JAVA DE-COMPLIER

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Contents• Introduction

• Examples of base program

• Steps to de-compile java

• Input of the program

• Algorithm

• Output of the program

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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.

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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

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Examples of base program

Consider the following example :

Public static int plus(int a, int b)

{

int c = a+b;

return c;

}

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• 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 :

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• If- else :

public int greaterThen(int intOne, int intTwo)

{

if (intOne > intTwo)

{

return 0;

}

else

{

return 1;

}

}

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• 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

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• Switch :

public int simpleSwitch(int intOne)

{

switch (intOne)

{

case 0: return 3;

case 1: return 2;

case 4: return 1;

default: return -1;

}

}

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• 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

}

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36: iconst_3

37: ireturn

38: iconst_2

39: ireturn

40: iconst_1

41: ireturn

42: iconst_m1

43: ireturn

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• While loop :

public void whileLoop()

{

int i = 0;

while (i < 2)

{

i++;

}

}

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• 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

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• 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.

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• Do- while :

public void doWhileLoop()

{

int i = 0;

do

{

i++;

}

while (i < 2);

}

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• 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

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• Try- catch block :public void tryCatchCatchFinally(int i){ Try{i = 2; }catch (RuntimeException e){i = 3;} finally {i = 4;} }

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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

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Algorithm

• Algorithm for Java de-complier.

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Output of the program

• The output of the de-complied byte code is java source code .

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Thank You

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