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Decompiling Java 221

Posted by timothy from the it'll-be-your-unbrewing dept.
Richard Rodger writes with a review of Godfrey Nolan's Decompiling Java. "I've just put this book down after reading it right through non-stop for four days. I haven't done that with a technical book since Learning Perl. Most techie book these days are quickie grab-bags, and you end up paying for a lot of dead trees that you aren't interested in." Read on for the rest of his review.
Decompiling Java
author Godfrey Nolan
pages 264
publisher apress
rating 8/10
reviewer Richard Rodger
ISBN 1590592654
summary Learn how decompilation works in order to properly protect your intellectual property.

If you are interested in Decompiling Java, then this book tell you exactly how to do that. There's no fluff and every chapter counts. I can safely concur that Fiachra's observations are indeed correct. You'd better be prepared for some serious hard core details, but then that's what you'd paid for. It is really great to read a book that doesn't end each chapter with a few links to the real material because the author couldn't be bothered to write it up.

So what do you get? As a battle-hardened Java coder of not a few years programming, I wanted to find out about the gory details of bytecodes and how to get at them. Now it's a subject I always knew I should know about, but never took the time to read up on it. Decompiling Java puts all that knowledge into one place.

Here's a quick run-through of the chapters so you know what you're getting:

Ch.1 Introduction
Decompilation isn't just another coding tool - there are other, real world issues like ending up in jail to think about. Godfrey proposes a sort of code-of-honour for decompilers. This book could so easily have been positioned for the fr33ky kod3r skript kiddie market, and I'm glad that the author and publishers took a mature and sensible approach to the subject. I have had to decompile purchased code because of bugs and I'm glad that someone took the time to think about an ethical framework for doing this.

Ch.2 Ghost in the Machine
A good and solid introduction to the JVM and the classfile format. If you're in the market for this book, you probably already know most of this, but a refresher course is always good. For me, it definitely sorted out a lot on internal hand-waving on the subject. Just remember kids, the only thing to fear is fear itself - it's only binary data after all.

Ch.3 Tools of the Trade
Although the author builds his only decompiler later in the book, it nice to get a chapter devoted to the existing toolset and the Java decompiler scene.

Ch. 4 Protecting your Source
For the honest developer, knowing how to decompile code is more about protecting your own source code than breaking someone else's (who wants to read other people's smelly code anyway!). This chapter is one of the most directly practical. I had always assumed that obfuscation was a magic fix that I could apply if necessary. In reality, good obfuscation is just like good encryption (that is, uncommon, difficult to verify, and still subject to lateral attacks). Even compiled bytecode has relatively low entropy, so the value of obfuscation must be considered carefully.

Ch.5 Decompiler Design
This is were it starts getting a wee bit technical. Decompilation, as you can imagine, is a bit of a black art, and there are many ways of doing it. Some of them involve scary maths and some involve scary coding and the rest both. But that's why you don't meet many people who can write decompilers. Godfrey does a great job of taking you on a practical run through this fog of decompilers. At the end of this chapter you will be able to decide for yourself what approach is best suited to your problem domain. Again, this material can be challenging but it's like boot camp: You just gotta.

Ch.6 Decompiler Implementation
If the previous chapter hurt your brain and scared you silly then this chapter will have you weeping for joy. The author takes a practical, effective, and most importantly, understandable approach to actually implementing a compiler. Now, as he freely admits, his design may encounter difficulties with edge effects and infrequently used idioms, but it will take you to the point where you can solve them yourself. I really had to smile at how simple and effective the approach taken here is - instead of the expected multiple passes and mind bending parse tree manipulation, we have a single-pass, source-generating decompiler for Java. You won't follow it all first time, but it does work and you can verify it for yourself. Like I said at the start, you don't get that empty feeling from this book, and this chapter is pretty much why. I bought a book about decompiling Java, and now I can.

Ch.7 Case Studies
This chapter addresses the "why" of decompiling, returning again to the moral questions raised at the start. It's more food for thought than prescriptive preaching though, which again is refreshing. I have admit to dipping into this chapter while reading the rest of the book - the human interest angle always works a treat!

Of course, no book is perfect. What I think could have helped a bit overall would have been a introductory chapter to bytecode. But it's not a great loss and bytecode is actually pretty simple once you get your head around it. Still it might have lessened the learning curve somewhat.

Decompiling Java is a great addition to that section of your bookshelf dedicated to serious books that will be around for a while. The JVM specification and Java bytecode are not going to change that much, so this book is something you'll be able to use for a long time. Personally the best thing about this book for me was that it took me to the next level. Not many books can do this. As a working coder, I pretty much put things like decompilation into the "too hard, just for academics, and I could never grok it", category. It's great when a book comes along that can can you out of that comfort zone.

You can purchase Decompiling Java from bn.com. Slashdot welcomes readers' book reviews -- to see your own review here, carefully read the book review guidelines, then visit the submission page.
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Decompiling Java

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  • no bytecode intro? (Score:4, Interesting)

    by MankyD ( 567984 ) on Wednesday November 03, 2004 @02:56PM (#10714822) Homepage
    So it's a book about reconstructing bytecode into human interprettable info, but it doesn't have an intro to them? That seems awfully strange. Are you sure you didn't miss something?

    • Re:no bytecode intro? (Score:5, Funny)

      by MankyD ( 567984 ) on Wednesday November 03, 2004 @03:03PM (#10714945) Homepage
      whoops. Sorry, I forgot first post was reserved for trolls.
    • **Ch.2 Ghost in the Machine
      A good and solid introduction to the JVM and the classfile format. If you're in the market for this book, you probably already know most of this, but a refresher course is always good. For me, it definitely sorted out a lot on internal hand-waving on the subject. Just remember kids, the only thing to fear is fear itself - it's only binary data after all.**

      maybe that one has the bytecode covered in short?

      btw.. for everyone thinking that obfuscators do a good job.. THEY DON'T! es
      • someone who doesn't have a clue about bytecode (or op codes) would be way out of their depth anyway.
      • Obfuscators DO work. They're certainly not foolproof, but they definitely make it more difficult to crack a program of any size.

        I'm not talking about tiny programs; but who even bothers decompiling tiny midlets? Isn't it obvious what they're doing? With tiny programs, if you know enough to be cracking Java programs, you might as well just write the thing out yourself. It's not magic.

        But for larger applications, any decent obfuscator can make it very time-consuming to decompile and edit the programs.

    • Re:no bytecode intro? (Score:4, Informative)

      by pjt33 ( 739471 ) on Wednesday November 03, 2004 @06:34PM (#10717682)
      More details aren't hard to find: the JVM specification [sun.com] is fairly readable and available from the Sun website.

  • Better Java Book (Score:4, Informative)

    by Pingular ( 670773 ) on Wednesday November 03, 2004 @02:56PM (#10714824)
    I've read both and I have to say Covert Java [amazon.com] is slightly more in-depth, but perhaps more for people more familiar with Java.

  • Been decompiling Java for awhile now... (Score:5, Funny)

    by twoslice ( 457793 ) on Wednesday November 03, 2004 @02:58PM (#10714853)
    Everytime I take a piss after my morning cup of joe...

  • another alternative = encrypted class files (Score:5, Interesting)

    by ardiri ( 245358 ) on Wednesday November 03, 2004 @03:01PM (#10714901) Homepage
    in 1999 i wrote a paper on security in set-top boxes (one of my first papers); yay. but, one approach we had was to build a custom class loader that would actually load encrypted classes.

    the details of the paper are:

    1999 - Security in Set-Top boxes
    European Multimedia, Embedded Systems and Electronic Commerce
    EMMSEC '99, Stockholm, SWEDEN
    June 21-23, 1999

    COPY: (pdf)
    http://www.ardiri.com/publications/emmsec99 .pdf

    there was a lot of interest on this topic back in the time :) i had a number of successful prototypes built - but, unless you build the class loader into hardware (ie: cannot access the .class file), its just another hurdle, nothing more.

    • Re:another alternative = encrypted class files (Score:4, Informative)

      by JavaNPerl ( 70318 ) * on Wednesday November 03, 2004 @04:24PM (#10716097)
      I believe encryption is probably not worth the effort for most people, it's not much of a hurdle to overcome. Ultimately all custom class loader execution paths will lead to a ClassLoader.defineClass call which can either be intercepted by creating a modified system class loader or by creating a JVMPI agent which listens for JVMPI_EVENT_CLASS_LOAD_HOOK events. If you can prevent these methods from being employed then you're probably operating in a secure environment where encryption would be overkill.

    • Doesn't work (Score:5, Informative)

      by lycono ( 173768 ) on Wednesday November 03, 2004 @04:34PM (#10716214)
      This approach to "security" in Java is so trivially easy to circumvent that its worthless.

      There are a number of papers and articles detailing why this type of approach to "IP security" is so misguided. One such article is here: http://www.javaworld.com/javaworld/javaqa/2003-05/ 01-qa-0509-jcrypt.html [javaworld.com]

      The crux is that at some point in time, you have to deliver the encrypted class to the JVM in an unencrypted format. Intercepting this delivery is incredibly easy (no expert knowledge required, the details for doing so are detailed in the article above), at which time someone can just write the unecrypted class file out to disk (or wherever they wish). Voila! All your IP are belong to us.

      • Re:Doesn't work (Score:4, Informative)

        by r7 ( 409657 ) on Wednesday November 03, 2004 @06:36PM (#10717700)
        > "security" in Java is so trivially easy to circumvent

        Are you confusing encryption with obfuscation? If not I agree that class-level encryption has no ROI.

        Obfuscation, on the other hand, is an excellent tool for protecting IP. I use Proguard http://proguard.sourceforge.net/ [sourceforge.net] via Ant and am happy with the result, having tried to grok the resulting byte code (using jad...) Good luck trying to work with that!

        R7
      • I have implemented an encrypted class loader that never delivers the unencrypted bytes to the java class. The author of this article is mostly correct, and I have read this article before. He states the following:

        until JVM architecture changes to, say, support class decoding inside native code...

        This is already possible and I have implemented it. It involves making direct calls to the jvm libraries from JNI rather than callbacks to java from JNI. There is still a way to get the bytes, but it invo

  • What ethical problems? (Score:5, Insightful)

    by jmorris42 ( 1458 ) * <jmorris@[ ]u.org ['bea' in gap]> on Wednesday November 03, 2004 @03:01PM (#10714909)
    Good review, but I have one major nit to pick.

    What ethical problems? Decompiling is perfectly moral and ethical. Whether it is illegal is a seperate and, for me, almost irelevant issue. If I legally own a copyrighted work I am allowed to read it, period and end of story. Corporate licences excepted, software is SOLD, not licensed despite the scary words on the box and the dread click through EULA.

    Hell, I learned assembly by writing a disassembler (in BASIC) and reading the Microsoft BASIC roms, then later reading the commented listings that ran in Color Computer Magazine. (TO avoid a copyright fight, and because M$ refused to grant them permission, CCM ran only the comments and memory locations, leaving the reader to run their own dissassembly for the opcodes.)

    The only ethical problem would be lifting the code and reusing it without permission and I think we all know that is wrong.
    • What ethical problems? Decompiling is perfectly moral and ethical. ... If I legally own a copyrighted work I am allowed to read it, period and end of story. Corporate licences excepted, software is SOLD, not licensed despite the scary words on the box and the dread click through EULA.

      I disagree here. I am a strong believer that people should be able to trade goods/services for prices/conditions they mutually agree upon. If I write software and say I will sell it to you for $x on condition that you do Y

      • Re:What ethical problems? (Score:5, Interesting)

        by jmorris42 ( 1458 ) * <jmorris@[ ]u.org ['bea' in gap]> on Wednesday November 03, 2004 @03:27PM (#10715288)
        > I think it is morally repugnant of you to break our agreement and decompile.

        While you are welcome to your delusions, but out here in the real world we have some things called laws. Specifically the Uniform Commercial Code and the Copyright laws.

        You will note that I excepted commercial licenses, since those are actual signed contracts and are legally binding.

        According to the Uniform Commercial Code if goods are exchanged in regular trade there can't be strings attached; i.e. if it looks like a sale it IS a sale. If I buy a copy of Microsoft Windows from Newegg.com I did just that, I BOUGHT a copy of Windows. That means I can do anything with that copy, including read it. I can even copy it in whole or in part so long as such activity falls under the backup exception written into the law or by Fair Use. Of course any other reproduction is forbidden by the artifical monopoly rights granted to the author by copyright. While I have a lot of problems with how copyrights are currently operated (eternal instead of "limited times" as prescribed by the Constituition) I don't have a major problem with that limitation.

        But think about it, what you are saying is that you can sell me a copyrighted work that I am forbidden to read myself. What a load of fetid dingos kidneys! Ford can't forbid me from taking apart a Caddy and not only making, but SELLING plans; but you think your algorithms are so freaking special that you want the government to put me in jail for the crime of reading them? What are you smoking?
        • But think about it, what you are saying is that you can sell me a copyrighted work that I am forbidden to read myself

          Sure. I was thinking of this as an example when I wrote my last post. I could write a book, and say to you, "Here's my book, it's $5, but if you buy it you can't read chapters 3, 7, or 9." And I'd have no problem with that. If you don't want to pay money for a book whose full contents I say you can't read, then fine - don't buy it. But don't buy it, knowing what terms I've laid out, and

          • If you don't want to pay money for a book whose full contents I say you can't read, then fine - don't buy it.

            You can certainly restrict your customers such terms -- as long as you convince the buyer to read and sign a legally binding contract prior to the sale.

            If you don't get a signature, you can't expect any restrictions.

        • Ahem. Last time I checked Ford didn't make Caddys. So I doubt they'd have any qualms about your taking one apart and selling the plans. They might even encourage it!
        • When you think about it - you can overwrite most laws (non-criminal laws) with a contract - so long as the contract is not illegal (i.e. a contract that between groups to rob a bank is not legal). A good example is contracts that break the "employment at will" clause that many states use.
          So if I write some material and I state that part of our agreement is that you cannot read certain sections of it - by purchasing it you are agreeing to my terms and can be held liable in a civil court. Remember - in term

        • First you imply that the actual laws are irrelevant to your views on morality:

          What ethical problems? Decompiling is perfectly moral and ethical. Whether it is illegal is a seperate and, for me, almost irelevant issue.

          Then when someone argues on moral grounds:

          If I write software and say I will sell it to you for $x on condition that you do Y (perhaps Y is not decompiling the source), and you agree to these terms, I think it is morally repugnant of you to break our agreement and decompile.

          • > First you imply that the actual laws are irrelevant to your views on
            > morality:

            When a society is correctly operating, laws codify morals. In our current dystopia of the Law divorced from Truth and Justice that isn't always the case. It is the Right, nay it is the Duty, of every citizen in a free society to violate an unjust law as an act of civil disobiedience.

            And yes, I have done so publicly, specifically by confessing to violating the DMCA by viewing DVDs on my laptop in a letter to President

            • If you don't like the conditions of the offer, don't buy the product.

              That is exactly what the Uniform Commercial Code is about, ensuring everyone knows and can agree in a meaningful way to the terms of the transaction. Without a signed contract specifying different terms though, the UCC says that a sale of goods transfers a clear title to those goods, meaning there can't be any conditions attached.

              You're talking about specific laws here. The grandparent post was merely saying that violating an a

              • > The grandparent post was merely saying that violating an agreement is
                > immoral.

                True enough but this whole matter revolves around whether an agreement exists. When people disagree on such matters, that is where the Law comes into the discussion and it is very clear on the point.

                The original poster holds that as the creator and owner of a work he has absolute power to dictate the terms and conditions it can be USED under, and that by purchasing his work I MUST agree to those terms. I hold that he
        • Ford can't forbid me from taking apart a Caddy and not only making, but SELLING plans

          Especially seeing how Cadillac is a GM brand.

      • Re:What ethical problems? (Score:5, Insightful)

        by YoJ ( 20860 ) on Wednesday November 03, 2004 @03:35PM (#10715364) Journal
        It's that mutually agreed condition part that is problematic. When I buy hardware or software, I don't normally agree to any conditions on it. If I did, your philosophy would have merit. There are even good examples of things you buy where you must agree to conditions (like cell phones); you read a contract and sign your name. Software and hardware companies want to have it both ways. They want an enforceable contract that users agree to, but they also want to present their wares in a friendly way that makes people think they are buying something normally.
        • Software and hardware companies want to have it both ways. They want an enforceable contract that users agree to, but they also want to present their wares in a friendly way that makes people think they are buying something normally.

          I agree with you fully. Burying what you can and can't do in a EULA, one that you can't read until you take the software home and start installing it, is not what I have in mind. If they wanted to impose such conditions on the software's use, it would behoove them to have on

      • (I wasn't the author of the grandparent post) What if I buy some software but do not get to see the EULA until after I have purchased it (say, it isn't available online) and, after purchasing it and reading the EULA, I am not permitted to return the product for a refund? Am I still obligated to follow the EULA in that case?
        • Personally I don't think a EULA that is hidden like that should be binding. I'm not pro-tricking consumers. If a seller wants to place limitations on how his product is used, he needs to make the limitations crystal clear to the potential buyer before any money changes hands. The buyer should then think about whether the goods/conditions are worth the price being asked, and make their decision based on that balance.
          • What about EULAs that are deliberately written in a style/technical language unreadable to the majority of the buyers ? Or written with intent to discourage complete reading (a EULA of 125 pages for instance, with 75% of its scentences longer than 10 lines and each one containing 15 buzwords that you have to go lookup online. NOBODY should be asked to read and accept all that. It's just plain stupid)
          • That sounds reasonable to me. Unfortunately, with the exception of EULAs that are available online, I do not know of a single local software store here that would allow you to return software because you did not accept the EULA (but had opened the software). Granted, I live in Edmonton, Alberta, Canada, so it may be different in the U.S., or even in other cities in Canada. But I have tried returning software before when I did not accept the EULA, to CompuSmart and to FutureShop, and neither place would a
    • Hell, I learned assembly by writing a disassembler (in BASIC) and reading the Microsoft BASIC roms, then later reading the commented listings that ran in Color Computer Magazine. (TO avoid a copyright fight, and because M$ refused to grant them permission, CCM ran only the comments and memory locations, leaving the reader to run their own dissassembly for the opcodes.)

      Regardless of the ethics, reading other people's code is, IMHO, the single best way to learn how coding works. And decompiling from objec
  • It has always been the case with Java (and in general many other interpretted/pcode generating languages) that enable them to be decompiled. I remember, back in old VB days, you could take a VB (pre 3.0) executable and decompile to get the original source. Of course, variable names were changed (since VB compiler changed them when converting to pcode).
    As systems get more open/advanced, the sources are more difficult to hide. In case of web apps, there is no need to decompile anything, the javascripts are
    • I didn't think it was possible to view the source of an ASP. So I did a search and came up with this [tek-tips.com].

      I'm not sure why you think of NTFS file streams. That's a complete different issue. How would your trick work if the ASP pages are on a FAT file system? NTFS streams are interesting: I once used them in a pratical joke to consume all of somebody's disk space. They couldn't see in Explorer where it went! Incidentally, it's too bad that Macs can't make use of them for their resource forks when browsing

    • Re:This is one of the features of Java (Score:2, Interesting)

      by Anonymous Coward
      Java doesn't really have to be that vulnerable. Perhaps code compiled with javac is weak, but it could easily be made much stronger.

      Why? Well, Java bytecode is a lot more flexible than the Java language. Take exceptions for example. In the language we handle exceptions with try-catch-finally grammar productions. But in the bytecode we have a table that specifes ranges of bytecode addresses that mapped via an exception to a exception handler. The cute thing is that a "range of bytecodes" has nothing t
    • As systems get more open/advanced, the sources are more difficult to hide. In case of web apps, there is no need to decompile anything, the javascripts are available for all to see in plain text. Even more advanced applications that use ASP pages ...

      Web applications are typically implemented server-side. Javascript is client-side code.

      Javascript != web applications

      Perhaps what you are referring to is the source for ASP and JSP/servlets. There have been bugs in servlet containers (specifically, I bel

    • Re:This is one of the features of Java (Score:4, Insightful)

      by almaw ( 444279 ) on Wednesday November 03, 2004 @04:52PM (#10716444) Homepage
      How can the parent be modded +3 insightful?!?

      Even more advanced applications that use ASP pages that execute on the server, can be seen by changing the URL to list the source rather than execute them

      Are you smoking crack?

      You can't arbitrarily get at source code on someone's web server. Do you think eBay would want you seeing the passwords to their database servers?

      Web apps aren't written in JavaScript. Sure, there might be some to drive calendar selection or something, but pretty much all real apps (shopping carts, etc.) are done server side.

      Please get a clue and stop spreading your FUD around.

      Additionally, this isn't a "feature" of Java. It's just a side-effect of its machine-independent bytecode. You could argue that it's not all that hard to reverse engineer compiled C - if you step it through a debugger you can see what it does fairly easily.

      Systems being more "advanced" (let's wave our hands a little bit more) won't make it any more difficult to hide the source. Many many people run Java on the server side of web apps. It will always be impossible to view the source for such applications (unless the developers put it up for the world to see, of course). As for being "open", what do you mean? If you mean, "open source" then, well, duh... :)
      • You can't arbitrarily get at source code on someone's web server. Do you think eBay would want you seeing the passwords to their database servers?

        I think he's referring to an old bug on IIS that would allow you to view the contents of a file on the server. I believe it was a sample ASP that MS included to demonstrate come capability of ASPs.

        I'm sure there are a few servers around that still have that enabled, but I'm sure most had that thing fixed 3-4 years ago.

  • DJ Decompiler and JAD (Score:3, Informative)

    by Anonymous Coward on Wednesday November 03, 2004 @03:08PM (#10715004)
    No Java developer should be without DJ Decompiler (which sits on top of JAD, the actual decompiler, command line only). Seriously, this book may be useful, but most people are way below needing to know any of this. If you do need to know it or are just curious, fine.

    Oh, and obfuscation, blah, any good IDE (like IntelliJ IDEA) is able to help you work around this junk.

  • Being able to decompile code.... (Score:5, Interesting)

    by tcopeland ( 32225 ) * <tom@NoSPaM.thomasleecopeland.com> on Wednesday November 03, 2004 @03:08PM (#10715008) Homepage
    ...can be handy when trying to figure out the advantage of one coding idiom over another. On the PMD [sf.net] project (a Java static analysis tool) there was a discussion [sourceforge.net] yesterday about code like this:
    if(logger.isLoggable(Level.FINEST) == true){
    // etc
    }
    which can be changed to
    if(logger.isLoggable(Level.FINEST)){
    // etc
    }
    to make it read (to me, anyway) a bit clearer.

    Anyhow, decompiling the classfile with "javap -c" shows that a couple of instructions get eliminated by dropping the explicit comparison to "true". So the classfile gets smaller, it loads faster, and (unless the JIT compiler is smart enough to do constant propagation on that conditional) it'll run faster, too.

    • Which is really surprising to me (Score:4, Insightful)

      by devphil ( 51341 ) on Wednesday November 03, 2004 @04:19PM (#10716037) Homepage


      who, as a compiler hacker, would have expected an optimization pass to transform the first form into the second form before generating the bytecode.

      Or more precisely, to understand that both forms are testing for the same thing, and to produce identical simplified bytecode.

      • > to produce identical simplified bytecode.

        Yeah. Another result - this code results in the "if" statement body being optimized away:

        class Test {
        void foo() {
        if (false) {
        System.out.println("boo!");
        }
        }
        }

        but this doesn't:

        class Test {
        void foo() {
        boolean b = false;
        if (b) {
        System.out.println("boo!");
        }
        }
        }

        But who knows - the JIT compiler may discard those bytecodes at runtime. Hard to say...

      • I'm astounded that Sun javac doesn't do the obvious optimization. I wrote a test program with and without the "== true" part, and here's the diffs (the test prog just println's the result). What the hell?

        < 15: iconst_1
        < 16: if_icmpne 23
        < 19: iconst_1
        < 20: goto 24
        < 23: iconst_0
        < 24: invokevirtual #6; //Method java/io/PrintStream.println:(Z)V
        < 27: return
        ---
        > 15: invokevirtual #6; //Method java/io/PrintStream.println:(Z)V
        > 18: return

      • who, as a compiler hacker, would have expected an optimization pass to transform the first form into the second form before generating the bytecode

        Almost all of the Java compilers out there do little to no optimizations while generating bytecode. Rather, the focus of the compilers is to generate bytecode that is easily understood and optimized by the virtual machine.

        Since the bytecode for Java is a well-defined standard, there are many more bytecode processors for Java compared to say object-code proc

      • On the other hand, also speaking as a compiler hacker, I should point out that just because an optimisation is possible, doesn't mean it's a good idea. When you add more optimisation, it costs. It's extra code that you have to write, test and maintain over the life of the compiler. If its entire purpose is to make the rare case of badly written code go faster, then you could quite legitimately turn around to the compiler user and say "don't do that".

        As an example, a compiler could quite easily optimise

    • Re:Being able to decompile code.... (Score:4, Informative)

      by AuMatar ( 183847 ) on Wednesday November 03, 2004 @04:28PM (#10716146)
      While I hate defending Java in any form, the two should be equivalent on a machine level. To see if a value is true (non-zero), you can do two things- subtract 0 and see if the result is 0, or AND it with 0xFFFF... To see if two things are both the same value, you subtract the two and check if the result is 0. The two are an equal number of instructions (and both test and cmp are quick instructions). On an asm level:

      mov eax, isLoggable
      cmp eax, 0 ;could also use test eax, 0xFFFFFFFF
      jnz skip_if ;if using test, do a jz skip_if ;etc
      skip_if: ;both branches continue here

      A check to true only changes the cmp to 1 instead of 0. It won't run any faster.
      • > the two should be equivalent on a
        > machine level.

        Yup, for assembler, that makes sense. But for Java bytecode, the explicit comparison is resulting in this:
        10: iconst_1
        11: if_icmpne 14
        vs
        10: ifeq 13
        So there's a least one extra instruction in there. Of course, again, what the JIT compiler might do with this is something else entirely.
    • Anyhow, decompiling the classfile with "javap -c" shows that a couple of instructions get eliminated by dropping the explicit comparison to "true". So the classfile gets smaller, it loads faster, and (unless the JIT compiler is smart enough to do constant propagation on that conditional) it'll run faster, too.

      The Java Language spec requires that a conforming compiler (not JIT, but source to bytecode compiler) do constant propagation.

      • > JLS requires [...] constant propagation

        Hm. I knew that the JLS required that in some circumstances - i.e., so that a switch statement can switch on a static final. What's the actual requirement, though? For example:
        int x = 2;
        int y = 4 * x; // can be int y = 4 * 2;
        int z = f(x, y);
        Is a conforming compiler also required to make that transformation?
    • javap does disassembly, not decompilation.

  • You didn't sell it. (Score:5, Insightful)

    by eddy ( 18759 ) on Wednesday November 03, 2004 @03:10PM (#10715034) Homepage Journal

    >knowing how to decompile code is more about protecting your own source code.

    There are many reasons to learn about, implement and use decompilers, but I don't think "to properly protect your intellectual property" should be one of them.

    I'm got somewhat interested in this book (never heard about it before), but I think I'm going to pass. Sounds like the decompiling described is too much of a one-trick pony -- which is fine, it's about decompiling java after all -- but I'd really like something like an extension and update of Cifuentes work in book form, with the lessons from the IDA team too.

    You know, from the beginning; starting with machine descriptions and disassembly for a generic front-end, efficent IR, and on up through the back end.

    Now that'd be a tome [worth paying for].

  • In Soviet Russia... (Score:3, Funny)

    by kkovach ( 267551 ) on Wednesday November 03, 2004 @03:15PM (#10715115)
    the decompiler compiles you!

    Er... um...

    the compiler decompiles you!

    Er...

    the java decompiles itself!

    Ah, whatever.

    - Kevin

  • I had always assumed that obfuscation was a magic fix that I could apply if necessary.

    Let me get this straight: the author recommends that 'honest' developers obfuscate their code?

    I've read programs that I thought were obfuscated, but later found out were just poorly written. Other times I've run into programmers who, tin hats firmly affixed, went to great lengths to make sure no one learned their Merlinesque techniques for getting the most out of BASIC.

    In context, the author seems to be talkin

    • Re:Maintainance nightmare (Score:5, Insightful)

      by jjgm ( 663044 ) on Wednesday November 03, 2004 @03:24PM (#10715227)
      I'm sure he's talking about obfuscating the bytecode, not the source code.

    • Re:Maintainance nightmare (Score:5, Informative)

      by nganju ( 821034 ) on Wednesday November 03, 2004 @03:36PM (#10715390)
      You've misunderstood. Java obfuscation is an automated process done with a third-party tool that rearranges Java BYTECODE, not source code. The idea being that someone that tries to decompile the BYTECODE will get a bunch of spaghetti. It doesn't take any extra time or energy by the developer, just 5 seconds to run the tool on your .class files.
      • >you've misunderstood...
        >byte code, not source code
        >feature of the language...

        My comments were intended to be more general than the Java language. Whether done with a command line switch, on an IDE checkbox at packaging time, or by writing a quickie program to do it yourself (which amortized is on the same order of time), doing anything to seal up your object code is counterproductive.

        Work to improve the code by revealing it, not by hiding it.

        Unstated and implicit was: what if you lose the sour
      • Besides that it removes the meta information in classes. Or renames actually. In Java, all class and member names are available (for reflection) after compilation. This makes the design rather obvious, even without decompilation. Renaming them to counters (a, b, c etc.) makes it pretty hard to see the design straight away. As long as you don't use these classes from outside, this does not do anything to functionality of the program.

        Normally, you would only do this after debugging your code. Obviously, it m
        • What's fun about decompiling obfuscated code is when you end up with variables and classes that have reserved names, e.g. a class called "if".

          I had to decompile and patch a ridiculously buggy JDBC driver for a commercial database which had been run through an obfuscator, and ran into that issue. Renaming was rather a hassle, I must say.

          I came to the conclusion that they had obfuscated their driver out of shame at the embarrasingly bad code, rather than to protect any intellectual property therein.
    • It's not really a waste of time because the source code remains entirely untouched... it's just one extra step in the build process, and it'd probably even be right there in the buildfile for the project, so it'd happen automatically whenever the source gets changed.

    • Opposite of debugging (Score:4, Funny)

      by BitwizeGHC ( 145393 ) on Wednesday November 03, 2004 @03:45PM (#10715523) Homepage
      The opposite of debugging is, of course, "embuggening".

      Hat tip to Jebediah Springfield.
    • I've read programs that I thought were obfuscated, but later found out were just poorly written.
      Some user has in their .signature here:
      "That's not encrypted - that's a perl script I'm
      working on." from crObar's now defunct matrix parody.

  • Consti-what? (Score:2, Funny)

    by Anonymous Coward
    non-stop reading for 4 days and the first thing he does is post on /.?

    I might have gone the bathroom, or perhaps had a snack. Maybe a nap.

  • Books online (Score:4, Informative)

    by truthsearch ( 249536 ) on Wednesday November 03, 2004 @03:35PM (#10715371) Homepage Journal
    Most techie book these days are quickie grab-bags, and you end up paying for a lot of dead trees that you aren't interested in.

    And so I suggest a service like O'Reilly's Safari Bookshelf [oreilly.com]. It includes the full text of over 2,000 technical books, many not published by them. No killing trees, far less money than buying books, plus full text search.

  • Jad... (Score:5, Informative)

    by david.given ( 6740 ) <dg@cowlark.com> on Wednesday November 03, 2004 @03:36PM (#10715389) Homepage Journal
    ...is pretty much the standard decompiler where I work. Alas, it's no longer free, as I've just found out when I searched for it's home page [tripod.com], but it works really well. I have, on occasion, used it as a pretty-printer for other people's code. It undoes obfuscation with ease.

    I have yet to try it on byte-code produced by non-Java languages, but I'd be interested to see the results...

    (It sucks that it's no longer free. The version I've got I installed through Debian, for goodness sake, years ago. Does anyone know any free alternatives that work as well?)

  • Sun has this info for free... (Score:4, Informative)

    by frankvl ( 817911 ) on Wednesday November 03, 2004 @03:37PM (#10715397)
    Sun has put the Java bytecode specification [sun.com] online for free..

    Reverse engineering in Java is as simple as the compile process itself. Besides there are already free tools available so why bother??

  • Decompiling Java by Godfrey Nolan [amazon.com] on Amazon.

    Another book on the subject is Covert Java : Techniques for Decompiling, Patching, and Reverse Engineering by Alex Kalinovsky [amazon.com]... probably more targetted at those who are already pretty familiar with things and want a more in-depth look.

    (Yes, Slashbots, those are affiliate links... that doesn't make them any less useful though, does it?)

    • (Yes, Slashbots, those are affiliate links... that doesn't make them any less useful though, does it?)

      Yes, it does make them less useful. Because now it is impossible to tell whether you are saying things like "a more in-depth look" because you really mean it, or because you stand to make a quick buck by making bogus claims about the book.

      Nothing personal, of course; you can probably see yourself why the rest of us simply can't know if you are being honest or running an astroturf con.

  • Nothing beats decompiling hacker code... (Score:3, Informative)

    by RZ-1 ( 770712 ) on Wednesday November 03, 2004 @04:36PM (#10716231) Homepage
    that was aimed to foil decompilers.

    Its starts off with public variable names like:
    public int YOU_DECOMPILING_NOOB =-1;
    public int NO_SKILLZ_4U=100;

    and then the obfusticator kicks in:
    where a1 and al(with an L) are switched around.
    The variable and method names look similar.

    if (a1.b1.x.y == al.b1.xl.y2){
    a1.v1.x.y &= al.b1.x1.y2 >> 0x4c;
    a1.b1.x( al.b1.x2 );
    }

    Ouch! Also, I think every decompiler has some weaknesses and isn't able to undo all code. I know Jad has some limitations. Unfortunately, I wasn't able to get the source of the code that broke the decompilers ;)

  • Why read a book. Just download Jode (Score:3, Informative)

    by JPyObjC Dude ( 772176 ) on Wednesday November 03, 2004 @05:48PM (#10717138)
    I have been decompiling Java regularily. Just get Jode Jode [sourceforge.net] Its very simple and effective. As long as the writers are not using ubfuscation tools, the code is fully readable in it's original form sans commenting.
  • Capture a java applet?

    By which I mean, there is a java applet running in my web browser. I'd like to decompile it and look over the source code. It's small enough I believe this would be informative. Is there a good way to do this?
  • System.out.println ("I'll give it a read ");

  • Want to keep your code to yourself? Use C++. (Score:3, Funny)

    by Futurepower(R) ( 558542 ) on Wednesday November 03, 2004 @06:44PM (#10717804) Homepage

    The books about decompiling Java are excellent advertisements for C++.

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