On Sat, 19 Mar 2022 19:34:23 GMT, Chris Plummer <cjplum...@openjdk.org> wrote:
> `isInNewGen()` is throwing an NPE because "heap" is null:
>
>
> public boolean isInNewGen() {
> return ((gen != null) && (gen == ((GenCollectedHeap)heap).getGen(0)));
> }
>
>
> The call came from here:
>
>
> } else if (isInHeap()) {
> if (isInTLAB()) {
> tty.print("In thread-local allocation buffer for thread (");
> getTLABThread().printThreadInfoOn(tty);
> tty.print(") ");
> getTLAB().printOn(tty); // includes "\n"
> } else {
> if (isInNewGen()) {
> tty.print("In new generation ");
> } else if (isInOldGen()) {
> tty.print("In old generation ");
> } else {
> tty.print("In unknown section of Java heap");
> }
> if (getGeneration() != null) {
> getGeneration().printOn(tty); // does not include "\n"
> }
> tty.println();
> }
>
>
> `isInHeap()` returns true if either "heap" or "gen" is non-null. If "gen" is
> non-null and "heap" is null, it is not safe to call `isInNewGen()`. Yet you
> can see from the code above that when `isInNewGen()` is called, the only
> guarantee is that "heap" or "gen" is non-null, not both, and it turns out
> that `PointerFinder.find()` only sets "gen" when the ptr is found to be in a
> generation of the heap. It does not set "heap". So the logic in
> `PointerFinder.find()` is not in agreement with the logic in
> `PointerLocation.printOn()` w.r.t. to the setting up and meaning of the "gen"
> and "heap" fields.
>
> The solution is pretty straight forward. Always set "heap" when the ptr is in
> the heap, even if it is in a generation (in which case "gen" is also set) or
> in a tlab (in which case "tlab" is also set). `isInHeap()` no longer requires
> that "gen" also be set, just "heap".
>
> I also noticed that the printlns in the following were not being triggered:
>
>
> if (isInNewGen()) {
> tty.print("In new generation ");
> } else if (isInOldGen()) {
> tty.print("In old generation ");
> } else {
>
>
> This was true even though "gen" was set and was indeed either pointing to the
> new gen or old gen. It turns out that the following returns a newly created
> Generation object:
>
> ` ((GenCollectedHeap)heap).getGen(0)`
>
> For this reason `equals()` must be used to compare them, not ==. The
> `VMObject.equals()` method is what ends up being called, and it compares the
> address associated with the underlying `VMObject`.
Thanks for the reviews!
-------------
PR: https://git.openjdk.java.net/jdk/pull/7873
