From b0818b6a7c96296d89e4630a5bc6a59fcb1a4f2c Mon Sep 17 00:00:00 2001
From: Jack Berg <jberg@newrelic.com>
Date: Tue, 7 Feb 2023 15:41:13 -0600
Subject: [PATCH] Avoid exemplar allocations if there are no measurements

---
 .../internal/exemplar/FixedSizeExemplarReservoir.java      | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/internal/exemplar/FixedSizeExemplarReservoir.java b/sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/internal/exemplar/FixedSizeExemplarReservoir.java
index a5b65382b59..05dfdd1afd3 100644
--- a/sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/internal/exemplar/FixedSizeExemplarReservoir.java
+++ b/sdk/metrics/src/main/java/io/opentelemetry/sdk/metrics/internal/exemplar/FixedSizeExemplarReservoir.java
@@ -20,6 +20,7 @@ abstract class FixedSizeExemplarReservoir<T extends ExemplarData> implements Exe
   private final ReservoirCell[] storage;
   private final ReservoirCellSelector reservoirCellSelector;
   private final BiFunction<ReservoirCell, Attributes, T> mapAndResetCell;
+  private volatile boolean hasMeasurements = false;
 
   /** Instantiates an exemplar reservoir of fixed size. */
   FixedSizeExemplarReservoir(
@@ -40,6 +41,7 @@ public void offerLongMeasurement(long value, Attributes attributes, Context cont
     int bucket = reservoirCellSelector.reservoirCellIndexFor(storage, value, attributes, context);
     if (bucket != -1) {
       this.storage[bucket].recordLongMeasurement(value, attributes, context);
+      this.hasMeasurements = true;
     }
   }
 
@@ -48,11 +50,15 @@ public void offerDoubleMeasurement(double value, Attributes attributes, Context
     int bucket = reservoirCellSelector.reservoirCellIndexFor(storage, value, attributes, context);
     if (bucket != -1) {
       this.storage[bucket].recordDoubleMeasurement(value, attributes, context);
+      this.hasMeasurements = true;
     }
   }
 
   @Override
   public List<T> collectAndReset(Attributes pointAttributes) {
+    if (!hasMeasurements) {
+      return Collections.emptyList();
+    }
     // Note: we are collecting exemplars from buckets piecemeal, but we
     // could still be sampling exemplars during this process.
     List<T> results = new ArrayList<>();
@@ -63,6 +69,7 @@ public List<T> collectAndReset(Attributes pointAttributes) {
       }
     }
     reservoirCellSelector.reset();
+    this.hasMeasurements = false;
     return Collections.unmodifiableList(results);
   }
 }