Re: Problems with Try/Catch Blocks (yay! thanks!)

Andrew,

Thank you very much for your patience.  Your link and explanation were 
exactly what I needed to finally get what I was doing wrong and fix the 
problem.    The classfile section of the VMSpec really helped me 
understand what happens when objects are created.   Again, thanks for 
your help. 

--vince


Andrew Huntwork wrote:

> because you _didn't_ do it with bcel.  You seem to think that changing
>
> invokespecial ArrayList.<init>
> to
> invokestatic ArrayList.getArrList
> is the same as changing
> new ArrayList()
> to
> ArrayList.getArrList
>
> It's not.
> new ArrayList()
> compiles to
> new ArrayList
> dup
> invokespecial ArrayList.<init>
>
> (as shown here: 
> http://java.sun.com/docs/books/vmspec/2nd-edition/html/ClassFile.doc.html#9839 
>
> )
>
> ArrayList.getArrList()
> compiles to
> invokestatic ArrayList.getArrList
>
> So you can't expect to replace 1 instruction from the first sequence 
> with the second sequence and expect things to be ok.  replace the 
> whole sequence, or at least pop the extra ArrayLists off the stack.
>
> Vincent Marquez wrote:
>
>> 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
>>
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>> 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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>>>
>>
>>
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