diff --git a/jdk/test/sun/management/jmxremote/startstop/ManagementAgentJcmd.java b/jdk/test/sun/management/jmxremote/startstop/ManagementAgentJcmd.java
index f273cd2ce4b..d2f231cdfc0 100644
--- a/jdk/test/sun/management/jmxremote/startstop/ManagementAgentJcmd.java
+++ b/jdk/test/sun/management/jmxremote/startstop/ManagementAgentJcmd.java
@@ -196,7 +196,9 @@ final class ManagementAgentJcmd {
             l.addToolArg(cmd);
         }
 
-        StringBuilder output = new StringBuilder();
+        // this buffer will get filled in different threads
+        //   -> must be the synchronized StringBuffer
+        StringBuffer output = new StringBuffer();
 
         AtomicBoolean portUnavailable = new AtomicBoolean(false);
         Process p = ProcessTools.startProcess(