1. Java 6 Decompiler
Joshua Cranmer

2. Why decompile? Source code may be lost but compiled code not Examples
Accidentally deleted source code (happened to me!)‏
Need to patch abandonware (happened to me!)‏
Security analysis (not happened to me)‏

3. Myths of decompilation
Decompilers are illegal
Just as legal as BitTorrent
If so, then why does IDA Pro exist?
Decompilation is impossible
Undecidable step is actually pre-disassembly (code v. data)‏
Decompilation is impractical
Based on the notion that it merely undos the steps a compiler does

4. Steps of decompilation
Signature recovery
Simple parser
Newer features make this more difficult
Stack analysis and variable recovery
“simple” without optimization or arbitrary scoping
Trivial decompilation
Example: fadd -> +
Control flow graph recovery
Most difficult portion
Direct translation impossible in some circumstances
Post-decompilation transformation
Changes legal syntax to sensible syntax

5. Signature Recovery Signatures are stored like (Ljava/lang/Object;I)V
Generics use a syntax like (TE;)V
Proposed Java 7 features are crazier
Enums, annotations, etc. use specific bits or binary JVM attributes (relatively simple)‏
Completed Q1

6. Stack Analysis
Used to infer about variables and unroll some optimizations
Uses Static Single Assignment (a “variable” can only be assigned once)‏
Variables are not presently unified, making ugliness
Most work done in late Q1 and Q2

7. Control Flow Graph Reconstruction
Hardest part of decompiling
Worked on during Q2, Q3, and Q4
Basic algorithm: create blocks and unify
Only unifications currently supported are if-else blocks
Couldn’t complete due to difficulty to get loops working

8. Example of CFG Reconstruction
Following is an if-else-block recovery A
<block A>
if <expression> {
<block B>
} else {
<block C> }
<block D> B C D

9. Post-decompilation Transformation
Not implemented
Idea is to take certain recognizable blocks of code and refactor them into common expressions
Examples:
Object.class (before Java 5)‏
Inner class private accessors
Bridge code
String concatenation

10. Future work Code is a horrible internal mess
Probably switch to building off of other open-source projects
Better type analysis and unification (especially generics)‏
Allow especially CFG recovery to be generified for other types of decompilation
???
Profit
Send any and all questions to