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

Let's see how to use javap tool or command.

Example #1

snippet

javap fully_class_name

Example #2

Example to use javap tool

snippet

javap java.lang.Object

Output

Compiled from "Object.java" public class java.lang.Object { public java.lang.Object(); public final native java.lang.Class<?> getClass(); public native int hashCode(); public boolean equals(java.lang.Object); protected native java.lang.Object clone() throws java.lang.CloneNotSupportedException; public java.lang.String toString(); public final native void notify(); public final native void notifyAll(); public final native void wait(long) throws java.lang.InterruptedException; public final void wait(long, int) throws java.lang.InterruptedException; public final void wait() throws java.lang.InterruptedException; protected void finalize() throws java.lang.Throwable; static {}; }

Example #3

Another example to use javap tool for your class

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

snippet

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

Example #4

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

snippet

javap Simple

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

javap -c command

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

Example

snippet

javap -c Simple

Output

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

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