JDK-8310913 Move ReferencedKeyMap to jdk.internal so it may be shared

[JimLaskey]

java.lang.runtime.ReferencedKeyMap was introduced to provide a concurrent caching scheme for Carrier objects. The technique used is generally useful for a variety of caching schemes and is being moved to be shared in other parts of the jdk. The MethodType interning case is one example.

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Issue

• JDK-8310913: Move ReferencedKeyMap to jdk.internal so it may be shared (Enhancement - P3)

Reviewers

• Naoto Sato (@naotoj - Reviewer) ⚠️ Review applies to a7217b66
• Roger Riggs (@RogerRiggs - Reviewer) ⚠️ Review applies to fbb78778
• Mandy Chung (@mlchung - Reviewer) ⚠️ Review applies to 8eff9183
• Chen Liang (@liach - Author) ⚠️ Review applies to fbb78778

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[mlbridge]

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[naotoj]

Looks fine to me. Good to see this class is being shared, as this can be used for refactoring the locale-related caches (#14404).

[naotoj]

Now that this is shared by other components, do we still need this warning?

[JimLaskey]

True that.

[openjdk]

@JimLaskey This change now passes all automated pre-integration checks.

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8310913: Move ReferencedKeyMap to jdk.internal so it may be shared

Reviewed-by: naoto, rriggs, mchung, liach

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[RogerRiggs]

LGTM

[mlchung]

MethodType's ConcurrentWeakInternSet uses NativeReferenceQueue which uses native monitors (that was added for virtual threads). This change uses the reference queue using j.u.c locks. Has you confirmed that this is not an issue? Or ReferencedKeySet can take a parameter to specify using the native reference queue?

In ReferencedKey::equals method, can use Reference::refersTo instead of Objects.equals.

[JimLaskey]

@mlchung I missed the use of newNativeReferenceQueue. This change has been sitting in my queue for a few years, I will adapt to the new reality. Thank you for pointing it out.

[mlchung]

WeakReferenceKey::equals and SoftReferenceKey::equals:

Instead of return Objects.equals(get(), obj);, suggest to do:

    return refersTo(obj);

[JimLaskey]

@mlchung Not sure I catch the nuance. refersTo doesn't work if the search key is constructed. For example, if the key reference is a record Pair(int a, int b) {}, then map.containsKey(new Pair(10, 20)) would locate with equals but not with refersTo.

[RogerRiggs]

@mlchung Not sure I catch the nuance. refersTo doesn't work if the search key is constructed. For example, if the key reference is a record Pair(int a, int b) {}, then map.containsKey(new Pair(10, 20)) would locate with equals but not with refersTo.

Using get() on a reference can momentarily create a strong reference to the item. refersTo() does not.
But since your map is holding proxies to the object, refersTo() would be to the proxy, not the object; so that's not an option.

[mlchung]

@RogerRiggs WeakReferenceKey is a WeakReference with the key as the referent. I might be missing something?

[RogerRiggs]

@RogerRiggs WeakReferenceKey is a WeakReference with the key as the referent. I might be missing something?

The comparison of the unwrapped key using refersTo is an identity comparison; but the usual comparison for Maps uses equals. Using refersTo would limit the use of ReferencedKeyMap to cases where the original key requires identity comparision. (I missed Jim's earlier comment that pointed this out.)

[mlchung]

thanks. It may worth adding a comment about equality comparison and can't use refersTo

[mlchung]

Looks good. Thanks for the update. Nit: the test can use the 2-arg factory method.

[liach]

Thank you for the updates.

[JimLaskey]

/integrate

[openjdk]

Going to push as commit 6af0af5.
Since your change was applied there have been 7 commits pushed to the master branch:

• 86783b9: 8301996: Move field resolution information out of the cpCache
• 5362ec9: 8312492: Remove THP sanity checks at VM startup
• e47a84f: 8312489: Increase jdk.jar.maxSignatureFileSize default which is too low for JARs such as WhiteSource/Mend unified agent jar
• 78f6799: 8293972: runtime/NMT/NMTInitializationTest.java#default_long-off failed with "Suspiciously long bucket chains in lookup table."
• 97b6883: 8295059: test/langtools/tools/javap 12 test classes use com.sun.tools.classfile library
• 3671d83: 8313252: Java_sun_awt_windows_ThemeReader_paintBackground release resources in early returns
• b60e0ad: 8313207: Remove MetaspaceShared::_has_error_classes

Your commit was automatically rebased without conflicts.

[openjdk]

@JimLaskey Pushed as commit 6af0af5.

💡 You may see a message that your pull request was closed with unmerged commits. This can be safely ignored.

[ExE-Boss]

This line is wrong, as intern(…) will never return null.

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This is the closest to the correct implementation,

Suggested change

	return intern(e) == null;
	return !contains(e) & intern(e) == e;

but may incorrectly return true in case of the following data race, assuming t1e == t2e:

1. Thread 1 calls contains(t1e) and gets false.
2. Thread 2 calls intern(t2e) and successfully adds t2e.
3. Thread 1 calls intern(t1e) and gets back t2e, which is == to t1e.
4. Thread 1 returns true, even though it was Thread 2 which modified the ReferencedKeySet.

[JimLaskey]

Good catch. Your solution might be correct but I think !contains(e) is redundant since that is how intern starts out.

   static <T> T intern(ReferencedKeyMap<T, ReferenceKey<T>> setMap, T key, UnaryOperator<T> interner) {
        T value = existingKey(setMap, key);
        if (value != null) {
            return value;
        }
        key = interner.apply(key);
        return internKey(setMap, key);
    }

Agree? So changing to return intern(e) == e; should be sufficient.

The other aspect of this is that internKey uses putIfAbsent which should prevent the race (assuming ConcurrentHashMap).

    /**
     * Attempt to add key to map.
     *
     * @param setMap    {@link ReferencedKeyMap} where interning takes place
     * @param key       key to add
     *
     * @param <T> type of key
     *
     * @return the old key instance if found otherwise the new key instance
     */
    private static <T> T internKey(ReferencedKeyMap<T, ReferenceKey<T>> setMap, T key) {
        ReferenceKey<T> entryKey = setMap.entryKey(key);
        T interned;
        do {
            setMap.removeStaleReferences();
            ReferenceKey<T> existing = setMap.map.putIfAbsent(entryKey, entryKey);
            if (existing == null) {
                return key;
            } else {
                // If {@code putIfAbsent} returns non-null then was actually a
                // {@code replace} and older key was used. In that case the new
                // key was not used and the reference marked stale.
                interned = existing.get();
                if (interned != null) {
                    entryKey.unused();
                }
            }
        } while (interned == null);
        return interned;
    }

Agree? Anyone else want to pipe in?

[RogerRiggs]

ok, intern(e) == e is sufficient.

[JimLaskey]

Created https://bugs.openjdk.org/browse/JDK-8325255

[ExE-Boss]

While intern(e) == e is (mostly) sufficient, it will return a false positive when add(e) is called and e is already present in the map, the correctest implementation would be some internal API in ReferencedKeyMap for implementing ReferencedKeySet::add.