What is Understanding javap tool?

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

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. }  

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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. }  

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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. }  

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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