[BUG-8620] Make DefaultJsonSerializer resilient to serializer init failures

Wrap each serializer instantiation in try-catch so that if one library's
initialization fails (e.g., Jackson's PropertyNamingStrategies fields
obfuscated by R8), the factory falls through to the next available
JSON library instead of crashing with NoClassDefFoundError.

Also add DefaultJsonSerializer unit tests.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: muzahidul-opti

core-api/src/main/java/com/optimizely/ab/event/internal/serializer/DefaultJsonSerializer.java

@@ -47,23 +47,45 @@ public static Serializer getInstance() {
      */
     private static @Nonnull
     Serializer create() {
-        Serializer serializer;
-
         if (isPresent("com.fasterxml.jackson.databind.ObjectMapper")) {
-            serializer = new JacksonSerializer();
-        } else if (isPresent("com.google.gson.Gson")) {
-            serializer = new GsonSerializer();
-        } else if (isPresent("org.json.simple.JSONObject")) {
-            serializer = new JsonSimpleSerializer();
-        } else if (isPresent("org.json.JSONObject")) {
-            serializer = new JsonSerializer();
-        } else {
-            throw new MissingJsonParserException("unable to locate a JSON parser. "
-                + "Please see <link> for more information");
+            try {
+                Serializer serializer = new JacksonSerializer();
+                logger.debug("using json serializer: {}", serializer.getClass().getSimpleName());
+                return serializer;
+            } catch (Throwable t) {
+                logger.warn("Jackson found on classpath but serializer initialization failed, trying next option.", t);
+            }
+        }
+        if (isPresent("com.google.gson.Gson")) {
+            try {
+                Serializer serializer = new GsonSerializer();
+                logger.debug("using json serializer: {}", serializer.getClass().getSimpleName());
+                return serializer;
+            } catch (Throwable t) {
+                logger.warn("Gson found on classpath but serializer initialization failed, trying next option.", t);
+            }
+        }
+        if (isPresent("org.json.simple.JSONObject")) {
+            try {
+                Serializer serializer = new JsonSimpleSerializer();
+                logger.debug("using json serializer: {}", serializer.getClass().getSimpleName());
+                return serializer;
+            } catch (Throwable t) {
+                logger.warn("json-simple found on classpath but serializer initialization failed, trying next option.", t);
+            }
+        }
+        if (isPresent("org.json.JSONObject")) {
+            try {
+                Serializer serializer = new JsonSerializer();
+                logger.debug("using json serializer: {}", serializer.getClass().getSimpleName());
+                return serializer;
+            } catch (Throwable t) {
+                logger.warn("org.json found on classpath but serializer initialization failed.", t);
+            }
         }
 
-        logger.debug("using json serializer: {}", serializer.getClass().getSimpleName());
-        return serializer;
+        throw new MissingJsonParserException("unable to locate a JSON parser. "
+            + "Please see <link> for more information");
     }
 
     private static boolean isPresent(@Nonnull String className) {

core-api/src/test/java/com/optimizely/ab/event/internal/serializer/DefaultJsonSerializerTest.java

@@ -0,0 +1,45 @@
+/**
+ *
+ *    Copyright 2026, Optimizely and contributors
+ *
+ *    Licensed under the Apache License, Version 2.0 (the "License");
+ *    you may not use this file except in compliance with the License.
+ *    You may obtain a copy of the License at
+ *
+ *        http://www.apache.org/licenses/LICENSE-2.0
+ *
+ *    Unless required by applicable law or agreed to in writing, software
+ *    distributed under the License is distributed on an "AS IS" BASIS,
+ *    WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ *    See the License for the specific language governing permissions and
+ *    limitations under the License.
+ */
+package com.optimizely.ab.event.internal.serializer;
+
+import org.junit.Test;
+import static org.junit.Assert.*;
+
+public class DefaultJsonSerializerTest {
+
+    @Test
+    public void getInstanceReturnsNonNull() {
+        Serializer serializer = DefaultJsonSerializer.getInstance();
+        assertNotNull("getInstance() should return a non-null serializer", serializer);
+    }
+
+    @Test
+    public void getInstanceReturnsSameInstance() {
+        Serializer first = DefaultJsonSerializer.getInstance();
+        Serializer second = DefaultJsonSerializer.getInstance();
+        assertSame("getInstance() should return the same singleton instance", first, second);
+    }
+
+    @Test
+    public void instanceCanSerialize() {
+        Serializer serializer = DefaultJsonSerializer.getInstance();
+        String result = serializer.serialize(java.util.Collections.singletonMap("test_key", "test_value"));
+        assertNotNull("Serializer should produce non-null output", result);
+        assertTrue("Serialized output should contain the key", result.contains("test_key"));
+        assertTrue("Serialized output should contain the value", result.contains("test_value"));
+    }
+}