Java Bytecode Verification impossible [2]

Name: skT45625 Date: 10/23/2000


java version "1.3.0"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.3.0)
Java HotSpot(TM) Client VM (build 1.3.0, mixed mode)


The following legal Java program is rejected by any Java Bytecode
verifier I have tried. The program shows that Java Bytecode Verification
is not possible as described in the Java Virtual Machine Specification.

Remark: The example program shows a different problem than in my
previous bug report. There are no "break" statements and labels involved
in this program.

public class Test {
  int test(boolean b) {
    int i;
    try {
      if (b) return 1;
      i = 2;
    } finally {
      if (b) i = 3;
    }
    return i;
  }
}

tomis> java -version
java version "1.3.0"
Java(TM) 2 Runtime Environment, Standard Edition (build 1.3.0)
Java HotSpot(TM) Client VM (build 1.3.0, mixed mode)
tomis> javac Test.java
tomis> java Test
Exception in thread "main" java.lang.VerifyError: (class: Test, method: test
signature: (Z)I) Register 2 contains wrong type

What is the problem? The variable i is modified by the subroutine S
corresponding to the finally block. The variable i, however, is
"unusable" at the end of S, since S is called before the "return 1" and
after the try block. When S is called before the "return 1" variable i
is "unusable" and it remains "unusable", since there is a path from the
beginning to the end of S which does not have an "istore i"
instruction. Hence, at the of the try block, the variable "i" is
unusable in the eyes of the bytecode verifier and therefore also at the
end of the method at the "return i" instruction.

Solution? Restrict the "rules of definite assignment" in the JLS for
16.2.14 "try statement" such that the above program is no longer legal.

 
http://java.sun.com/docs/books/jls/second_edition/html/defAssign.doc.html#26242

  V is definitely assigned after the try statement iff the following
  is true:
     V is definitely assigned after the finally block.
(Review ID: 111235)
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