javadecompiler

how to identify ternary operator from java's bytecode

i am working on a java’s bytecode project, and need to identify ternary operator and nested ternary operator.

I got two questions

  1. how to determine an if statement is ternary operator?based on stack variables?
  2. how to determine an if statement’ s consumed value on stack oprand is a ternary operator’s result

example:

((a>b ? 0 : 1 ) > (a > c ? 10 : 20) ? 100 : 101

here is a full example of ternary operator chain:

source code:

public void ddddd()
{
    int m,k, n, z;
    Random r = new Random();
    boolean p = (r.nextInt() > 100 || (30 > r.nextInt() ? (r.nextInt() > 4000 ? 1 : 0) : (r.nextInt() > 2000 ? 100 : 20)) <  (r.nextInt() > 1000 ? 0 : 1));
}

structed bytecode:

[

   7          astore] java/util/Random var1(1) = <init>(v0)
 [   9           aload] java/util/Random var1(1) = (stack_var)var1
 [  11   invokevirtual] I v11 = var1.nextInt()
 [  14          bipush] (I)v14 = 100
 if { // block_id: 16 16 => 89  parent_id: 0
    [  16       if_icmpgt] v11 > v14 : goto => 85
    if_false_block { // block_id: 15 19 => 89  parent_id: 16
        [  19          bipush] (I)v19 = 30
        [  21           aload] java/util/Random var1(1) = (stack_var)var1
        [  23   invokevirtual] I v23 = var1.nextInt()
        if { // block_id: 9 26 => 64  parent_id: 15
            [  26       if_icmple] v19 <= v23 : goto => 48
            if_false_block { // block_id: 8 29 => 45  parent_id: 9
                [  29           aload] java/util/Random var1(1) = (stack_var)var1
                [  31   invokevirtual] I v31 = var1.nextInt()
                [  34          sipush] (I)v34 = 4000
                if { // block_id: 6 37 => 45  parent_id: 8
                    [  37       if_icmple] v31 <= v34 : goto => 44
                    if_false_block { // block_id: 5 40 => 41  parent_id: 6
                        [  40        iconst_1] (I)v40 = 1
                        [  41            goto] goto: 66
                    }
                    if_true_block { // block_id: 4 44 => 45  parent_id: 6
                        [  44        iconst_0] (I)v44 = 0
                        [  45            goto] goto: 66
                    }
                }
            }
            if_true_block { // block_id: 7 48 => 64  parent_id: 9
                [  48           aload] java/util/Random var1(1) = (stack_var)var1
                [  50   invokevirtual] I v50 = var1.nextInt()
                [  53          sipush] (I)v53 = 2000
                if { // block_id: 3 56 => 64  parent_id: 7
                    [  56       if_icmple] v50 <= v53 : goto => 64
                    if_false_block { // block_id: 2 59 => 61  parent_id: 3
                        [  59          bipush] (I)v59 = 100
                        [  61            goto] goto: 66
                    }
                    if_true_block { // block_id: 1 64 => 64  parent_id: 3
                        [  64          bipush] (I)v64 = 20
                    }
                }
            }
        }
        [  66           aload] java/util/Random var1(1) = (stack_var)var1
        [  68   invokevirtual] I v68 = var1.nextInt()
        [  71          sipush] (I)v71 = 1000
        if { // block_id: 12 74 => 81  parent_id: 15
            [  74       if_icmple] v68 <= v71 : goto => 81
            if_false_block { // block_id: 11 77 => 78  parent_id: 12
                [  77        iconst_0] (I)v77 = 0
                [  78            goto] goto: 82
            }
            if_true_block { // block_id: 10 81 => 81  parent_id: 12
                [  81        iconst_1] (I)v81 = 1
            }
        }
        if { // block_id: 14 82 => 89  parent_id: 15
            [  82       if_icmpge] v40 >= v77 : goto => 89
            [  85        iconst_1] (I)v85 = 1
            [  86            goto] goto: 90
            if_true_block { // block_id: 13 89 => 89  parent_id: 14
                [  89        iconst_0] (I)v89 = 0
            }
        }
    }
 }
 [  90          istore] I var2(1) = (stack_var)var2
 [  92          return] return
Solution

  • The reason why I want to identify the ternary operator is that I need to covert the bytecode to the source code. A single ternary operator is easier to handle, and normally the result of the ternary operator will be stored or returned, but nested ternary operator chain difficult to identify.

    The method I have founded is not something like SSA FORM. Instead, guess based on the control flow graph

    A single ternary operator is identified according to the control flow graph. Whether a ternary operator can be contracted to one line depends on whether there is an uncontracted ternary operator among the children in the control flow graph. This recursive processing can shrink a nested type of ternary operator into one line.

    And many many thanks user85421 told me that new versions java alreay make switch as multiple return operator on operand stack.