Fix #4561 (revert #4430) (#4562)

Author: cowtowncoder

release-notes/VERSION-2.x

@@ -8,6 +8,11 @@ Project: jackson-databind
 
 -
 
+2.17.2 (not yet released)
+
+#4561: Issues using jackson-databind 2.17.1 with Reactor
+ (reported by @wdallastella)
+
 2.17.1 (04-May-2024)
 
 #4428: `ByteBuddy` scope went beyond `test` in version 2.17.0

src/main/java/com/fasterxml/jackson/databind/deser/DeserializerCache.java

@@ -1,7 +1,6 @@
 package com.fasterxml.jackson.databind.deser;
 
 import java.util.HashMap;
-import java.util.concurrent.locks.ReentrantLock;
 
 import com.fasterxml.jackson.annotation.JsonFormat;
 import com.fasterxml.jackson.databind.*;
@@ -53,12 +52,6 @@ public final class DeserializerCache
     protected final HashMap<JavaType, JsonDeserializer<Object>> _incompleteDeserializers
         = new HashMap<JavaType, JsonDeserializer<Object>>(8);
 
-
-    /**
-     * We hold an explicit lock while creating deserializers to avoid creating duplicates.
-     */
-    private final ReentrantLock _incompleteDeserializersLock = new ReentrantLock();
-
     /*
     /**********************************************************
     /* Life-cycle
@@ -249,12 +242,10 @@ protected JsonDeserializer<Object> _createAndCacheValueDeserializer(Deserializat
             DeserializerFactory factory, JavaType type)
         throws JsonMappingException
     {
-        /* Only one thread to construct deserializers at any given point in time;
-         * limitations necessary to ensure that only completely initialized ones
-         * are visible and used.
-         */
-        _incompleteDeserializersLock.lock();
-        try {
+        // Only one thread to construct deserializers at any given point in time;
+        // limitations necessary to ensure that only completely initialized ones
+        // are visible and used.
+        synchronized (_incompleteDeserializers) {
             // Ok, then: could it be that due to a race condition, deserializer can now be found?
             JsonDeserializer<Object> deser = _findCachedDeserializer(type);
             if (deser != null) {
@@ -277,8 +268,6 @@ protected JsonDeserializer<Object> _createAndCacheValueDeserializer(Deserializat
                     _incompleteDeserializers.clear();
                 }
             }
-        } finally {
-            _incompleteDeserializersLock.unlock();
         }
     }