perf: eliminate UnmodifiableList overhead and add HttpHeaders toLowerCase fast-path

Change 1: Replace vendored UnmodifiableList with Collections.unmodifiableList()
- UnmodifiableList wraps via 4-level virtual dispatch chain
  (UnmodifiableList -> AbstractSerializableListDecorator ->
   AbstractListDecorator -> AbstractCollectionDecorator)
- AbstractListDecorator.decorated() called on every list operation
  accounts for ~1.02% CPU (398 samples at 1t-c128)
- Collections.unmodifiableList() returns a simple wrapper that
  JIT can inline efficiently

Change 2: Add isAsciiLowerCase fast-path in HttpHeaders
- HTTP/2 headers are already lowercase per protocol spec
- Most x-ms-* custom headers are already lowercase
- Skip String.toLowerCase(Locale.ROOT) when string is already
  all-lowercase via simple char range check
- StringLatin1.toLowerCase accounts for ~1.29% CPU (500 samples)

Files changed:
- LocationCache.java: field types + return types + constructors
- GlobalEndpointManager.java: return types
- ClientRetryPolicy.java: variable type
- GlobalPartitionEndpointManagerForPerPartitionCircuitBreaker.java: variable types
- HttpHeaders.java: set() and getHeader() fast-path

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: Annie Liang

sdk/cosmos/azure-cosmos-tests/src/test/java/com/azure/cosmos/GlobalPartitionEndpointManagerForPPAFUnitTests.java

@@ -14,7 +14,6 @@
 import com.azure.cosmos.implementation.ResourceType;
 import com.azure.cosmos.implementation.RxDocumentServiceRequest;
 import com.azure.cosmos.implementation.SerializationDiagnosticsContext;
-import com.azure.cosmos.implementation.apachecommons.collections.list.UnmodifiableList;
 import com.azure.cosmos.implementation.guava25.collect.ImmutableList;
 import com.azure.cosmos.implementation.perPartitionAutomaticFailover.GlobalPartitionEndpointManagerForPerPartitionAutomaticFailover;
 import com.azure.cosmos.implementation.perPartitionAutomaticFailover.PartitionLevelAutomaticFailoverInfo;
@@ -87,12 +86,12 @@ public void beforeClass() {
             .collect(Collectors.toList());
 
         Mockito.when(this.singleWriteAccountGlobalEndpointManagerMock.getAvailableReadRoutingContexts()).thenReturn(availableReadRegionalRoutingContexts);
-        Mockito.when(this.singleWriteAccountGlobalEndpointManagerMock.getApplicableReadRegionalRoutingContexts(Mockito.anyList())).thenReturn(new UnmodifiableList<>(availableReadRegionalRoutingContexts));
+        Mockito.when(this.singleWriteAccountGlobalEndpointManagerMock.getApplicableReadRegionalRoutingContexts(Mockito.anyList())).thenReturn(Collections.unmodifiableList(availableReadRegionalRoutingContexts));
         Mockito.when(this.singleWriteAccountGlobalEndpointManagerMock.canUseMultipleWriteLocations()).thenReturn(false);
         Mockito.when(this.singleWriteAccountGlobalEndpointManagerMock.canUseMultipleWriteLocations(Mockito.any())).thenReturn(false);
 
         Mockito.when(this.multiWriteAccountGlobalEndpointManagerMock.getAvailableReadRoutingContexts()).thenReturn(availableReadRegionalRoutingContexts);
-        Mockito.when(this.multiWriteAccountGlobalEndpointManagerMock.getApplicableReadRegionalRoutingContexts(Mockito.anyList())).thenReturn(new UnmodifiableList<>(availableReadRegionalRoutingContexts));
+        Mockito.when(this.multiWriteAccountGlobalEndpointManagerMock.getApplicableReadRegionalRoutingContexts(Mockito.anyList())).thenReturn(Collections.unmodifiableList(availableReadRegionalRoutingContexts));
         Mockito.when(this.multiWriteAccountGlobalEndpointManagerMock.canUseMultipleWriteLocations()).thenReturn(true);
         Mockito.when(this.multiWriteAccountGlobalEndpointManagerMock.canUseMultipleWriteLocations(Mockito.any())).thenReturn(true);
     }

sdk/cosmos/azure-cosmos-tests/src/test/java/com/azure/cosmos/GlobalPartitionEndpointManagerForPPCBUnitTests.java

@@ -14,7 +14,6 @@
 import com.azure.cosmos.implementation.ResourceType;
 import com.azure.cosmos.implementation.RxDocumentServiceRequest;
 import com.azure.cosmos.implementation.SerializationDiagnosticsContext;
-import com.azure.cosmos.implementation.apachecommons.collections.list.UnmodifiableList;
 import com.azure.cosmos.implementation.perPartitionAutomaticFailover.PerPartitionAutomaticFailoverInfoHolder;
 import com.azure.cosmos.implementation.perPartitionCircuitBreaker.GlobalPartitionEndpointManagerForPerPartitionCircuitBreaker;
 import com.azure.cosmos.implementation.perPartitionCircuitBreaker.LocationHealthStatus;
@@ -214,8 +213,8 @@ public void recordHealthyToHealthyWithFailuresStatusTransition(String partitionL
                 maxExclusive,
                 LocationEastUs2EndpointToLocationPair.getKey());
 
-            Mockito.when(this.globalEndpointManagerMock.getReadEndpoints()).thenReturn((UnmodifiableList<RegionalRoutingContext>) UnmodifiableList.unmodifiableList(applicableReadWriteEndpoints));
-            Mockito.when(this.globalEndpointManagerMock.getWriteEndpoints()).thenReturn((UnmodifiableList<RegionalRoutingContext>) UnmodifiableList.unmodifiableList(applicableReadWriteEndpoints));
+            Mockito.when(this.globalEndpointManagerMock.getReadEndpoints()).thenReturn(Collections.unmodifiableList(applicableReadWriteEndpoints));
+            Mockito.when(this.globalEndpointManagerMock.getWriteEndpoints()).thenReturn(Collections.unmodifiableList(applicableReadWriteEndpoints));
 
             globalPartitionEndpointManagerForCircuitBreaker
                 .handleLocationExceptionForPartitionKeyRange(request, new RegionalRoutingContext(LocationEastUs2EndpointToLocationPair.getKey()), false);
@@ -284,8 +283,8 @@ public void recordHealthyWithFailuresToUnavailableStatusTransition(String partit
                 maxExclusive,
                 LocationEastUs2EndpointToLocationPair.getKey());
 
-            Mockito.when(this.globalEndpointManagerMock.getApplicableWriteRegionalRoutingContexts(Mockito.anyList())).thenReturn((UnmodifiableList<RegionalRoutingContext>) UnmodifiableList.unmodifiableList(applicableReadWriteEndpoints));
-            Mockito.when(this.globalEndpointManagerMock.getApplicableReadRegionalRoutingContexts(Mockito.anyList())).thenReturn((UnmodifiableList<RegionalRoutingContext>) UnmodifiableList.unmodifiableList(applicableReadWriteEndpoints));
+            Mockito.when(this.globalEndpointManagerMock.getApplicableWriteRegionalRoutingContexts(Mockito.anyList())).thenReturn(Collections.unmodifiableList(applicableReadWriteEndpoints));
+            Mockito.when(this.globalEndpointManagerMock.getApplicableReadRegionalRoutingContexts(Mockito.anyList())).thenReturn(Collections.unmodifiableList(applicableReadWriteEndpoints));
 
             int exceptionCountToHandle
                 = globalPartitionEndpointManagerForCircuitBreaker.getConsecutiveExceptionBasedCircuitBreaker().getAllowedExceptionCountToMaintainStatus(LocationHealthStatus.HealthyWithFailures, readOperationTrue);
@@ -361,8 +360,8 @@ public void recordUnavailableToHealthyTentativeStatusTransition(String partition
                 maxExclusive,
                 LocationEastUs2EndpointToLocationPair.getKey());
 
-            Mockito.when(this.globalEndpointManagerMock.getApplicableWriteRegionalRoutingContexts(Mockito.anyList())).thenReturn((UnmodifiableList<RegionalRoutingContext>) UnmodifiableList.unmodifiableList(applicableReadWriteEndpoints));
-            Mockito.when(this.globalEndpointManagerMock.getApplicableReadRegionalRoutingContexts(Mockito.anyList())).thenReturn((UnmodifiableList<RegionalRoutingContext>) UnmodifiableList.unmodifiableList(applicableReadWriteEndpoints));
+            Mockito.when(this.globalEndpointManagerMock.getApplicableWriteRegionalRoutingContexts(Mockito.anyList())).thenReturn(Collections.unmodifiableList(applicableReadWriteEndpoints));
+            Mockito.when(this.globalEndpointManagerMock.getApplicableReadRegionalRoutingContexts(Mockito.anyList())).thenReturn(Collections.unmodifiableList(applicableReadWriteEndpoints));
 
             int exceptionCountToHandle
                 = globalPartitionEndpointManagerForCircuitBreaker.getConsecutiveExceptionBasedCircuitBreaker().getAllowedExceptionCountToMaintainStatus(LocationHealthStatus.HealthyWithFailures, readOperationTrue);
@@ -449,8 +448,8 @@ public void recordHealthyTentativeToHealthyStatusTransition(String partitionLeve
                 maxExclusive,
                 LocationEastUs2EndpointToLocationPair.getKey());
 
-            Mockito.when(this.globalEndpointManagerMock.getApplicableWriteRegionalRoutingContexts(Mockito.anyList())).thenReturn((UnmodifiableList<RegionalRoutingContext>) UnmodifiableList.unmodifiableList(applicableReadWriteEndpoints));
-            Mockito.when(this.globalEndpointManagerMock.getApplicableReadRegionalRoutingContexts(Mockito.anyList())).thenReturn((UnmodifiableList<RegionalRoutingContext>) UnmodifiableList.unmodifiableList(applicableReadWriteEndpoints));
+            Mockito.when(this.globalEndpointManagerMock.getApplicableWriteRegionalRoutingContexts(Mockito.anyList())).thenReturn(Collections.unmodifiableList(applicableReadWriteEndpoints));
+            Mockito.when(this.globalEndpointManagerMock.getApplicableReadRegionalRoutingContexts(Mockito.anyList())).thenReturn(Collections.unmodifiableList(applicableReadWriteEndpoints));
 
             int exceptionCountToHandle
                 = globalPartitionEndpointManagerForCircuitBreaker.getConsecutiveExceptionBasedCircuitBreaker().getAllowedExceptionCountToMaintainStatus(LocationHealthStatus.HealthyWithFailures, readOperationTrue);
@@ -545,8 +544,8 @@ public void recordHealthyTentativeToUnavailableTransition(String partitionLevelC
                 maxExclusive,
                 LocationEastUs2EndpointToLocationPair.getKey());
 
-            Mockito.when(this.globalEndpointManagerMock.getApplicableWriteRegionalRoutingContexts(Mockito.anyList())).thenReturn((UnmodifiableList<RegionalRoutingContext>) UnmodifiableList.unmodifiableList(applicableReadWriteEndpoints));
-            Mockito.when(this.globalEndpointManagerMock.getApplicableReadRegionalRoutingContexts(Mockito.anyList())).thenReturn((UnmodifiableList<RegionalRoutingContext>) UnmodifiableList.unmodifiableList(applicableReadWriteEndpoints));
+            Mockito.when(this.globalEndpointManagerMock.getApplicableWriteRegionalRoutingContexts(Mockito.anyList())).thenReturn(Collections.unmodifiableList(applicableReadWriteEndpoints));
+            Mockito.when(this.globalEndpointManagerMock.getApplicableReadRegionalRoutingContexts(Mockito.anyList())).thenReturn(Collections.unmodifiableList(applicableReadWriteEndpoints));
 
             int exceptionCountToHandle
                 = globalPartitionEndpointManagerForCircuitBreaker.getConsecutiveExceptionBasedCircuitBreaker().getAllowedExceptionCountToMaintainStatus(LocationHealthStatus.HealthyWithFailures, readOperationTrue);
@@ -639,8 +638,8 @@ public void allRegionsUnavailableHandling(String partitionLevelCircuitBreakerCon
                 maxExclusive,
                 LocationEastUs2EndpointToLocationPair.getKey());
 
-            Mockito.when(this.globalEndpointManagerMock.getApplicableWriteRegionalRoutingContexts(Mockito.anyList())).thenReturn((UnmodifiableList<RegionalRoutingContext>) UnmodifiableList.unmodifiableList(applicableReadWriteEndpoints));
-            Mockito.when(this.globalEndpointManagerMock.getApplicableReadRegionalRoutingContexts(Mockito.anyList())).thenReturn((UnmodifiableList<RegionalRoutingContext>) UnmodifiableList.unmodifiableList(applicableReadWriteEndpoints));
+            Mockito.when(this.globalEndpointManagerMock.getApplicableWriteRegionalRoutingContexts(Mockito.anyList())).thenReturn(Collections.unmodifiableList(applicableReadWriteEndpoints));
+            Mockito.when(this.globalEndpointManagerMock.getApplicableReadRegionalRoutingContexts(Mockito.anyList())).thenReturn(Collections.unmodifiableList(applicableReadWriteEndpoints));
 
             int exceptionCountToHandle
                 = globalPartitionEndpointManagerForCircuitBreaker
@@ -724,8 +723,8 @@ public void multiContainerBothWithSinglePartitionHealthyToUnavailableHandling(St
                 maxExclusive,
                 LocationEastUs2EndpointToLocationPair.getKey());
 
-            Mockito.when(this.globalEndpointManagerMock.getApplicableWriteRegionalRoutingContexts(Mockito.anyList())).thenReturn((UnmodifiableList<RegionalRoutingContext>) UnmodifiableList.unmodifiableList(applicableReadWriteEndpoints));
-            Mockito.when(this.globalEndpointManagerMock.getApplicableReadRegionalRoutingContexts(Mockito.anyList())).thenReturn((UnmodifiableList<RegionalRoutingContext>) UnmodifiableList.unmodifiableList(applicableReadWriteEndpoints));
+            Mockito.when(this.globalEndpointManagerMock.getApplicableWriteRegionalRoutingContexts(Mockito.anyList())).thenReturn(Collections.unmodifiableList(applicableReadWriteEndpoints));
+            Mockito.when(this.globalEndpointManagerMock.getApplicableReadRegionalRoutingContexts(Mockito.anyList())).thenReturn(Collections.unmodifiableList(applicableReadWriteEndpoints));
 
             int exceptionCountToHandle
                 = globalPartitionEndpointManagerForCircuitBreaker.getConsecutiveExceptionBasedCircuitBreaker().getAllowedExceptionCountToMaintainStatus(LocationHealthStatus.HealthyWithFailures, readOperationTrue);
@@ -821,8 +820,8 @@ public void allRegionsUnavailableHandlingWithMultiThreading(String partitionLeve
                 .map(uriStringPair -> new RegionalRoutingContext(uriStringPair.getLeft()))
                 .collect(Collectors.toList());
 
-            Mockito.when(this.globalEndpointManagerMock.getApplicableWriteRegionalRoutingContexts(Mockito.anyList())).thenReturn((UnmodifiableList<RegionalRoutingContext>) UnmodifiableList.unmodifiableList(applicableReadWriteEndpoints));
-            Mockito.when(this.globalEndpointManagerMock.getApplicableReadRegionalRoutingContexts(Mockito.anyList())).thenReturn((UnmodifiableList<RegionalRoutingContext>) UnmodifiableList.unmodifiableList(applicableReadWriteEndpoints));
+            Mockito.when(this.globalEndpointManagerMock.getApplicableWriteRegionalRoutingContexts(Mockito.anyList())).thenReturn(Collections.unmodifiableList(applicableReadWriteEndpoints));
+            Mockito.when(this.globalEndpointManagerMock.getApplicableReadRegionalRoutingContexts(Mockito.anyList())).thenReturn(Collections.unmodifiableList(applicableReadWriteEndpoints));
 
             RxDocumentServiceRequest requestCentralUs = constructRxDocumentServiceRequestInstance(
                 readOperationTrue ? OperationType.Read : OperationType.Create,

sdk/cosmos/azure-cosmos-tests/src/test/java/com/azure/cosmos/ProactiveConnectionManagementTest.java

@@ -15,7 +15,6 @@
 import com.azure.cosmos.implementation.RxDocumentServiceRequest;
 import com.azure.cosmos.implementation.TestConfigurations;
 import com.azure.cosmos.implementation.Utils;
-import com.azure.cosmos.implementation.apachecommons.collections.list.UnmodifiableList;
 import com.azure.cosmos.implementation.apachecommons.lang.tuple.ImmutablePair;
 import com.azure.cosmos.implementation.caches.AsyncCache;
 import com.azure.cosmos.implementation.caches.AsyncCacheNonBlocking;
@@ -181,7 +180,7 @@ public void openConnectionsAndInitCachesWithContainer(ProactiveConnectionManagem
 
             cosmosAsyncContainer.openConnectionsAndInitCaches(proactiveConnectionRegionCount).block();
 
-            UnmodifiableList<RegionalRoutingContext> readEndpoints =
+            List<RegionalRoutingContext> readEndpoints =
                 globalEndpointManager.getReadEndpoints();
 
             List<URI> proactiveConnectionEndpoints = readEndpoints.subList(
@@ -345,7 +344,7 @@ public void openConnectionsAndInitCachesWithCosmosClient_And_PerContainerConnect
             ConcurrentHashMap<String, ?> routingMap = getRoutingMap(rxDocumentClient);
             ConcurrentHashMap<String, ?> collectionInfoByNameMap = getCollectionInfoByNameMap(rxDocumentClient);
             Set<String> endpoints = ConcurrentHashMap.newKeySet();
-            UnmodifiableList<RegionalRoutingContext> readEndpoints = globalEndpointManager.getReadEndpoints();
+            List<RegionalRoutingContext> readEndpoints = globalEndpointManager.getReadEndpoints();
 
             List<URI> proactiveConnectionEndpoints = readEndpoints.subList(
                     0,
@@ -495,7 +494,7 @@ public void openConnectionsAndInitCachesWithCosmosClient_And_PerContainerConnect
             ConcurrentHashMap<String, ?> routingMap = getRoutingMap(rxDocumentClient);
             ConcurrentHashMap<String, ?> collectionInfoByNameMap = getCollectionInfoByNameMap(rxDocumentClient);
             Set<String> endpoints = ConcurrentHashMap.newKeySet();
-            UnmodifiableList<RegionalRoutingContext> readEndpoints = globalEndpointManager.getReadEndpoints();
+            List<RegionalRoutingContext> readEndpoints = globalEndpointManager.getReadEndpoints();
             List<URI> proactiveConnectionEndpoints = readEndpoints.subList(
                 0,
                 Math.min(readEndpoints.size(), proactiveContainerInitConfig.getProactiveConnectionRegionsCount()))
@@ -666,7 +665,7 @@ public void openConnectionsAndInitCachesWithCosmosClient_And_PerContainerConnect
             ConcurrentHashMap<String, ?> routingMap = getRoutingMap(rxDocumentClient);
             ConcurrentHashMap<String, ?> collectionInfoByNameMap = getCollectionInfoByNameMap(rxDocumentClient);
             Set<String> endpoints = ConcurrentHashMap.newKeySet();
-            UnmodifiableList<RegionalRoutingContext> readEndpoints = globalEndpointManager.getReadEndpoints();
+            List<RegionalRoutingContext> readEndpoints = globalEndpointManager.getReadEndpoints();
             List<URI> proactiveConnectionEndpoints = readEndpoints.subList(
                     0,
                     Math.min(readEndpoints.size(), proactiveContainerInitConfig.getProactiveConnectionRegionsCount()))

sdk/cosmos/azure-cosmos-tests/src/test/java/com/azure/cosmos/implementation/RegionScopedSessionContainerConcurrencyTest.java

@@ -4,7 +4,6 @@
 package com.azure.cosmos.implementation;
 
 import com.azure.cosmos.GatewayTestUtils;
-import com.azure.cosmos.implementation.apachecommons.collections.list.UnmodifiableList;
 import com.azure.cosmos.implementation.guava25.collect.ImmutableList;
 import com.azure.cosmos.implementation.routing.RegionalRoutingContext;
 import com.azure.cosmos.models.ModelBridgeInternal;
@@ -21,6 +20,7 @@
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.HashMap;
+import java.util.Collections;
 import java.util.List;
 import java.util.Map;
 import java.util.Objects;
@@ -253,8 +253,8 @@ public void concurrentSetAndResolveTokens(String profileName,
         for (URI u : orderedReadEndpoints) {
             endpointBuilder.add(new RegionalRoutingContext(u));
         }
-        UnmodifiableList<RegionalRoutingContext> endpoints =
-            new UnmodifiableList<>(endpointBuilder.build());
+        List<RegionalRoutingContext> endpoints =
+            Collections.unmodifiableList(endpointBuilder.build());
 
         Mockito.when(globalEndpointManagerMock.getReadEndpoints()).thenReturn(endpoints);
         Mockito.when(globalEndpointManagerMock.getRegionName(Mockito.eq(EAST_US), Mockito.any()))