Reachability with a strict reading of the JLS

Question

Hi,
I have a small question, please tell me what you think.
Given the following codepublic class ReachableTest {
     public static void main (String[] args) {
          ExpensiveObject o = new ExpensiveObject ();
          byte[][] arr = new byte[100][];
          for (int i = 0; i < 100; i++) {
               arr[i] = new byte[1000000];
class ExpensiveObject {
     private final byte[] expensive = new byte[100000000];
     @Override
     public String toString () {
          return expensive.toString ();
}at which point is the ExpensiveObject eligible for garbage collection? Running the example with java -Xmx100m ReachableTest gives an OutOfMemoryError as the Sun JVM won't collect the variable o before the end of the method. My question is, given the following note in the JLS, could a conformant JVM collect the ExpensiveObject before the loop begins?
"A reachable object is any object that can be accessed in any potential continuing computation from any live thread. Optimizing transformations of a program can be designed that reduce the number of objects that are reachable to be less than those which would naively be considered reachable. For example, a compiler or code generator may choose to set a variable or parameter that will no longer be used to null to cause the storage for such an object to be potentially reclaimable sooner." http://java.sun.com/docs/books/jls/third_edition/html/execution.html#44762

Answer 1

My question
is, given the following note in the JLS, could a
conformant JVM collect the ExpensiveObject before the
loop begins?Yes. Indeed, with the code as given, a comformant JVM could conceivably not bother to instantiate ExpensiveObject in the first place, since its constructor has no side effects, and the reference is never used.
The new Java Memory Model, which is part of Java 1.5, somewhat changes the rules about reachability, particularly as regards references held in static fields, but it wouldn't affect your example.
Sylvia.