Convert ConcurrentHashMap which does service loading to HashMap w…

…ith `ReentrantLock` (helidon-io#8977)

* Convert ConcurrentHashMap which does service loading to HashMap with ReentrantLock

Signed-off-by: Tim Quinn <[email protected]>

* Use a RW lock

* Replace the contentious ConcurrentHashMap with a HashMap (with the lock) (which was the whole point after all)

---------

Signed-off-by: Tim Quinn <[email protected]>

common/configurable/src/main/java/io/helidon/common/configurable/ObserverManager.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2022, 2023 Oracle and/or its affiliates.
+ * Copyright (c) 2022, 2024 Oracle and/or its affiliates.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,12 +17,15 @@
 package io.helidon.common.configurable;
 
 import java.lang.System.Logger.Level;
+import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.ServiceLoader;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.concurrent.ExecutorService;
 import java.util.concurrent.atomic.AtomicInteger;
+import java.util.concurrent.locks.ReadWriteLock;
+import java.util.concurrent.locks.ReentrantReadWriteLock;
 import java.util.function.Supplier;
 import java.util.stream.Collectors;
 
@@ -50,7 +53,8 @@ class ObserverManager {
     private static final LazyValue<List<ExecutorServiceSupplierObserver>> OBSERVERS = LazyValue
             .create(ObserverManager::loadObservers);
 
-    private static final Map<Supplier<? extends ExecutorService>, SupplierInfo> SUPPLIERS = new ConcurrentHashMap<>();
+    private static final Map<Supplier<? extends ExecutorService>, SupplierInfo> SUPPLIERS = new HashMap<>();
+    private static final ReadWriteLock SUPPLIERS_LOCK = new ReentrantReadWriteLock();
 
     // A given supplier category can have multiple suppliers, so keep track of the next available index by category.
     private static final Map<String, AtomicInteger> SUPPLIER_CATEGORY_NEXT_INDEX_VALUES = new ConcurrentHashMap<>();
@@ -71,13 +75,18 @@ private ObserverManager() {
     static void registerSupplier(Supplier<? extends ExecutorService> supplier,
                                  String supplierCategory,
                                  String executorServiceCategory) {
-        int supplierIndex = SUPPLIER_CATEGORY_NEXT_INDEX_VALUES.computeIfAbsent(supplierCategory, key -> new AtomicInteger())
-                .getAndIncrement();
-        SUPPLIERS.computeIfAbsent(supplier,
-                                  s -> SupplierInfo.create(s,
-                                                           executorServiceCategory,
-                                                           supplierCategory,
-                                                           supplierIndex));
+        SUPPLIERS_LOCK.writeLock().lock();
+        try {
+            int supplierIndex = SUPPLIER_CATEGORY_NEXT_INDEX_VALUES.computeIfAbsent(supplierCategory, key -> new AtomicInteger())
+                    .getAndIncrement();
+            SUPPLIERS.computeIfAbsent(supplier,
+                                      s -> SupplierInfo.create(s,
+                                                               executorServiceCategory,
+                                                               supplierCategory,
+                                                               supplierIndex));
+        } finally {
+            SUPPLIERS_LOCK.writeLock().unlock();
+        }
     }
 
     /**
@@ -90,7 +99,13 @@ static void registerSupplier(Supplier<? extends ExecutorService> supplier,
      * @throws IllegalStateException if the supplier has not previously registered itself
      */
     static <E extends ExecutorService> E registerExecutorService(Supplier<E> supplier, E executorService) {
-        SupplierInfo supplierInfo = SUPPLIERS.get(supplier);
+        SUPPLIERS_LOCK.readLock().lock();
+        SupplierInfo supplierInfo;
+        try {
+            supplierInfo = SUPPLIERS.get(supplier);
+        } finally {
+            SUPPLIERS_LOCK.readLock().unlock();
+        }
         if (supplierInfo == null) {
             throw new IllegalStateException("Attempt to register an executor service to an unregistered supplier");
         }