields(); 
        for (int i = 0; i < f.length; i++) { 
            System.out.println(f[i]); 
        } 
  
        // Print constructor of class 
        System.out.println("\nConstructors of class"); 
  
        Constructor cons[] = class_name.getDeclaredConstructors(); 
        for (int i = 0; i < cons.length; i++) { 
            System.out.println(cons[i]); 
        } 
  
        // Print methods of class 
        System.out.println("\nMethods of class"); 
  
        Method method[] = class_name.getDeclaredMethods(); 
        for (int i = 0; i < method.length; i++) 
            System.out.println(method[i]); 
    } 
} 

Output:

Fields of class
int test_class.a
double test_class.d
char test_class.c
java.lang.String test_class.s

Constructors of class
test_class(int, double, char, java.lang.String)
test_class()

Methods of class
void test_class.printData()
void test_class.setData()

Example 2: In this example we use this user-defined javap tool on pre-defined Java classes.

// The program implements 
// user defined javap tool 
  
// Import Reflection 
import java.lang.reflect.*; 
  
// Custom javap tool 
public class javapcustom { 
  
    public static void main(String[] args) 
        throws Exception 
    { 
  
        // You can replace args[0] with 
        // the name of predefined Class 
        // on which you want to use javap tool 
        // eg: replace args[0] with "java.lang.String" 
        Class class_name = Class.forName(args[0]); 
  
        // Print field of class 
        System.out.println("Fields of class"); 
        Field f[] = class_name.getDeclaredFields(); 
        for (int i = 0; i < f.length; i++) { 
            System.out.println(f[i]); 
        } 
  
        // Print constructor of class 
        System.out.println("\nConstructors of class"); 
        Constructor cons[] = class_name.getDeclaredConstructors(); 
        for (int i = 0; i < cons.length; i++) { 
            System.out.println(cons[i]); 
        } 
  
        // Print methods of class 
        System.out.println("\nMethods of class"); 
        Method method[] = class_name.getDeclaredMethods(); 
        for (int i = 0; i < method.length; i++) 
            System.out.println(method[i]); 
    } 
} 