javap(1)

💅 javap(1) — Basic Tools, bestie edition

okay so quick vibe check: this whole page lives in the Basic Tools section of the manual, and it's all about javap. Buckle up, we're going in 🫶

✨ NAME ✨

javap — disassembles one or more class files. That's it, that's the whole gig. 💅

✨ SYNOPSIS ✨

javap [options] classfile...

options 🎀

the command-line options, duh. go look at OPTIONS below, i got you ✨

classfile 🎀

one or more classes, separated by spaces, that get processed for annotations like DocFooter.class. and like — you're not stuck with one format, bestie. you can specify a class that can be found in the class path, or by its file name, or even with a URL like file:///home/user/myproject/src/DocFooter.class. versatile queen 👑

✨ DESCRIPTION ✨

so the javap command disassembles one or more class files. the output depends on which options you used — different options, different lore 🌈

when you use no options at all, javap prints the package, protected and public fields, and methods of the classes you passed in. and it prints its output to stdout, obviously 💖

✨ OPTIONS ✨

okay this is the part everyone actually came for, let's go 🔥

• -help, --help, -? — prints a help message for the javap command. three whole ways to ask for help, pick your fighter 🫶
• -version — prints release information. which era of Java are we in, iykyk ⭐
• -l — prints line and local variable tables 🔍
• -public — shows only public classes and members. the main character energy filter 👑
• -protected — shows only protected and public classes and members 🌸
• -private, -p — shows all classes and members. no secrets, we're going feral, nothing is hidden 💀
• -Joption — passes the specified option to the JVM. for example:

javap -J-version
javap -J-Djava.security.manager -J-Djava.security.policy=MyPolicy MyClassName

for more information about JVM options, see the command documentation 📚

• -s — prints internal type signatures 🧚
• -sysinfo — shows system information (path, size, date, MD5 hash) of the class being processed. the whole receipts, literally 🧾
• -constants — shows static final constants 💜
• -c — prints disassembled code — like, the actual instructions that comprise the Java bytecodes, for each of the methods in the class. this is the good stuff, no cap 🔥
• -verbose — prints stack size, number of locals and arguments for methods. yes she tells you everything 🙈
• -classpath path — specifies the path the javap command uses to look up classes. overrides the default or the CLASSPATH environment variable when it is set 💅
• -bootclasspath path — specifies the path from which to load bootstrap classes. by default, the bootstrap classes are the classes that implement the core Java platform located in jre/lib/rt.jar and several other JAR files 📦
• -extdir dirs — overrides the location at which installed extensions are searched for. the default location for extensions is the value of java.ext.dirs 🦋

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✨ EXAMPLE ✨

ok so first compile the following DocFooter class 👇

import java.awt.*;
import java.applet.*;

public class DocFooter extends Applet {
        String date;
        String email;

        public void init() {
                resize(500,100);
                date = getParameter("LAST_UPDATED");
                email = getParameter("EMAIL");
        }

        public void paint(Graphics g) {
                g.drawString(date + " by ",100, 15);
                g.drawString(email,290,15);
        }
}

the output from the javap DocFooter.class command yields the following ✨

Compiled from "DocFooter.java"
public class DocFooter extends java.applet.Applet {
  java.lang.String date;
  java.lang.String email;
  public DocFooter();
  public void init();
  public void paint(java.awt.Graphics);
}

and the output from the javap -c DocFooter.class command yields the following — yes this is the spicy one 🔥

Compiled from "DocFooter.java"
public class DocFooter extends java.applet.Applet {
  java.lang.String date;
  java.lang.String email;

  public DocFooter();
    Code:
       0: aload_0
       1: invokespecial #1                  // Method
java/applet/Applet."<init>":()V
       4: return

  public void init();
    Code:
       0: aload_0
       1: sipush        500
       4: bipush        100
       6: invokevirtual #2                  // Method resize:(II)V
       9: aload_0
      10: aload_0
      11: ldc           #3                  // String LAST_UPDATED
      13: invokevirtual #4                  // Method
 getParameter:(Ljava/lang/String;)Ljava/lang/String;
      16: putfield      #5                  // Field date:Ljava/lang/String;
      19: aload_0
      20: aload_0
      21: ldc           #6                  // String EMAIL
      23: invokevirtual #4                  // Method
 getParameter:(Ljava/lang/String;)Ljava/lang/String;
      26: putfield      #7                  // Field email:Ljava/lang/String;
      29: return

  public void paint(java.awt.Graphics);
    Code:
       0: aload_1
       1: new           #8                  // class java/lang/StringBuilder
       4: dup
       5: invokespecial #9                  // Method
 java/lang/StringBuilder."<init>":()V
       8: aload_0
       9: getfield      #5                  // Field date:Ljava/lang/String;
      12: invokevirtual #10                 // Method
 java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      15: ldc           #11                 // String  by
      17: invokevirtual #10                 // Method
 java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
      20: invokevirtual #12                 // Method
 java/lang/StringBuilder.toString:()Ljava/lang/String;
      23: bipush        100
      25: bipush        15
      27: invokevirtual #13                 // Method
 java/awt/Graphics.drawString:(Ljava/lang/String;II)V
      30: aload_1
      31: aload_0
      32: getfield      #7                  // Field email:Ljava/lang/String;
      35: sipush        290
      38: bipush        15
      40: invokevirtual #13                 // Method
java/awt/Graphics.drawString:(Ljava/lang/String;II)V
      43: return
}

✨ SEE ALSO ✨

the friend group 🫶 go check them out:

• java(1)
• javac(1)
• javadoc(1)
• jdb(1)
• jdeps(1)

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and that's the whole page, bestie 🌸 this was the JDK 8 era of things, dated 8 August 2014 — vintage but still iconic, mwah 💋✨