![What is a Java Decompiler? transforms Java bytecode into Java source code public class HelloWorld { public static void main(String[] args) { System.out.println("Hello World"); } } public class HelloWorld extends java.lang.Object{ public HelloWorld(); Code: 0: aload_0 1: invokespecial #1; //Method java/lang/Object."<init>":()V 4: return public static void main(java.lang.String[]); Code: 0: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream; 3: ldc #3; //String Hello World 5: invokevirtual #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V 8: return }](https://image.slidesharecdn.com/scam2009-100708073734-phpapp01/85/Decompiling-Java-SCAM2009-Presentation-3-320.jpg)
Decompiling Java - SCAM2009 Presentation
- 1. Decompiling Java James Hamilton, PhD Student 0 1 0 0 0 0 1 1 0 1 1 0 0 0 1 0 0 1 1 0 1 0 1 0 0 1 0 1 0 1 0 1 1 0 0 1 0 0 0 1 1 1 0 1
- 2. What we did We tested 10 different decompilers with the latests Java class file format using 10 different test programs. In 2003, Emmerik tested the first 8 + 2 other, unobtainable ones.
- 3. What is a Java Decompiler? transforms Java bytecode into Java source code public class HelloWorld { public static void main(String[] args) { System.out.println("Hello World"); } } public class HelloWorld extends java.lang.Object{ public HelloWorld(); Code: 0: aload_0 1: invokespecial #1; //Method java/lang/Object."<init>":()V 4: return public static void main(java.lang.String[]); Code: 0: getstatic #2; //Field java/lang/System.out:Ljava/io/PrintStream; 3: ldc #3; //String Hello World 5: invokevirtual #4; //Method java/io/PrintStream.println:(Ljava/lang/String;)V 8: return }
- 4. What are they for? Static Single Assignment for Decompilation, Michael Van Emmerik, 2007
- 7. Assumption about how the bytecode was generated Java compiler generated bytecode most likely Sun's javac aribtrary bytecode generated by another language to Java bytecode compiler
- 8. generated manually by a programmer
- 9. transformed Java compiled bytecode
- 10. Our evaluation 10 decompilers tested
- 11. 10 programs testing different problem areas 6 javac generated
- 12. 4 abitrary based on a 2003 survey
- 14. + indication of the general effectiveness of a Java decompiler.
- 15. - manual inspection and subjective scale. Emmerik, 2003, http://www.program-transformation.org/Transform/JavaDecompilerTests
- 16. Results many of the current decompilers cannot parse the latest class files 4 obsolete
- 17. 3 unmaintained no currently maintained commercial decompilers
- 18. best decompiler? depends on the tool which generated the class file Dava for abitrary bytecode
- 19. Java Decompiler or JODE for javac bytecode
- 20. The sorts of problems the Java decompilers had Type inference for local variables – giving the tighest possible type to local variables
- 22. Control flow
- 23. Where decompilers failed public class TypeSet { public void m1(Integer i, String s) { untyped x; if(i != null) x = i; else x = s; m2(x); } public void m2(Comparable x) { } } this example is easy but multiple inheritance via interfaces complicates things*.
- 24. is this really a problem, if all we need is compilable code?
- 25. can we just make everything an object, and use typecasts? invokevirtual TypeSet/m2(Ljava/lang/Comparable;)V Example from: Java and the Java Virtual Machine: Definition, Verification and Validation, Stärk et al, 2001 * see Efficient Inference of Static Types for Java Bytecode, Gagnon et al. 2000 and Efficient local type inference, Bellamy et al. 2008. Type Inference
- 26. most decompilers, except Dava, do not expect this. System.out.println("a"); label_0: { try { System.out.println("b"); }catch (RuntimeException $r9) { System.out.println("g"); break label_0; } try { System.out.println("c"); }catch (RuntimeException $r9) { System.out.println("g"); break label_0; }catch (Exception $r6) { System.out.println("e"); break label_0; } try { System.out.println("d"); }catch (Exception $r6) { System.out.println("e"); } } //end label_0: System.out.println("f"); Example from: Decompiling Java Bytecode: Problems, Traps and Pitfalls, Miecznikowski et al, 2002 Where decompilers failed Intersecting try-catch blocks
- 27. public static int foo(int i, int j) { while (true) { try{ while (i < j) i = j++/i; }catch (RuntimeException re) { i = 10; continue; } break; } return j; } Example from: Decompiling Java Bytecode: Problems, Traps and Pitfalls, Miecznikowski et al, 2002 Where decompilers failed Control Flow
- 28. Not much improvement in 6 years all decompilers aren't very good Problems caused by newer class file specification
- 29. Different decompilers for different problems can we produce a decompiler which combines the best of javac decompilers and abitrary decompilers?
- 30. do we really need to perform type inference if the only condition is that we require re-compilable code?
- 31. are the problems with the Java decompilers fundamental or just bugs? Conclusion Questions