diff --git a/poi/src/main/java/org/apache/poi/hpsf/ClassIDPredefined.java b/poi/src/main/java/org/apache/poi/hpsf/ClassIDPredefined.java
index 61d6566c66..963e447de4 100644
--- a/poi/src/main/java/org/apache/poi/hpsf/ClassIDPredefined.java
+++ b/poi/src/main/java/org/apache/poi/hpsf/ClassIDPredefined.java
@@ -112,7 +112,7 @@ public enum ClassIDPredefined {
     }
 
     private final String externalForm;
-    private ClassID classId;
+    private volatile ClassID classId;
     private final String fileExtension;
     private final String contentType;
 
diff --git a/poi/src/main/java/org/apache/poi/ss/format/CellDateFormatter.java b/poi/src/main/java/org/apache/poi/ss/format/CellDateFormatter.java
index 24a95b023f..081863be78 100644
--- a/poi/src/main/java/org/apache/poi/ss/format/CellDateFormatter.java
+++ b/poi/src/main/java/org/apache/poi/ss/format/CellDateFormatter.java
@@ -44,7 +44,7 @@ public class CellDateFormatter extends CellFormatter {
 
     private static final int NUM_MILLISECONDS_IN_DAY = 1000 * 60 * 60 * 24;
 
-    private static CellDateFormatter SIMPLE_DATE_FORMATTER;
+    private static volatile CellDateFormatter SIMPLE_DATE_FORMATTER;
 
     class DatePartHandler implements CellFormatPart.PartHandler {
         private int mStart = -1;