javap(1)

🦋 javap(1) — Basic Tools 🦋

💖 NAME 💖

javap — disassembles one or more class files. periodt. 💅

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✨ SYNOPSIS ✨

javap [options] classfile...

• options — the command-line options. See OPTIONS.
• classfile — one or more classes separated by spaces to be processed for annotations such as DocFooter.class. You can specify a class that can be found in the class path, by its file name or with a URL such as file:///home/user/myproject/src/DocFooter.class.

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🌸 DESCRIPTION 🌸

okay so bestie, the javap command disassembles one or more class files. The output depends on the options used. When no options are used, then the javap command prints the package, protected and public fields, and methods of the classes passed to it. The javap command prints its output to stdout. no cap, it's giving bytecode decoder energy. 🔥

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🎀 OPTIONS 🎀

• -help, --help, -? — prints a help message for the javap command.
• -version — prints release information.
• -l — prints line and local variable tables.
• -public — shows only public classes and members.
• -protected — shows only protected and public classes and members.
• -private, -p — shows all classes and members.
• -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.
• -constants — shows static final constants.
• -c — prints disassembled code, for example, the instructions that comprise the Java bytecodes, for each of the methods in the class.
• -verbose — prints stack size, number of locals and arguments for methods.
• -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 🧚

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);
}

The output from javap -c 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();
    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
}

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🌈 SEE ALSO 🌈

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

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this page is from JDK 8, dated 8 August 2014 — javap(1) says bye bestie, go disassemble something iconic 💖✨