Understanding javap tool (Reflection Session-3)

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