Consider the class public class Demo { public static void main(String[] args) { Object[] objs = new Integer[1]; objs[0] = o(); } private static Object o() { return null; } } It compiles fine, and java -verify Demo executes without errors. However, java org.apache.bcel.verifier.Verifier Demo complains: Pass 3b, method number 1 ['public static void main(String[] arg0)']: VERIFIED_REJECTED Constraint violated in method 'public static void main(String[] arg0)': Instruction AASTORE constraint violated: The type of 'value' ('java.lang.Object') is not assignment compatible to the components of the array 'arrayref' refers to. ('java.lang.Integer') etc. The problem is that while the vmspec2 says "The type of every value stored into an array of type reference by an aastore instruction must be assignment compatible (ยง2.6.7) with the component type of the array", the class file doesn't contain information on the compile-time component type of the array. I suspect the native verifier does nothing about this requirement, leaving the VM to throw an ArrayStoreException if problems arise. Enver Haase asked me to mention in this report that while the error message says "assignment compatible to" and the vmspec2 says "assignment compatible with", it's not a case of doing the check the wrong way round.
Here is my understanding of the problem: 1. Verification type of objs is Object[] 2. Run-time type of objs (in this particular case) is Integer[] 3. Verification type of the result of o() is Object Verification should pass, since it is purely concerned about verification type. However a run-time error should be thrown if o() return a non-null value. In this case since o() return null it shouldn't. scheng@innaworks.com
Not quite. A run-time error should be thrown if o() return a non-null value which is not an Integer.