Decompiled Java from APK - while loops that do nothing

Question

I decompiled an apk and see many while loops that return immediately only to be followed by other code:

while (true){
   return;
   if (!cond1){
     continue;
   }
   if (cond2){
     continue;
   }
}

If you wanted to produce this code in a decompile, what Java code would you write to get there?

Note. The decompile process is apktool -> baksmali -> smali -> dex2jar

EDIT

I can't actually get at the original Java bytecode from the Android APK (at least I don't know how to). It may be that my tools are doing a poor reverse-engineering job, but here is what the output of smali is:

:goto_8
return-void

.line 40
:sswitch_9
const/4 v0, 0x0

iput v0, p0, Lcom/sec/android/app/camera/command/ContextualTagSelectCommand;->mContextualTag:I

goto :goto_8

.line 44
:sswitch_d
const/4 v0, 0x1

iput v0, p0, Lcom/sec/android/app/camera/command/ContextualTagSelectCommand;->mContextualTag:I

goto :goto_8

Which corresponds to:

while (true)
{
  return;
  this.mContextualTag = 0;
  continue;
  this.mContextualTag = 1;
}

Yes, I know. This is the primary reason I asked the question. — Error 454, Aug 27 '12 at 16:56
@MarkoTopolnik: It might have been compiled using another compiler. — Mark Byers, Aug 27 '12 at 16:56
@Error454: can you show us the bytecode or whatever APK uses? Maybe the decompiler got confused? — Tomasz Nurkiewicz, Aug 27 '12 at 16:57
It looks like the decompilation of some goto instructions, but without a good inference of the actual Java code. The bigger picture may show that the line `if (!cond1)` is being jumped into from outside. — Marko Topolnik, Aug 27 '12 at 16:57
The bytecode is pretty much as expected: several gotos to the earlier point. Now look the wider context of the method code, there must be some gotos to :sswitch_9 and :sswitch_d. — Marko Topolnik, Aug 27 '12 at 17:35

score 2 · Accepted Answer · answered Aug 27 '12 at 18:09

2

Your bytecode is a fragment of a compiled switch statement. Specifically, it was implemented using the sparse switch Dalvik bytecode instruction. You are showing only two switch cases, where both assign to mContextualTag and then execute the return statement. This return is placed at a single point in the program and the switch cases jump to it. The labels sswitch_9 and sswitch_d may be indicating the relative offsets from the sswitch instruction itself.

answered Aug 27 '12 at 18:09

Marko Topolnik

195,646
29
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436

Yep. This is a switch instruction. The switch data (the cases and targets) will be at the bottom of the method, with a .sparse-switch directive, and there should be a sparse_switch instruction that references that switch data. – JesusFreke Aug 27 '12 at 23:41
@JesusFreke That agrees with my thinking. I imagined a catch-segment from regular Java, that also occurs at the bottom of the method. The decompiler should have inlined that code into the location of the `sswitch` instruction instead of trying to decompile it as normal program flow. – Marko Topolnik Aug 28 '12 at 07:38

Decompiled Java from APK - while loops that do nothing

1 Answers1