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javac: inconsistency in created code
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Posted: Jan 30, 2014 12:25 AM
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maybe there is a specialist for java internals (Bill?). I have a problem understanding the compiled code for the following java code
public class TestException
{
public static void main(String[] args)
{
StringBuilder sb = null;
try
{
sb = new StringBuilder(null);
}
catch (NullPointerException t)
{
}
if (sb != null)
{
System.out.print(sb.toString());
}
}
}
This is compiled to:
stack=3, locals=3, args_size=1
0: aconst_null
1: astore_1
2: new #2 // class java/lang/StringBuilder
5: dup
6: aconst_null
7: invokespecial #3 // Method java/lang/StringBuilder."<init>":(Ljava/lang/String;)V
10: astore_1
11: goto 15
14: astore_2
15: aload_1
16: ifnull 29
19: getstatic #5 // Field java/lang/System.out:Ljava/io/OutStream;
22: aload_1
23: invokevirtual #6 // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
26: invokevirtual #7 // Method java/io/OutStream.print:(Ljava/lang/String;)V
29: return
Exception table:
from to target type
2 11 14 Class java/lang/NullPointerException
My problem: at line 7, the stack content is
(top-2) ref to StringBuilder instance
(top-1) ref to StringBuilder instance
(top) null
After invoking StringBuilder(String), the last two arguments will be removed, so stack contains only
(top) ref to StringBuilder instance
This is ok for normal execution, but when having an exception (catch starts at address 14), stack will look like
(top-1) ref to StringBuilder instance
(top) ref to Exception instance
After storing the exception ref in local var 2, so at end of catch, there is still the ref to the stringbuilder left on stack.
Can someone point out where I go wrong?
Background:
I try to write a jvm, and this let grow the stack over it's estimated size (with side effects to data behind this stack frame...
Or do I really need to do a special handing for this case (new, dup, <init>) in the exception handling?
Thanks, Gunter
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