--- old/jdk/src/share/classes/com/sun/java/util/jar/pack/PackerImpl.java	2013-02-07 17:06:46.000000000 +0100
+++ new/jdk/src/share/classes/com/sun/java/util/jar/pack/PackerImpl.java	2013-02-07 17:06:46.000000000 +0100
@@ -84,7 +84,7 @@
      * @param out an OutputStream
      * @exception IOException if an error is encountered.
      */
-    public void pack(JarFile in, OutputStream out) throws IOException {
+    public synchronized void pack(JarFile in, OutputStream out) throws IOException {
         assert(Utils.currentInstance.get() == null);
         TimeZone tz = (props.getBoolean(Utils.PACK_DEFAULT_TIMEZONE))
                       ? null
@@ -119,7 +119,7 @@
      * @param out an OutputStream
      * @exception IOException if an error is encountered.
      */
-    public void pack(JarInputStream in, OutputStream out) throws IOException {
+    public synchronized void pack(JarInputStream in, OutputStream out) throws IOException {
         assert(Utils.currentInstance.get() == null);
         TimeZone tz = (props.getBoolean(Utils.PACK_DEFAULT_TIMEZONE)) ? null :
             TimeZone.getDefault();