The javap command disassembles a class file. The javap command displays information about the fields,constructors and methods present in a class file.

Table of Contents

Syntax to use javap tool

Let’s see how to use javap tool or command.
1. javap fully_class_name

Example to use javap tool

1. javap java.lang.Object

Output:

1. Compiled from "Object.java" 
2. public class java.lang.Object { 
3. public java.lang.Object(); 
4. public final native java.lang.Class<?> getClass(); 
5. public native int hashCode(); 
6. public boolean equals(java.lang.Object); 
7. protected native java.lang.Object clone() throws java.lang.CloneNotSupportedException; 
8. public java.lang.String toString(); 
9. public final native void notify(); 
10. public final native void notifyAll(); 
11. public final native void wait(long) throws java.lang.InterruptedException; 
12. public final void wait(long, int) throws java.lang.InterruptedException; 
13. public final void wait() throws java.lang.InterruptedException; 
14. protected void finalize() throws java.lang.Throwable; 
15. static {}; 
16. }

Another example to use javap tool for your class

Let’s use the javap command for our java file.

1. class Simple{ 
2. public static void main(String args[]){ 
3. System.out.println("hello java"); 
4. } 
5. }

Now let’s use the javap tool to disassemble the class file.

1. javap Simple

Output:

1. Compiled from ".java" 
2. class Simple { 
3. Simple(); 
4. public static void main(java.lang.String[]); 
5. }

javap -c command

You can use the javap -c command to see disassembled code. The code that reflects the java bytecode.

1. javap -c Simple
Output:

1. Compiled from ".java" 
2. class Simple { 
3. Simple(); 
4. Code: 
5. 0: aload_0 
6. 1: invokespecial #1 // Method java/lang/Object."<init>":()V 
7. 4: return 
8. 
9. public static void main(java.lang.String[]); 
10. Code: 
11. 0: getstatic #2 // Field java/lang/System.out:Ljava/io/PrintStream; 
12. 3: ldc #3 // String hello java 
13. 5: invokevirtual #4 // Method java/io/PrintStream.println:(Ljava/lang/String;)V 
14. 8: return 
15. }

Options of javap tool

The important options of javap tool are as follows.

Option	Description
-help	prints the help message.
-l	prints line number and local variable
-c	disassembles the code
-s	prints internal type signature
-sysinfo	shows system info (path, size, date, MD5 hash)
-constants	shows static final constants
-version	shows version information

Creating a program that works as javap tool

Following methods of java.lang.Class class can be used to display the metadata of a class.

Method	Description
public Field[] getDeclaredFields()throws SecurityException	returns an array of Field objects reflecting all the fields declared by the class or interface represented by this Class object.
public Constructor[] getDeclaredConstructors()throws SecurityException	returns an array of Constructor objects reflecting all the constructors declared by the class represented by this Class object.
public Method[] getDeclaredMethods()throws SecurityException	returns an array of Method objects reflecting all the methods declared by the class or interface represented by this Class object.

Example of creating javap tool

Let’s create a program that works like javap tool.

1. import java.lang.reflect.*; 
2. 
3. public class MyJavap{ 
4. public static void main(String[] args)throws Exception { 
5. Class c=Class.forName(args[0]); 
6. 
7. System.out.println("Fields........"); 
8. Field f[]=c.getDeclaredFields(); 
9. for(int i=0;i<f.length;i++) 
10. System.out.println(f[i]); 
11. 
12. System.out.println("Constructors........"); 
13. Constructor con[]=c.getDeclaredConstructors(); 
14. for(int i=0;i<con.length;i++) 
15. System.out.println(con[i]); 
16. 
17. System.out.println("Methods........"); 
18. Method m[]=c.getDeclaredMethods(); 
19. for(int i=0;i<m.length;i++) 
20. System.out.println(m[i]); 
21. } 
22. }

At runtime, you can get the details of any class, it may be user-defined or pre-defined class.

java 127 1

java 128

So, this brings us to the end of blog. This Tecklearn ‘Understanding javap tool’ blog helps you with commonly asked questions if you are looking out for a job in Java Programming. If you wish to learn Java and build a career Java Programming domain, then check out our interactive, Java and JEE Training, that comes with 24*7 support to guide you throughout your learning period. Please find the link for course details:

Java and JEE Training

Java and JEE Training

About the Course

Java and JEE Certification Training is designed by professionals as per the industrial requirements and demands. This training encompasses comprehensive knowledge on basic and advanced concepts of core Java & J2EE along with popular frameworks like Hibernate, Spring & SOA. In this course, you will gain expertise in concepts like Java Array, Java OOPs, Java Function, Java Loops, Java Collections, Java Thread, Java Servlet, and Web Services using industry use-cases and this will help you to become a certified Java expert.

Why Should you take Java and JEE Training?

• Java developers are in great demand in the job market. With average pay going between $90,000/- to $120,000/- depending on your experience and the employers.
• Used by more than 10 Million developers worldwide to develop applications for 15 Billion devices.
• Java is one of the most popular programming languages in the software world. Rated #1 in TIOBE Popular programming languages index (15th Consecutive Year)

What you will Learn in this Course?

Introduction to Java

• Java Fundamentals
• Introduction to Java Basics
• Features of Java
• Various components of Java language
• Benefits of Java over other programming languages
• Key Benefits of Java

Installation and IDE’s for Java Programming Language

• Installation of Java
• Setting up of Eclipse IDE
• Components of Java Program
• Editors and IDEs used for Java Programming
• Writing a Simple Java Program

Data Handling and Functions

• Data types, Operations, Compilation process, Class files, Loops, Conditions
• Using Loop Constructs
• Arrays- Single Dimensional and Multi-Dimensional
• Functions
• Functions with Arguments

OOPS in Java: Concept of Object Orientation

• Object Oriented Programming in Java
• Implement classes and objects in Java
• Create Class Constructors
• Overload Constructors
• Inheritance
• Inherit Classes and create sub-classes
• Implement abstract classes and methods
• Use static keyword
• Implement Interfaces and use it

Polymorphism, Packages and String Handling

• Concept of Static and Run time Polymorphism
• Function Overloading
• String Handling –String Class
• Java Packages

Exception Handling and Multi-Threading

• Exception handling
• Various Types of Exception Handling
• Introduction to multi-threading in Java
• Extending the thread class
• Synchronizing the thread

File Handling in Java

• Input Output Streams
• Java.io Package
• File Handling in Java

Java Collections

• Wrapper Classes and Inner Classes: Integer, Character, Boolean, Float etc
• Applet Programs: How to write UI programs with Applet, Java.lang, Java.io, Java.util
• Collections: ArrayList, Vector, HashSet, TreeSet, HashMap, HashTable

Java Database Connectivity (JDBC)

• Introduction to SQL: Connect, Insert, Update, Delete, Select
• Introduction to JDBC and Architecture of JDBC
• Insert/Update/Delete/Select Operations using JDBC
• Batch Processing Transaction
• Management: Commit and Rollback

Java Enterprise Edition – Servlets

• Introduction to J2EE
• Client Server architecture
• URL, Port Number, Request, Response
• Need for servlets
• Servlet fundamentals
• Setting up a web project in Eclipse
• Configuring and running the web app with servlets
• GET and POST request in web application with demo
• Servlet lifecycle
• Servlets Continued
• Session tracking and filter
• Forward and include Servlet request dispatchers

Java Server Pages (JSP)

• Fundamentals of Java Server Page
• Writing a code using JSP
• The architecture of JSP
• JSP Continued
• JSP elements: Scriptlets, expressions, declaration
• JSP standard actions
• JSP directives
• Introduction to JavaBeans
• ServletConfig and ServletContext
• Servlet Chaining
• Cookies Management
• Session Management

Hibernate

• Introduction to Hibernate
• Introduction to ORM
• ORM features
• Hibernate as an ORM framework
• Hibernate features
• Setting up a project with Hibernate framework
• Basic APIs needed to do CRUD operations with Hibernate
• Hibernate Architecture

POJO (Plain Old Java Object)

• POJO (Plain Old Java Object)
• Persistent Objects
• Lifecycle of Persistent Object

Spring

• Introduction to Spring
• Spring Fundamentals
• Advanced Spring

Got a question for us? Please mention it in the comments section and we will get back to you.

432