Is it allowed for unused variables to be optimized away by the compiler?

Question

Let's assume I have the following code:
public void myMethod() {
   final Object x = new Object();
   // I never refer to x.
   // other code, for example:
   System.out.println("lala");
}The question I have is: what is the earliest possible moment that x can be garbage collected?
I have checked the JLS and JVM spec, but I cannot find the answer. It seems to me, that the assignment to x causes a local variable slot in the stack frame to be assigned a reference to x, and therefore, x cannot be garbage collected until after the method returns since there is a strong reference to x during the entire method call. But maybe the compiler might be allowed to compile away the unused local variable, resulting simply in the code: new Object(), which would then create an object which immediately has no strong references, and can be collected before the println. Does anybody know the correct answer?
Regards,
Sebastiaan

Answer 1

sebster wrote:
On what do you base this assertion? Do you have any references?The problem is that I can't remember if I read it in a document related to garbage collection or if it was in the VM specification (the language specification does not specify that a GC must exist if I remember it correctly).
One thing that shows what I told you currently is true is that this code:
public class ByteCode {
    public void myMethod() {
        final Object x = new Object();
        System.out.println("lala");
}Compiles into this using the Sun compiler for Java 6.
public ByteCode();
  Code:
   Stack=1, Locals=1, Args_size=1
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   return
  LineNumberTable:
   line 21: 0
public void myMethod();
  Code:
   Stack=2, Locals=2, Args_size=1
   0:   new     #2; //class java/lang/Object
   3:   dup
   4:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   7:   astore_1
   8:   getstatic       #3; //Field java/lang/System.out:Ljava/io/PrintStream;
   11:  ldc     #4; //String lala
   13:  invokevirtual   #5; //Method java/io/PrintStream.println:(Ljava/lang/String;)V
   16:  return
  LineNumberTable:
   line 23: 0
   line 25: 8
   line 27: 16
}Note the call to astore_1 that is the assignment to the local variable.
Kaj
Ps. looks like the forum has a bug in the handling of the pre tag. :(
Edited by: kajbj on Feb 14, 2008 5:09 PM