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Use IdentityHashMap for consistent JIT optimization in AbstractCompositeMeter #6846

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Use IdentityHashMap for consistent JIT optimization in AbstractCompositeMeter #6846

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11 changes: 7 additions & 4 deletions ...er-core/src/main/java/io/micrometer/core/instrument/composite/AbstractCompositeMeter.java
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Original file line number Diff line number Diff line change
Expand Up @@ -20,17 +20,20 @@
import io.micrometer.core.instrument.MeterRegistry;
import org.jspecify.annotations.Nullable;

import java.util.Collections;
import java.util.IdentityHashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.concurrent.atomic.AtomicBoolean;

abstract class AbstractCompositeMeter<T extends Meter> extends AbstractMeter implements CompositeMeter {

private static final IdentityHashMap<MeterRegistry, Meter> EMPTY_CHILDREN = new IdentityHashMap<>(0);

private final AtomicBoolean childrenGuard = new AtomicBoolean();

private Map<MeterRegistry, T> children = Collections.emptyMap();
// Enforcing type of Map to explicitly be constrained to one type may help JIT optimizations.
@SuppressWarnings("unchecked")
private IdentityHashMap<MeterRegistry, T> children = (IdentityHashMap<MeterRegistry, T>) EMPTY_CHILDREN;

private volatile @Nullable T noopMeter;

Expand Down Expand Up @@ -72,7 +75,7 @@ public final void add(MeterRegistry registry) {
for (;;) {
if (childrenGuard.compareAndSet(false, true)) {
try {
Map<MeterRegistry, T> newChildren = new IdentityHashMap<>(children);
IdentityHashMap<MeterRegistry, T> newChildren = new IdentityHashMap<>(children);
newChildren.put(registry, newMeter);
this.children = newChildren;
break;
Expand All @@ -95,7 +98,7 @@ public final void remove(MeterRegistry registry) {
for (;;) {
if (childrenGuard.compareAndSet(false, true)) {
try {
Map<MeterRegistry, T> newChildren = new IdentityHashMap<>(children);
IdentityHashMap<MeterRegistry, T> newChildren = new IdentityHashMap<>(children);
newChildren.remove(registry);
this.children = newChildren;
break;
Expand Down