[ISSUE #2082] Use StringBuilder instead of StringBuffer. (#2087)

* stringbuilder replace
Co-authored-by: Nqunlin.yi <qunlin.yi@beibei.com>

common/src/main/java/org/apache/rocketmq/common/protocol/NamespaceUtil.java

@@ -39,19 +39,19 @@ public class NamespaceUtil {
             return resourceWithNamespace;
         }
 
-        StringBuffer strBuffer = new StringBuffer();
+        StringBuilder stringBuilder = new StringBuilder();
         if (isRetryTopic(resourceWithNamespace)) {
-            strBuffer.append(MixAll.RETRY_GROUP_TOPIC_PREFIX);
+            stringBuilder.append(MixAll.RETRY_GROUP_TOPIC_PREFIX);
         }
         if (isDLQTopic(resourceWithNamespace)) {
-            strBuffer.append(MixAll.DLQ_GROUP_TOPIC_PREFIX);
+            stringBuilder.append(MixAll.DLQ_GROUP_TOPIC_PREFIX);
         }
 
         String resourceWithoutRetryAndDLQ = withOutRetryAndDLQ(resourceWithNamespace);
         int index = resourceWithoutRetryAndDLQ.indexOf(NAMESPACE_SEPARATOR);
         if (index > 0) {
             String resourceWithoutNamespace = resourceWithoutRetryAndDLQ.substring(index + 1);
-            return strBuffer.append(resourceWithoutNamespace).toString();
+            return stringBuilder.append(resourceWithoutNamespace).toString();
         }
 
         return resourceWithNamespace;
@@ -91,17 +91,17 @@ public class NamespaceUtil {
         }
 
         String resourceWithoutRetryAndDLQ = withOutRetryAndDLQ(resourceWithOutNamespace);
-        StringBuffer strBuffer = new StringBuffer();
+        StringBuilder stringBuilder = new StringBuilder();
 
         if (isRetryTopic(resourceWithOutNamespace)) {
-            strBuffer.append(MixAll.RETRY_GROUP_TOPIC_PREFIX);
+            stringBuilder.append(MixAll.RETRY_GROUP_TOPIC_PREFIX);
         }
 
         if (isDLQTopic(resourceWithOutNamespace)) {
-            strBuffer.append(MixAll.DLQ_GROUP_TOPIC_PREFIX);
+            stringBuilder.append(MixAll.DLQ_GROUP_TOPIC_PREFIX);
         }
 
-        return strBuffer.append(namespace).append(NAMESPACE_SEPARATOR).append(resourceWithoutRetryAndDLQ).toString();
+        return stringBuilder.append(namespace).append(NAMESPACE_SEPARATOR).append(resourceWithoutRetryAndDLQ).toString();
 
     }