Re: Problems with Try/Catch Blocks

Well, i've been playing with this a little in my spare time and there is 
still a problem.  This may be due to my lack of experience in regards to 
Java's exception handling, but I don't understand why the code would 
compile and run when I manually change the ArrayList() constructor to a 
static method returning a new ArrayList, but not when I do it with 
BCEL.  For reference, here is the original code I wrote by hand for the 
test case:

-------------------------------------------------------------------------------------------------
    public static java.util.ArrayList getArrList();
     Code:
       0:   new     #2; //class java/util/ArrayList
       3:   dup
       4:   invokespecial   #3; //Method java/util/ArrayList."<init>":()V
       7:   areturn
   
    public static void main(java.lang.String[]);
      Code:
       0:   getstatic       #4; //Field 
java/lang/System.out:Ljava/io/PrintStream;
       3:   ldc     #5; //String test
       5:   invokevirtual   #6; //Method 
java/io/PrintStream.println:(Ljava/lang/String;)V
       8:   new     #2; //class java/util/ArrayList
       11:  dup
       12:  invokespecial   #3; //Method java/util/ArrayList."<init>":()V
       15:  astore_1
       16:  goto    20
       19:  astore_1
       20:  return
      Exception table:
       from   to  target type
         8    16    19   Class java/lang/Exception
----------------------------------------------------------------------------------
and the actual java that was compiled to whats up above:
    public static void main( String [] args ) {
        System.out.println( "test" );
        try {
            ArrayList arr = new ArrayList( );
        } catch ( Exception e ) {
        }
    }
-----------------------------------------------------------------------------------

Next, I ran it through my BCEL program that modifies it.  The output is 
exactly the same except for 13, which is now
13:  invokestatic    #41; //Method 
java/util/ArrayList.getArrList:(I)Ljava/util/ArrayList;
Here is the output again from the "after BCEL modification"

-------------------------------------------------------------------------------------
    public static void main(java.lang.String[]);
      Code:
       0:   getstatic       #4; //Field 
java/lang/System.out:Ljava/io/PrintStream;
       3:   ldc     #5; //String test
       5:   invokevirtual   #6; //Method 
java/io/PrintStream.println:(Ljava/lang/String;)V
       8:   new     #2; //class java/util/ArrayList
       11:  dup
       12:  invokestatic    #37; //Method 
java/util/ArrayList.getArrList:()Ljava/util/ArrayList;
       15:  astore_1
       16:  goto    20
       19:  astore_1
       20:  return
      Exception table:
       from   to  target type
         8    16    19   Class java/lang/Exception
------------------------------------------------------------------------------------------------


I'm confused because I don't see how changing that one line would cause 
the class to not be verified correctly.  I'm also stumped because when I 
change my code manually to the following, compile, and run, it works fine.
    try {
       ArrayList arr = getArrList();
    } catch ( Exception e ) {
    }


Is this a problem with BCEL or a problem with my lack of Exception 
knowledge?  I'm leaning towards the later, but I'm quite stumped.  If 
anyone has any clues or ideas on how I could solve this problem, I'd 
appreciate it very much.   Thank you for your time.

--vince






















Andrew Huntwork wrote:

> Vincent Marquez wrote:
>
>> method name = public static void main(String[] arg0)
>>    getstatic java.lang.System.out Ljava/io/PrintStream;
>>    ldc "test"
>>    invokevirtual java/io/PrintStream/println(Ljava/lang/String;)V
>>    new java/util/ArrayList
>>    dup
>>    invokespecial java/util/ArrayList/<init>()V
>>    astore_1
>>    goto -> 20
>
>
> is this right?
> try {
>
>>    astore_1
>>    getstatic java.lang.System.out Ljava/io/PrintStream;
>>    ldc "test test test"
>>    invokevirtual java/io/PrintStream/println(Ljava/lang/String;)V
>
> } catch(...
>
> is this offset 20?
>
>>    return
>> =======================================================
>
>
> if i'm right then your problem is...
>
>>
>> ===============  AFTER ==================================
>> method name = public void <init>()
>>    aload_0
>>    invokespecial java/lang/Object/<init>()V
>>    return
>> method name = public static java.util.ArrayList getArrList()
>>    new java/util/ArrayList
>>    dup
>>     invokestatic 
>> java/util/ArrayList/getArrList()Ljava/util/ArrayList; LoadClassType = 
>> java.util.ArrayList
>>    areturn
>> method name = public static void main(String[] arg0)
>>    getstatic java.lang.System.out Ljava/io/PrintStream;
>>    ldc "test"
>>    invokevirtual java/io/PrintStream/println(Ljava/lang/String;)V
>>    new java/util/ArrayList
>>    dup
>
>
> this dup ^.  It leaves an extra object on the stack that will be there 
> when you get to 20 via the goto -> 20 below but not if you get there 
> via the exception catch block.  the stack in an exception handler is 
> always just the exception object.  everything on the stack at the 
> instruction that threw the exception gets tossed.
>
>>     invokestatic 
>> java/util/ArrayList/getArrList()Ljava/util/ArrayList; LoadClassType = 
>> java.util.ArrayList
>>    astore_1
>>    goto -> 20
>>    astore_1
>>    getstatic java.lang.System.out Ljava/io/PrintStream;
>>    ldc "test test test"
>>    invokevirtual java/io/PrintStream/println(Ljava/lang/String;)V
>>    return
>> ====================================================================
>
>
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