Cache entity codecs in a ConcurrentHashMap (plan item 2.8)

core/src/main/java/dev/morphia/mapping/codec/MorphiaCodecProvider.java

@@ -1,10 +1,10 @@
 package dev.morphia.mapping.codec;
 
 import java.util.ArrayList;
-import java.util.HashMap;
 import java.util.List;
 import java.util.Map;
 import java.util.ServiceLoader;
+import java.util.concurrent.ConcurrentHashMap;
 
 import com.mongodb.lang.Nullable;
 
@@ -35,7 +35,8 @@
  */
 @MorphiaInternal
 public class MorphiaCodecProvider implements CodecProvider {
-    private final Map<Class<?>, Codec<?>> codecs = new HashMap<>();
+    // Looked up concurrently. Not computeIfAbsent: building a codec looks up other codecs, which may land back here.
+    private final Map<Class<?>, Codec<?>> codecs = new ConcurrentHashMap<>();
     private final Mapper mapper;
     private final Conversions conversions;
     private final List<PropertyCodecProvider> propertyCodecProviders = new ArrayList<>();
@@ -74,7 +75,11 @@ public <T> Codec<T> get(Class<T> type, CodecRegistry registry) {
             if (model.hasLifecycle(PreLoad.class) || model.hasLifecycle(PostLoad.class) || mapper.hasListeners()) {
                 codec.setDecoder(new LifecycleDecoder(codec));
             }
-            codecs.put(type, codec);
+            // another thread may have built one meanwhile; keep the first so every caller shares it
+            MorphiaCodec<T> existing = (MorphiaCodec<T>) codecs.putIfAbsent(type, codec);
+            if (existing != null) {
+                codec = existing;
+            }
         }
 
         return codec;

core/src/test/java/dev/morphia/test/mapping/codec/harness/CodecHarnessTest.java

@@ -13,6 +13,10 @@
 import java.util.List;
 import java.util.Map;
 import java.util.Set;
+import java.util.concurrent.CountDownLatch;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.Future;
 import java.util.concurrent.TimeUnit;
 import java.util.function.Supplier;
 
@@ -25,6 +29,7 @@
 import dev.morphia.config.MorphiaConfig;
 import dev.morphia.mapping.DiscriminatorFunction;
 import dev.morphia.mapping.MapperType;
+import dev.morphia.mapping.codec.MorphiaCodecProvider;
 import dev.morphia.mapping.codec.pojo.EntityModel;
 import dev.morphia.mapping.codec.pojo.PropertyModel;
 import dev.morphia.mapping.codec.pojo.critter.CritterEntityModel;
@@ -72,6 +77,7 @@
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertInstanceOf;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
+import static org.junit.jupiter.api.Assertions.assertSame;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.junit.jupiter.api.Assertions.fail;
 
@@ -226,6 +232,36 @@ void modelHashesAreStableAcrossEncoding() {
         }
     }
 
+    @Test
+    void concurrentCodecLookupsShareOneCodec() throws Exception {
+        int threads = 8;
+        ExecutorService executor = Executors.newFixedThreadPool(threads);
+        try {
+            for (MapperType mapper : MapperType.values()) {
+                MorphiaDatastore datastore = datastore(Variant.DEFAULTS, mapper);
+                for (int round = 0; round < 20; round++) {
+                    MorphiaCodecProvider provider = new MorphiaCodecProvider(datastore);
+                    CountDownLatch start = new CountDownLatch(1);
+                    List<Future<Codec<HarnessScalars>>> results = new ArrayList<>();
+                    for (int i = 0; i < threads; i++) {
+                        results.add(executor.submit(() -> {
+                            start.await();
+                            return provider.get(HarnessScalars.class, datastore.getCodecRegistry());
+                        }));
+                    }
+                    start.countDown();
+
+                    Codec<HarnessScalars> first = results.get(0).get();
+                    for (Future<Codec<HarnessScalars>> result : results) {
+                        assertSame(first, result.get(), mapper + ": every lookup should get the one cached codec");
+                    }
+                }
+            }
+        } finally {
+            executor.shutdownNow();
+        }
+    }
+
     @Test
     void readsLoadOnlyProperties() {
         // @LoadOnly values are never written, so the fixtures can't show that they are read

design/mapping-game-plan.md

@@ -179,7 +179,7 @@ structural item covers are moved to that item (see Phases 4 and 5).
   This also removes the deep `equals` cost on `CopyOnWriteArraySet.add` and in `EntityEncoder`.
 - Audit tests for any that rely on structural equality.
 
-### 2.8 `MorphiaCodecProvider.codecs` is a plain `HashMap` (D8)
+### ~~2.8 `MorphiaCodecProvider.codecs` is a plain `HashMap` (D8)~~
 - Switch to `ConcurrentHashMap`, using `get` + `putIfAbsent`. Not `computeIfAbsent`, because codec construction
   recurses into other codec lookups.