timsort (original) (raw)

Martin Buchholz martinrb at google.com
Mon Jul 6 23:12:03 UTC 2009

No one actually said NO, but both Alan and Andrew strongly hinted that a backward compatibility mode would be good. So I kept the old implementation and allow it to be selectable via a system property. There's nothing more compatible than the legacy implementation.

/**
 * Old merge sort implementation can be selected (for
 * compatibility with broken comparators) using a system property.
 * Cannot be a static boolean in the enclosing class due to
 * circular dependencies.  To be removed in a future release.
 */
static final class LegacyMergeSort {
    private static final boolean userRequested =
        java.security.AccessController.doPrivileged(
            new sun.security.action.GetBooleanAction(
                "java.util.Arrays.useLegacyMergeSort")).booleanValue();
}

The sort method bodies now look like:

    if (LegacyMergeSort.userRequested)
        legacyMergeSort(a);
    else
        ComparableTimSort.sort(a);

New webrev at: http://cr.openjdk.java.net/~martin/webrevs/openjdk7/timsort/

If I don't hear from anyone soon (e.g. for CCC approval) I'll commit to the tl forest.

Martin

On Tue, Jun 30, 2009 at 11:28, Alan Bateman<Alan.Bateman at sun.com> wrote:

Martin Buchholz wrote:

: As you suggested, I added the Classpath exception wording to TimSort.java and ComparableTimSort.java. I excised the old mergesort code from Arrays.java. webrev regenerated. Thanks for doing this. Adding all-or-nothing wording would be good to add, perhaps to the class-level javadoc.  But it doesn't have to be part of this change. I brought it up because it looks like (and you can correct me) that the IAE may be thrown after some reordering of the array has taken place. This might be unexpected.

The JDK project has unusually high compatibility concerns. I and Josh believe the introduction of a possible IAE if the comparator doesn't satisfy its contract is the right thing, but we'd also be willing to change the code to swallow the IAE or to do so conditionally based on the value of a system property. Keep in mind that a call to foreign code can throw any throwable at all, so a rogue comparator can cause arbitrary behavior even today. I think most people would agree that that catching these otherwise-silent failures is a good thing. The problem is the customer that upgrades the JDK on a production system and gets burnt by some broken third party code that "used" to work. Having some way to restore existing behavior would make this an easier sell. The suggestion from Jason to change it to an assertion might be worth thinking about too but I suspect that few developers test with assertions enabled. -Alan.

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