Reduce iterator allocations on hot paths (#10745)

Author: johnbley

docs/contributing/style-guideline.md

@@ -137,7 +137,14 @@ It is ok to use `Optional` in places where it does not leak into public API sign
 Also, avoid `Optional` usage on the hot path (instrumentation code), unless the instrumented library
 itself uses it.
 
-## `java.util.stream.Stream` usage
-
-Avoid streams on the hot path (instrumentation code), unless the instrumented library itself uses
-them.
+## Hot path constraints
+
+Avoid allocations whenever possible on the hot path (instrumentation code).
+This includes `Iterator` allocations from collections; note that
+`for(SomeType t : plainJavaArray)` does not allocate an iterator object.
+Non-allocating stream api usage on the hot path is acceptable but may not
+fit the surrounding code style; this is a judgement call.  Note that
+some stream apis make it much more difficult to allocate efficiently
+(e.g., `collect` with presized sink data structures involves
+convoluted `Supplier` code, or lambdas passed to `forEach` might be
+capturing/allocating lambdas).

instrumentation-api/src/main/java/io/opentelemetry/instrumentation/api/instrumenter/Instrumenter.java

@@ -16,9 +16,6 @@
 import io.opentelemetry.instrumentation.api.internal.InstrumenterUtil;
 import io.opentelemetry.instrumentation.api.internal.SupportabilityMetrics;
 import java.time.Instant;
-import java.util.ArrayList;
-import java.util.List;
-import java.util.ListIterator;
 import java.util.concurrent.TimeUnit;
 import javax.annotation.Nullable;
 
@@ -72,25 +69,25 @@ public static <REQUEST, RESPONSE> InstrumenterBuilder<REQUEST, RESPONSE> builder
   private final SpanNameExtractor<? super REQUEST> spanNameExtractor;
   private final SpanKindExtractor<? super REQUEST> spanKindExtractor;
   private final SpanStatusExtractor<? super REQUEST, ? super RESPONSE> spanStatusExtractor;
-  private final List<? extends SpanLinksExtractor<? super REQUEST>> spanLinksExtractors;
-  private final List<? extends AttributesExtractor<? super REQUEST, ? super RESPONSE>>
-      attributesExtractors;
-  private final List<? extends ContextCustomizer<? super REQUEST>> contextCustomizers;
-  private final List<? extends OperationListener> operationListeners;
+  private final SpanLinksExtractor<? super REQUEST>[] spanLinksExtractors;
+  private final AttributesExtractor<? super REQUEST, ? super RESPONSE>[] attributesExtractors;
+  private final ContextCustomizer<? super REQUEST>[] contextCustomizers;
+  private final OperationListener[] operationListeners;
   private final ErrorCauseExtractor errorCauseExtractor;
   private final boolean enabled;
   private final SpanSuppressor spanSuppressor;
 
+  @SuppressWarnings({"rawtypes", "unchecked"})
   Instrumenter(InstrumenterBuilder<REQUEST, RESPONSE> builder) {
     this.instrumentationName = builder.instrumentationName;
     this.tracer = builder.buildTracer();
     this.spanNameExtractor = builder.spanNameExtractor;
     this.spanKindExtractor = builder.spanKindExtractor;
     this.spanStatusExtractor = builder.spanStatusExtractor;
-    this.spanLinksExtractors = new ArrayList<>(builder.spanLinksExtractors);
-    this.attributesExtractors = new ArrayList<>(builder.attributesExtractors);
-    this.contextCustomizers = new ArrayList<>(builder.contextCustomizers);
-    this.operationListeners = builder.buildOperationListeners();
+    this.spanLinksExtractors = builder.spanLinksExtractors.toArray(new SpanLinksExtractor[0]);
+    this.attributesExtractors = builder.attributesExtractors.toArray(new AttributesExtractor[0]);
+    this.contextCustomizers = builder.contextCustomizers.toArray(new ContextCustomizer[0]);
+    this.operationListeners = builder.buildOperationListeners().toArray(new OperationListener[0]);
     this.errorCauseExtractor = builder.errorCauseExtractor;
     this.enabled = builder.enabled;
     this.spanSuppressor = builder.buildSpanSuppressor();
@@ -193,12 +190,12 @@ private Context doStart(Context parentContext, REQUEST request, @Nullable Instan
     Span span = spanBuilder.setParent(context).startSpan();
     context = context.with(span);
 
-    if (!operationListeners.isEmpty()) {
+    if (operationListeners.length != 0) {
       // operation listeners run after span start, so that they have access to the current span
       // for capturing exemplars
       long startNanos = getNanos(startTime);
-      for (OperationListener operationListener : operationListeners) {
-        context = operationListener.onStart(context, attributes, startNanos);
+      for (int i = 0; i < operationListeners.length; i++) {
+        context = operationListeners[i].onStart(context, attributes, startNanos);
       }
     }
 
@@ -231,12 +228,10 @@ private void doEnd(
     }
     span.setAllAttributes(attributes);
 
-    if (!operationListeners.isEmpty()) {
+    if (operationListeners.length != 0) {
       long endNanos = getNanos(endTime);
-      ListIterator<? extends OperationListener> i =
-          operationListeners.listIterator(operationListeners.size());
-      while (i.hasPrevious()) {
-        i.previous().onEnd(context, attributes, endNanos);
+      for (int i = operationListeners.length - 1; i >= 0; i--) {
+        operationListeners[i].onEnd(context, attributes, endNanos);
       }
     }
 

instrumentation-api/src/main/java/io/opentelemetry/instrumentation/api/instrumenter/SpanSuppressors.java

@@ -63,10 +63,10 @@ public boolean shouldSuppress(Context parentContext, SpanKind spanKind) {
 
   static final class BySpanKey implements SpanSuppressor {
 
-    private final Set<SpanKey> spanKeys;
+    private final SpanKey[] spanKeys;
 
     BySpanKey(Set<SpanKey> spanKeys) {
-      this.spanKeys = spanKeys;
+      this.spanKeys = spanKeys.toArray(new SpanKey[0]);
     }
 
     @Override

instrumentation-api/src/main/java/io/opentelemetry/instrumentation/api/semconv/http/CapturedHttpHeadersUtil.java

@@ -5,8 +5,6 @@
 
 package io.opentelemetry.instrumentation.api.semconv.http;
 
-import static java.util.Collections.unmodifiableList;
-
 import io.opentelemetry.api.common.AttributeKey;
 import java.util.List;
 import java.util.Locale;
@@ -24,9 +22,11 @@ final class CapturedHttpHeadersUtil {
   private static final ConcurrentMap<String, AttributeKey<List<String>>> responseKeysCache =
       new ConcurrentHashMap<>();
 
-  static List<String> lowercase(List<String> names) {
-    return unmodifiableList(
-        names.stream().map(s -> s.toLowerCase(Locale.ROOT)).collect(Collectors.toList()));
+  static String[] lowercase(List<String> names) {
+    return names.stream()
+        .map(s -> s.toLowerCase(Locale.ROOT))
+        .collect(Collectors.toList())
+        .toArray(new String[0]);
   }
 
   static AttributeKey<List<String>> requestAttributeKey(String headerName) {

instrumentation-api/src/main/java/io/opentelemetry/instrumentation/api/semconv/http/HttpCommonAttributesExtractor.java

@@ -37,8 +37,8 @@ abstract class HttpCommonAttributesExtractor<
 
   final GETTER getter;
   private final HttpStatusCodeConverter statusCodeConverter;
-  private final List<String> capturedRequestHeaders;
-  private final List<String> capturedResponseHeaders;
+  private final String[] capturedRequestHeaders;
+  private final String[] capturedResponseHeaders;
   private final Set<String> knownMethods;
 
   HttpCommonAttributesExtractor(