optimize joinrows function

doriswriter/src/main/java/com/dorisdb/connector/datax/plugin/writer/doriswriter/manager/DorisStreamLoadVisitor.java

@@ -100,10 +100,25 @@ public class DorisStreamLoadVisitor {
             if (null != props && props.containsKey("row_delimiter")) {
                 lineDelimiter = DorisDelimiterParser.parse(String.valueOf(props.get("row_delimiter")), "\n");
             }
-            return (String.join(lineDelimiter, rows) + lineDelimiter).getBytes(StandardCharsets.UTF_8);
+            StringBuilder sb = new StringBuilder();
+            for (String row : rows) {
+                sb.append(row).append(lineDelimiter);
+            }
+            return sb.toString().getBytes(StandardCharsets.UTF_8);
         }
         if (DorisWriterOptions.StreamLoadFormat.JSON.equals(writerOptions.getStreamLoadFormat())) {
-            return new StringBuilder("[").append(String.join(",", rows)).append("]").toString().getBytes(StandardCharsets.UTF_8);
+            StringBuilder sb = new StringBuilder();
+            sb.append("[");
+            boolean isFirstElement = true;
+            for (String row : rows) {
+                if (!isFirstElement) {
+                    sb.append(",");
+                }
+                sb.append(row);
+                isFirstElement = false;
+            }
+            sb.append("]");
+            return sb.toString().getBytes(StandardCharsets.UTF_8);
         }
         throw new RuntimeException("Failed to join rows data, unsupported `format` from stream load properties:");
     }