diff --git a/testing/src/main/java/io/grpc/internal/testing/AbstractTransportTest.java b/testing/src/main/java/io/grpc/internal/testing/AbstractTransportTest.java
index 6ae05d073f..b784aad4c5 100644
--- a/testing/src/main/java/io/grpc/internal/testing/AbstractTransportTest.java
+++ b/testing/src/main/java/io/grpc/internal/testing/AbstractTransportTest.java
@@ -1169,7 +1169,7 @@ public abstract class AbstractTransportTest {
     String largeMessage;
     {
       int size = 1 * 1024;
-      StringBuffer sb = new StringBuffer(size);
+      StringBuilder sb = new StringBuilder(size);
       for (int i = 0; i < size; i++) {
         sb.append('a');
       }