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; --- Copyright Jonathan Meyer 1996. All rights reserved. -----------------
; File: jasmin/examples/VerifyTest1.j
; Author: Jonathan Meyer, 10 July 1996
; Purpose: Trys to pull one on the verifier
; -------------------------------------------------------------------------
; This file illustrates the bytecode verifier at work - the
; code in the example() method below seems reasonable, but
; Java's bytecode verifier will fail the code because the two points leading
; to the Loop label (from the top of the method and from the ifne
; statement) have different stack states. Instead, a different approach
; must be adopted - e.g. by allocating an array, or simply writing:
;
; aconst_null
; aconst_null
; aconst_null
; aconst_null
; Note that many interpreters will run this code OK if you don't use
; a verifier. The code itself is well behaved (it doesn't trash the
; interpreter), but the approach it uses is disallowed by the verifier.
;
; Compile the example, then run it using:
;
; % java -verify VerifyTest1
; VERIFIER ERROR VerifyTest1.example()V:
; Inconsistent stack height 1 != 0
;
.class public examples/VerifyTest1
.super java/lang/Object
.method public <init>()V
aload_0
invokenonvirtual java/lang/Object/<init>()V
return
.end method
.method public example()V
.limit locals 2
.limit stack 10
; this tries to push four nulls onto the stack
; using a loop - Java's verifier will fail this program
iconst_4 ; store 4 in local variable 1 (used as a counter)
istore_1
Loop:
aconst_null ; push null onto the stack
iinc 1 -1 ; decrement local variable 4 (the counter variable)
iload_1
ifne Loop ; jump back to Loop unless the variable has reached 0
return
.end method
.method public static main([Ljava/lang/String;)V
; - do nothing : this is only to illustrate the bytecode verifier at work.
return
.end method
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