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Decompiling Java
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 IntroductionDecompilation 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.
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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.
Democrat delenda est
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 available for all to see in plain text. 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 (I dont remember the exact syntax, but I think it is related to the alternate data stream in NTFS)
That is the reason, we have copyright. On a more personal note, I think it serves the community if someone can see your implementation in code, get inspiration and either correct mistakes or expand on the code.
That's ok, you didn't get FP anyhow.
>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].
Belief is the currency of delusion.
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 talking about obfuscating object code. Yikes! What's the opposite of debugging? Buggery?
Encrypting object code to make it harder to reverse engineer is a giant waste of time. Here are more productive ways to spend the the same amount of energy:
In fact, I can't think of many worse wastes of time than making a compiled program hard to understand.
sigs, as if you care.
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.
You cannot apply a technological solution to a sociological problem. (Edwards' Law)
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.
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. I posted more on this in another thread, so let me just say you really have to try it out before you say obfuscators don't work. They definitely DO work at foiling the average cracker who won't spend hours and hours reconstructing a $100 piece of software.