Harakim

IDA Pro does a perfect decompilation. It's a nice program.

DaWei

Nothing does a perfect decompilation unless it's studded with information that renders it ineffictive as a truly good implementation. If you don't understand this concept, I invite you to behave as a VonNeuman processor divorced from wasteful hints.

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Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code.
Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Harakim

Java classes include all of the information needed to decompile the class. It doesn't compile to machine code.

DaWei

That's why it's inferior, when push comes to shove. Any implementation which produces even byte code, tops, isn't going to include the labels which are programmer-helpful. I can show you dozens of pieces of executing code which cannot be traced back to high-livel constructs. That says all that there needs to be said about the subject. Reasonable and accurate decompilation presupposes worthless inefficiencies. If that flips your skirt, so be it.

__________________
Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code.
Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

null_ptr0

As others said, your best bet is obfuscation; might I recommend the Zelix KlassMaster obfuscater.

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iload_0 iconst_1 ishl or
iload_0 iconst_2 idiv or
iload_0 iconst_2 iconst_1 imul idiv
[1] & [2] use the smallest stack size

DaWei

Obfuscation is the last resort of the inadequate.

__________________
Abstraction doesn't make it impossible to write bad code; it makes it possible to write superior code.
Contributor's Corner: Grumpy on C++ Exceptions DaWei on Pointers

Harakim

It is my understanding and experience that Java classes (since they use String-based reflection) contain a description of all classes, methods, and member variables. My experience has also shown that they contain local-scoped variable names, though I see no reason this would always be the case. Of course, when the JIT runs, it doesn't include this information unless you're doing a lot of reflection.

If you could give me a class file that does not contain this information, I would appreciate it as it would help me to learn more about compilers.

lectricpharaoh

I expect that one reason Java is so straightforward to decompile is because the designers wanted to defer most optimizations until the app gets deployed on the target machine. Some information, such as high-level looping constructs, might be helpful to the JIT compiler's optimizer. Why the hell they'd include other, irrelevant information is beyond me, though. Examples might include the original names of variables and references; very often, these names are a strong hint as to the object's purpose.

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A man's knowledge is like an expanding sphere, the surface corresponding to the boundary between the known and the unknown. As the sphere grows, so does its surface; the more a man learns, the more he realizes how much he does not know. Hence, the most ignorant man thinks he knows it all. - L. Sprague de Camp

Harakim

Class names, method names, and class level variable names are used by Java's reflection facilities. For instance, Java programmers will probably recognize Class.forName( String className ) which is commonly used for loading jdbc drivers.

I don't know why they used strings for reflection, but that is why the metadata is kept there.