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Support filtering shared class-info by arbitrary predicates #16

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Merged
mcculls merged 2 commits into from
Oct 7, 2025
Merged

Support filtering shared class-info by arbitrary predicates #16

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0a4923d
Support filtering shared class-info by arbitrary predicates
mcculls Oct 7, 2025
9de9d48
Merge branch 'main' into mcculls/smart-class-loader-key-filtering
mcculls Oct 7, 2025
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42 changes: 42 additions & 0 deletions utils/src/main/java/datadog/instrument/utils/ClassInfoCache.java
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Original file line number Diff line number Diff line change
Expand Up @@ -8,6 +8,7 @@

import java.util.Arrays;
import java.util.concurrent.atomic.AtomicLong;
import java.util.function.IntPredicate;

/**
* Shares class information from multiple classloaders in a single cache.
Expand Down Expand Up @@ -132,6 +133,47 @@ public T find(CharSequence className, int classLoaderKeyId) {
}
}

/**
* Finds information for the given class-name, filtered by class-loader key.
*
* @param className the class-name
* @param classLoaderKeyFilter the filter for the class-loader key
* @return information shared under the class-name and filtered class-loader
* @see ClassLoaderIndex#getClassLoaderKeyId(ClassLoader)
*/
@SuppressWarnings("unchecked")
public T find(CharSequence className, IntPredicate classLoaderKeyFilter) {
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@mcculls mcculls Oct 7, 2025 •
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Note this is structurally very similar to the other find method, but I don't want to deduplicate the code for performance reasons. The other method uses a fast primitive operator inside the loop, whereas this involves a method call.

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@bric3 bric3 Oct 8, 2025 •
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Might be worth leaving a text comment here, for anyone opening this source file, and not having the PR context :)

final int hash = className.hashCode();
final SharedInfo[] shared = this.shared;
final int slotMask = this.slotMask;

// try to find matching slot, rehashing after each attempt
for (int i = 1, h = hash; true; i++, h = rehash(h)) {
int slot = slotMask & h;
SharedInfo existing = shared[slot];
if (existing != null) {
if (existing.className.contentEquals(className)) {
// apply filter to class-loader key, -1 always matches
if (existing.classLoaderKeyId < 0
|| classLoaderKeyFilter.test(existing.classLoaderKeyId)) {
// use global TICKS as a substitute for access time
// TICKS is only incremented in 'share' for performance reasons
existing.accessed = TICKS.get();
return (T) existing.classInfo;
}
// fall-through and quit; name matched but class-loader didn't
} else if (i < MAX_HASH_ATTEMPTS) {
continue; // rehash and try again
}
}
// quit search when:
// * we find an empty slot (we know there won't be further info)
// * we find a slot with the same name but different class-loader
// * we've exhausted all hash attempts
return null;
}
}

/**
* Shares information for the given class-name, under the given class-loader key.
*
Expand Down
38 changes: 38 additions & 0 deletions utils/src/test/java/datadog/instrument/utils/ClassInfoCacheTest.java
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Original file line number Diff line number Diff line change
Expand Up @@ -8,6 +8,7 @@

import java.util.HashSet;
import java.util.Set;
import java.util.function.IntPredicate;
import org.junit.jupiter.api.Test;

class ClassInfoCacheTest {
Expand All @@ -19,6 +20,11 @@ void basicOperation() {
ClassLoader myCL = newCL();
ClassLoader notMyCL = newCL();

int myCLKey = ClassLoaderIndex.getClassLoaderKeyId(myCL);

IntPredicate myCLFilter = sameCLKey(myCLKey);
IntPredicate notMyCLFilter = myCLFilter.negate();

assertNull(cache.find("example.test.MyGlobalClass"));
assertNull(cache.find("example.test.MyLocalClass"));
assertNull(cache.find("example.test.NotMyClass"));
Expand All @@ -31,6 +37,14 @@ void basicOperation() {
assertNull(cache.find("example.test.MyLocalClass", notMyCL));
assertNull(cache.find("example.test.NotMyClass", notMyCL));

assertNull(cache.find("example.test.MyGlobalClass", myCLFilter));
assertNull(cache.find("example.test.MyLocalClass", myCLFilter));
assertNull(cache.find("example.test.NotMyClass", myCLFilter));

assertNull(cache.find("example.test.MyGlobalClass", notMyCLFilter));
assertNull(cache.find("example.test.MyLocalClass", notMyCLFilter));
assertNull(cache.find("example.test.NotMyClass", notMyCLFilter));

cache.share("example.test.MyGlobalClass", "my global data");
cache.share("example.test.MyLocalClass", "my local data", myCL);

Expand All @@ -46,6 +60,14 @@ void basicOperation() {
assertNull(cache.find("example.test.MyLocalClass", notMyCL));
assertNull(cache.find("example.test.NotMyClass", notMyCL));

assertEquals("my global data", cache.find("example.test.MyGlobalClass", myCLFilter));
assertEquals("my local data", cache.find("example.test.MyLocalClass", myCLFilter));
assertNull(cache.find("example.test.NotMyClass", myCLFilter));

assertEquals("my global data", cache.find("example.test.MyGlobalClass", notMyCLFilter));
assertNull(cache.find("example.test.MyLocalClass", notMyCLFilter));
assertNull(cache.find("example.test.NotMyClass", notMyCLFilter));

cache.clear();

assertNull(cache.find("example.test.MyGlobalClass"));
Expand All @@ -59,6 +81,14 @@ void basicOperation() {
assertNull(cache.find("example.test.MyGlobalClass", notMyCL));
assertNull(cache.find("example.test.MyLocalClass", notMyCL));
assertNull(cache.find("example.test.NotMyClass", notMyCL));

assertNull(cache.find("example.test.MyGlobalClass", myCLFilter));
assertNull(cache.find("example.test.MyLocalClass", myCLFilter));
assertNull(cache.find("example.test.NotMyClass", myCLFilter));

assertNull(cache.find("example.test.MyGlobalClass", notMyCLFilter));
assertNull(cache.find("example.test.MyLocalClass", notMyCLFilter));
assertNull(cache.find("example.test.NotMyClass", notMyCLFilter));
}

@Test
Expand Down Expand Up @@ -111,12 +141,20 @@ void overflow() {
for (int i = 0; i < 300; i++) {
if (overwritten.contains(i)) {
assertNull(cache.find("example.MyClass" + i), "rem " + i);
assertNull(cache.find("example.MyClass" + i, i), "rem " + i);
assertNull(cache.find("example.MyClass" + i, sameCLKey(i)), "rem " + i);
} else {
assertEquals(i, cache.find("example.MyClass" + i), "add " + i);
assertEquals(i, cache.find("example.MyClass" + i, i), "add " + i);
assertEquals(i, cache.find("example.MyClass" + i, sameCLKey(i)), "add " + i);
}
}
}

private static IntPredicate sameCLKey(int clKey) {
return k -> k == clKey;
}

private static ClassLoader newCL() {
return new ClassLoader() {};
}
Expand Down