Is unused object available for garbage collection when it's still visible in stack?

Question

In the following example there are two functionally equivalent methods:

public class Question {

    public static String method1() {
        String s = new          


        

Answer 1

If you're talking about the interpreter, then in the second case S1 remains "referenced" until the method exits and the stack frame is rolled up. (That is, in the standard interpreter -- it's entirely possible for GC to use liveness info from method verification. And, in addition (and more likely), javac may do its own liveness analysis and "share" interpreter slots based on that.)

In the case of the JITC, however, an even mildly optimizing one might recognize that S1 is unused and recycle that register for S2. Or it might not. The GC will examine register contents, and if S1 has been reused for something else then the old S1 object will be reclaimed (if not otherwise referenced). If the S1 location has not been reused then the S1 object might not be reclaimed.

"Might not" because, depending on the JVM, the JITC may or may not provide the GC with a map of where object references are "live" in the program flow. And this map, if provided, may or may not precisely identify the end of the "live range" (the last point of reference) of S1. Many different possibilities.

Note that this potential variability does not violate any Java principles -- GC is not required to reclaim an object at the earliest possible opportunity, and there's no practical way for a program to be sensitive to precisely when an object is reclaimed.