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.

Example to use javap tool

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 {};
}

Another example to use javap tool for your class

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

FileName: Simple.java

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

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

Output:

Compiled from "Simple.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.

Output:

Compiled from "Simple.java"  
class Simple {  
  Simple();  
    Code:  
       0: aload_0         
       1: invokespecial #1                  // Method java/lang/Object."":()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

Let's see how one can use these options with the help of an example. For the following file (ABC.java) we will use the above-mentioned options.

FileName: ABC.java

public class ABC
{
// main method
public static void main(String argvs[])
{
// declaring an integer array
int arr[] = {6, 7, 8, 6, 8, 0, 4};

// caculating size of the array
int size = arr.length;

// printing size of the array
System.out.println("The size of the array is " + size );

System.out.println("The 8th index of the array is " + arr[8] );

}
}

Command: javap -c ABC

Output:

Compiled from "ABC.java"
public class ABC {
  public ABC();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method java/lang/Object."":()V
       4: return

  public static void main(java.lang.String[]);
    Code:
       0: bipush        7
       2: newarray       int
       4: dup
       5: iconst_0
       6: bipush        6
       8: iastore
       9: dup
      10: iconst_1
      11: bipush        7
      13: iastore
      14: dup
      15: iconst_2
      16: bipush        8
      18: iastore
      19: dup
      20: iconst_3
      21: bipush        6
      23: iastore
      24: dup
      25: iconst_4
      26: bipush        8
      28: iastore
      29: dup
      30: iconst_5
      31: iconst_0
      32: iastore
      33: dup
      34: bipush        6
      36: iconst_4
      37: iastore
      38: astore_1
      39: aload_1
      40: arraylength
      41: istore_2
      42: getstatic     #7                  // Field java/lang/System.out:Ljava/io/PrintStream;
      45: iload_2
      46: invokedynamic #13,  0             // InvokeDynamic #0:makeConcatWithConstants:(I)Ljava/lang/String;
      51: invokevirtual #17                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      54: getstatic     #7                  // Field java/lang/System.out:Ljava/io/PrintStream;
      57: aload_1
      58: bipush        8
      60: iaload
      61: invokedynamic #23,  0             // InvokeDynamic #1:makeConcatWithConstants:(I)Ljava/lang/String;
      66: invokevirtual #17                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V
      69: return
}

Command: javap -l ABC

Output:

Compiled from "ABC.java"
public class ABC {
  public ABC();
    LineNumberTable:
      line 1: 0

  public static void main(java.lang.String[]);
    LineNumberTable:
      line 6: 0
      line 9: 39
      line 12: 42
      line 14: 54
      line 16: 69
}

Command: javap -s ABC

Output:

Compiled from "ABC.java"
public class ABC {
  public ABC();
    descriptor: ()V

  public static void main(java.lang.String[]);
    descriptor: ([Ljava/lang/String;)V
}

Command: javap -sysinfo ABC

Output:

Classfile /C:/Users/Nikhil Kumar/Documents/ABC.class
  Last modified Sep 11, 2021; size 970 bytes
  SHA-256 checksum 576adf03386399a4691e0ce5b6c5aa5d964b082a1a61299bac5632942e413312
  Compiled from "ABC.java"
public class ABC {
  public ABC();
  public static void main(java.lang.String[]);
}

Command: javap -constants ABC

Output:

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

Command: javap -version ABC

Output:

14
Compiled from "ABC.java"
public class ABC {
  public ABC();
  public static void main(java.lang.String[]);
}

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Java Reflection

Understanding javap tool

Syntax to use javap tool

Example to use javap tool

Another example to use javap tool for your class

javap -c command

Options of javap tool

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