"javap -c -private" - Disassembling Java Bytecode Class

Java Tools Tutorials - Herong's Tutorial Examples

∟javap - The Java Class File Disassembler

∟"javap -c -private" - Disassembling Java Bytecode Class

This section provides a tutorial example of how to disassemble Java bytecodes with 'javap -c -private' command. The disassembled codes are JVM execution instructions.

Actually, the main function of "javap" is to disassemble Java bytecodes with the "-c" option. The disassembled codes are execution instructions of JVM (Java Virtual Machine).

Now, let's try "javap -c -private" with the bytecode compiled from Circle.java source code.

herong> javap -c -private Circle

Compiled from "Circle.java"
public class Circle extends java.lang.Object{
public java.lang.String uom;

private int x;

private int y;

private int r;

public Circle();
  Code:
   0:  aload_0
   1:  invokespecial  #1; //Method java/lang/Object."<init>":()V
   4:  aload_0
   5:  ldc         #2; //String Centimeter
   7:  putfield  #3; //Field uom:Ljava/lang/String;
   10:  aload_0
   11:  iconst_0
   12:  putfield  #4; //Field x:I
   15:  aload_0
   16:  iconst_0
   17:  putfield  #5; //Field y:I
   20:  aload_0
   21:  iconst_1
   22:  putfield  #6; //Field r:I
   25:  return

public void setRadius(int);
  Code:
   0:  aload_0
   1:  iload_1
   2:  putfield  #6; //Field r:I
   5:  return

public void setCenter(int, int);
  Code:
   0:  aload_0
   1:  iload_1
   2:  putfield  #4; //Field x:I
   5:  aload_0
   6:  iload_2
   7:  putfield  #5; //Field y:I
   10:  return

...

private double getArea();
  Code:
   0:  ldc2_w      #18; //double 3.14159d
   3:  aload_0
   4:  getfield  #6; //Field r:I
   7:  i2d
   8:  dmul
   9:  aload_0
   10:  getfield  #6; //Field r:I
   13:  i2d
   14:  dmul
   15:  dreturn

}

Note that:

I used "-c" and "-private" options together to disassemble both private and public methods.
All class level variables are printed out in the disassembled codes as comments like "//Field r:I".
Constants are printed out in the disassembled codes as comments like "//double 3.14159d".
String constants are also printed out in the disassembled codes as comments like "//String Centimeter". So don't distribute any Java bytecode compiled from source codes that contain any password strings. People can easy find the passwords using the "javap" tool.
If you want to understand each statement listed in the disassembled code, you need to read the "JVM Specification" at https://docs.oracle.com/javase/specs/jvms/se12/jvms12.pdf.