Remove computeIfAbsent from the warm path

Author: alextwoods

build-tools/src/main/resources/software/amazon/awssdk/spotbugs-suppressions.xml

@@ -536,4 +536,15 @@
         </Or>
         <Bug pattern="NP_LOAD_OF_KNOWN_NULL_VALUE"/>
     </Match>
+
+    <!-- Intentional benign-race get-then-put on ConcurrentHashMap. SdkField instances are
+         static final, and the registry always returns the same marshaller for a given
+         (location, marshallingType) pair, so concurrent puts are idempotent. Using get()
+         instead of computeIfAbsent() avoids the latter's bucket-level synchronization
+         overhead on every call. -->
+    <Match>
+        <Class name="software.amazon.awssdk.protocols.json.internal.marshall.JsonProtocolMarshaller"/>
+        <Method name="marshallFieldViaRegistry"/>
+        <Bug pattern="AT_OPERATION_SEQUENCE_ON_CONCURRENT_ABSTRACTION"/>
+    </Match>
 </FindBugsFilter>

core/protocols/aws-json-protocol/src/main/java/software/amazon/awssdk/protocols/json/internal/marshall/JsonProtocolMarshaller.java

@@ -422,8 +422,16 @@ private void marshallFieldViaRegistry(SdkField<?> field, Object val) {
                                .marshall(val, marshallerContext, field.locationName(), (SdkField<Object>) field);
             return;
         }
-        JsonMarshaller<Object> marshaller = MARSHALLER_CACHE.computeIfAbsent(field,
-            f -> MARSHALLER_REGISTRY.getMarshaller(f.location(), f.marshallingType(), val));
+        // Use get-before-put instead of computeIfAbsent. ConcurrentHashMap.get() is a single lock-free
+        // volatile read, whereas computeIfAbsent() has additional overhead even on cache hits (bucket-level
+        // synchronization bookkeeping). The benign-race on first access is safe: SdkField instances are
+        // static final, and the registry always returns the same marshaller for a given (location, type) pair,
+        // so concurrent puts are idempotent.
+        JsonMarshaller<Object> marshaller = MARSHALLER_CACHE.get(field);
+        if (marshaller == null) {
+            marshaller = MARSHALLER_REGISTRY.getMarshaller(field.location(), field.marshallingType(), val);
+            MARSHALLER_CACHE.put(field, marshaller);
+        }
         marshaller.marshall(val, marshallerContext, field.locationName(), (SdkField<Object>) field);
     }