Java Decompiler
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This document discusses the process of de-compiling Java code. It explains that a decompiler reverse engineers executable Java code back into its original source code format. The document then provides examples of how different Java constructs like if/else statements, loops, and try/catch blocks are represented in bytecode. It also outlines the basic steps a decompiler takes, which are parsing the bytecode, recreating signatures, and converting it back into executable Java source code.
Java Decompiler
- 2. Contents • Introduction • Examples of base program • Steps to de-compile java • Input of the program • Algorithm • Output of the program 2 13-03-2015
- 3. Introduction • A de-compiler is a computer program that performs the reverse operation to that of a compiler. • The term de-compiler is most commonly applied to a program which translates executable programs (the output from a compiler) into source code in a high level language, when compiled, will produce an executable whose behavior is the same as the original executable program. 13-03-2015 3
- 4. Steps to de-compilation • Parse Java class file • Recreate signatures • Convert byte code to Java executable code Verify byte code Perform trivial conversion Reconstruct control structures Scope & Type Variables • Simplify code 13-03-2015 4
- 5. Examples of base program Consider the following example : Public static int plus(int a, int b) { int c = a+b; return c; } 13-03-2015 5
- 6. • Byte code : Public static int plus(int, int) Code : Stack=2, locals=3, arguments =2; 0 : iload_0 // load int a 1 : iload_1 // load int b 2 : 13-03-2015 6
- 7. • If- else : public int greaterThen(int intOne, int intTwo) { if (intOne > intTwo) { return 0; } else { return 1; } } 13-03-2015 7
- 8. • The above method results in following byte code : • 0: iload_1 • 1: iload_2 • 2: if_icmple 7 • 5: iconst_0 • 6: ireturn • 7:iconst_1 • 8: ireturn 13-03-2015 8
- 9. • Switch : public int simpleSwitch(int intOne) { switch (intOne) { case 0: return 3; case 1: return 2; case 4: return 1; default: return -1; } } 13-03-2015 9
- 10. • The above code produces following byte code : 0: iload_1 1: tableswitch { default: 42 min: 0 max: 4 0: 36 1: 38 2: 42 3: 42 4: 40 } 13-03-2015 10
- 11. 36: iconst_3 37: ireturn 38: iconst_2 39: ireturn 40: iconst_1 41: ireturn 42: iconst_m1 43: ireturn 13-03-2015 11
- 12. • While loop : public void whileLoop() { int i = 0; while (i < 2) { i++; } } 13-03-2015 12
- 13. • The above code produces following byte code : 0: iconst_0 1: istore_1 2: iload_1 3: iconst_2 4: if_icmpge 13 7: iinc 1, 1 10: goto 2 13: return 13-03-2015 13
- 14. • For loop : public void forLoop() { for(int i = 0; i < 2; i++) { } } The byte code produced by the for loop is similar to while loop. 13-03-2015 14
- 15. • Do- while : public void doWhileLoop() { int i = 0; do { i++; } while (i < 2); } 13-03-2015 15
- 16. • The above code generates the following byte code : 0: iconst_0 1: istore_1 2: iinc 1, 1 5: iload_1 6: iconst_2 7: if_icmplt 2 10: return 13-03-2015 16
- 17. • Try- catch block : public void tryCatchCatchFinally(int i) { Try { i = 2; } catch (RuntimeException e) { i = 3; } finally { i = 4; } } 13-03-2015 17
- 18. Input of the program • Input of the program is the byte code generated by Java Virtual Machine(JVM). Consider the following byte code : 0: iload_0 1: iload_1 2: iadd 3: istore_2 4 : iload_2 5 : return 13-03-2015 18
- 19. Algorithm • Algorithm for Java de-complier. 13-03-2015 19
- 20. Output of the program • The output of the de-complied byte code is java source code . 13-03-2015 20