Java Decompiler HOW-TO

Al Dev (Alavoor Vasudevan) alavoor[AT]yahoo.com

v4.5, 20 Sep 2002

This document will help you to de-compile the Java class programs. This documents gives a list of decompilers which can reverse engineer the Java class files and generate the Java source code files. Thie is very helpful if you do not have the Java source code file and have only the Java class files. The information in this document applies to all the operating sytems where Java language/Java VM runs.

1. Introduction

2. How can I trust Java Decompiler ??!!

3. Related URLs

4. Other Formats of this Document

5. Copyright

1. Introduction

(The latest version of this document is at http://www.milkywaygalaxy.freeservers.com. You may want to check there for changes).

Java compiler compiles the Java source code files (*.java) into binaries files (*.class). You would use the Java de-compiler to convert java class files into source code files (*.java).

Java de-compiler is very useful especially if you have *.class files and you do not have access to the source code. Some vendors do not ship the source code for java class files, in which case you use the java decompiler to look at the source code.

See also the "javap" command from Sun Microsystems. This command is available as soon as you install the JDK from Sun Microsystems. At unix prompt type - 

bash$ javap -help
bash$ javap -c  <filename>.class

The following are the list of Java decompilers available -

2. How can I trust Java Decompiler ??!!

For 100% assurance you need a SCIENTIFIC way to validate and trust the Java Decompiler program. The method described in this section will enable the decompiler program to be accepted as "trust-worthy" and reliable.

In order to verify that the decompiler program is regenerating the Java source-code properly, use the following technique -

Generate the class file from the generated source code using the compiler - 

bash$ mv myprogram.class myprogram_orig.class
bash$ javac myprogram.java

Now use the unix 'diff' command to compare the two class files - 

bash$ diff myprogram.class myprogram_orig.class
Both these files MUST BE IDENTICAL. This verifies that decompiler program is working perfectly. On DOS or Windows 95 you may want to use the free Cygnus Cygwin 'diff' or 'MKS' utilities.

This step gives 100% guarantee and 100% quality assurance and life term or long term WARRANTY on decompiler programs.

It is strongly recommended that you do these steps every time you run decompiler programs.

3. Related URLs

Visit following locators which are related to Java -

4. Other Formats of this Document

This document is published in 14 different formats namely - DVI, Postscript, Latex, Adobe Acrobat PDF, LyX, GNU-info, HTML, RTF(Rich Text Format), Plain-text, Unix man pages, single HTML file, SGML (Linuxdoc format), SGML (Docbook format), MS WinHelp format.

This howto document is located at -

You can also find this document at the following mirrors sites -

The document is written using a tool called "SGML-Tools" which can be got from - http://www.sgmltools.org Compiling the source you will get the following commands like

4.1 Acrobat PDF format

PDF file can be generated from postscript file using either acrobat distill or Ghostscript. And postscript file is generated from DVI which in turn is generated from LaTex file. You can download distill software from http://www.adobe.com. Given below is a sample session: 

bash$ man sgml2latex
bash$ sgml2latex filename.sgml
bash$ man dvips
bash$ dvips -o filename.ps filename.dvi
bash$ distill filename.ps
bash$ man ghostscript
bash$ man ps2pdf
bash$ ps2pdf input.ps output.pdf
bash$ acroread output.pdf &
Or you can use Ghostscript command ps2pdf. ps2pdf is a work-alike for nearly all the functionality of Adobe's Acrobat Distiller product: it converts PostScript files to Portable Document Format (PDF) files. ps2pdf is implemented as a very small command script (batch file) that invokes Ghostscript, selecting a special "output device" called pdfwrite. In order to use ps2pdf, the pdfwrite device must be included in the makefile when Ghostscript was compiled; see the documentation on building Ghostscript for details.

4.2 Convert Linuxdoc to Docbook format

This document is written in linuxdoc SGML format. The Docbook SGML format supercedes the linuxdoc format and has lot more features than linuxdoc. The linuxdoc is very simple and is easy to use. To convert linuxdoc SGML file to Docbook SGML use the program ld2db.sh and some perl scripts. The ld2db output is not 100% clean and you need to use the clean_ld2db.pl perl script. You may need to manually correct few lines in the document.

The ld2db.sh is not 100% clean, you will get lots of errors when you run 
        bash$ ld2db.sh file-linuxdoc.sgml db.sgml
        bash$ cleanup.pl db.sgml > db_clean.sgml
        bash$ gvim db_clean.sgml 
        bash$ docbook2html db.sgml
And you may have to manually edit some of the minor errors after running the perl script. For e.g. you may need to put closing tag < /Para> for each < Listitem>

4.3 Convert to MS WinHelp format

You can convert the SGML howto document to Microsoft Windows Help file, first convert the sgml to html using: 

        bash$ sgml2html xxxxhowto.sgml     (to generate html file)
        bash$ sgml2html -split 0   xxxxhowto.sgml (to generate a single page html file)
Then use the tool HtmlToHlp. You can also use sgml2rtf and then use the RTF files for generating winhelp files.

4.4 Reading various formats

In order to view the document in dvi format, use the xdvi program. The xdvi program is located in tetex-xdvi*.rpm package in Redhat Linux which can be located through ControlPanel | Applications | Publishing | TeX menu buttons. To read dvi document give the command - 

        xdvi -geometry 80x90 howto.dvi
        man xdvi
And resize the window with mouse. To navigate use Arrow keys, Page Up, Page Down keys, also you can use 'f', 'd', 'u', 'c', 'l', 'r', 'p', 'n' letter keys to move up, down, center, next page, previous page etc. To turn off expert menu press 'x'.

You can read postscript file using the program 'gv' (ghostview) or 'ghostscript'. The ghostscript program is in ghostscript*.rpm package and gv program is in gv*.rpm package in Redhat Linux which can be located through ControlPanel | Applications | Graphics menu buttons. The gv program is much more user friendly than ghostscript. Also ghostscript and gv are available on other platforms like OS/2, Windows 95 and NT, you view this document even on those platforms.

To read postscript document give the command - 

                gv howto.ps
                ghostscript howto.ps

You can read HTML format document using Netscape Navigator, Microsoft Internet explorer, Redhat Baron Web browser or any of the 10 other web browsers.

You can read the latex, LyX output using LyX a X-Windows front end to latex.

5. Copyright

Copyright policy is GNU/GPL as per LDP (Linux Documentation project). LDP is a GNU/GPL project. Additional requests are - you retain the author's name, email address and this copyright notice on all the copies. If you make any changes or additions to this document then you should intimate all the authors of this document.