[improve][common] Optimize TopicName.get() to reduce lock contention on cache lookup

pulsar-common/src/main/java/org/apache/pulsar/common/naming/TopicName.java

@@ -71,12 +71,32 @@ public static TopicName get(String domain, String tenant, String namespace, Stri
         return TopicName.get(name);
     }
 
+    /**
+     * Get or create a TopicName from the cache.
+     *
+     * <p>Optimization over {@code computeIfAbsent}: avoids holding the ConcurrentHashMap bin-lock
+     * while constructing a new TopicName object. The construction (string splitting / parsing) is
+     * pure CPU work and can be done outside the lock. In the typical steady-state (cache hit) this
+     * method does a single volatile read via {@code get()} and returns immediately with no
+     * synchronization overhead.
+     *
+     * <p>In the cache-miss case, two threads racing on the same key may both construct a
+     * {@code TopicName} instance, but only one wins the {@code putIfAbsent} and the loser's
+     * instance is simply discarded. This is safe because {@code TopicName} is immutable and
+     * construction is cheap compared to the lock-contention / context-switch cost of
+     * {@code computeIfAbsent}.
+     */
     public static TopicName get(String topic) {
+        // Fast path: already cached — single volatile read, no lock.
         TopicName tp = cache.get(topic);
         if (tp != null) {
             return tp;
         }
-        return cache.computeIfAbsent(topic, k -> new TopicName(k));
+        // Slow path: construct outside the bin-lock to avoid blocking other threads.
+        TopicName newTp = new TopicName(topic);
+        TopicName existing = cache.putIfAbsent(topic, newTp);
+        // If another thread raced us and already inserted, use its instance (keeps identity stable).
+        return existing != null ? existing : newTp;
     }
 
     public static TopicName getPartitionedTopicName(String topic) {