--- a/jdk/src/share/classes/sun/misc/URLClassPath.java	Mon Jun 29 13:10:33 2009 +0100
+++ b/jdk/src/share/classes/sun/misc/URLClassPath.java	Mon Jun 29 15:05:15 2009 +0100
@@ -135,7 +135,9 @@
      * If the URL specified is null or is already in the list of
      * URLs, then invoking this method has no effect.
      */
-    public void addURL(URL url) {
+    public synchronized void addURL(URL url) {
+        if (closed)
+            return;
         synchronized (urls) {
             if (url == null || path.contains(url))
                 return;

--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/jdk/test/java/net/URLClassLoader/B6827999.java	Mon Jun 29 15:05:15 2009 +0100
@@ -0,0 +1,68 @@
+/*
+ * Copyright 2006 Sun Microsystems, Inc.  All Rights Reserved.
+ * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
+ *
+ * This code is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 only, as
+ * published by the Free Software Foundation.
+ *
+ * This code is distributed in the hope that it will be useful, but WITHOUT
+ * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
+ * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+ * version 2 for more details (a copy is included in the LICENSE file that
+ * accompanied this code).
+ *
+ * You should have received a copy of the GNU General Public License version
+ * 2 along with this work; if not, write to the Free Software Foundation,
+ * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
+ *
+ * Please contact Sun Microsystems, Inc., 4150 Network Circle, Santa Clara,
+ * CA 95054 USA or visit www.sun.com if you need additional information or
+ * have any questions.
+ */
+
+/*
+ * @test
+ * @bug 6827999
+ * @run main/othervm B6827999
+ * @summary Fix for 6827999
+ */
+
+import java.net.*;
+import java.io.*;
+
+/**
+ */
+
+public class B6827999
+{
+    public static void main(String[] args) throws Exception {
+
+        URL[] urls = new URL[] {new URL("http://foobar.jar") };
+        MyURLClassLoader ucl = new MyURLClassLoader(urls);
+
+        ucl.addURL(new URL("http://foo/bar.jar"));
+        urls = ucl.getURLs();
+
+        if (urls.length != 2)
+            throw new RuntimeException("Failed:(1)");
+        ucl.close();
+
+        ucl.addURL(new URL("http://foo.bar/bar.jar"));
+
+        if (ucl.getURLs().length != 2) {
+            throw new RuntimeException("Failed:(2)");
+        }
+
+    }
+
+    static class MyURLClassLoader extends URLClassLoader {
+        public MyURLClassLoader(URL[] urls) {
+            super(urls);
+        }
+        public void addURL(URL url) {
+            super.addURL(url);
+        }
+    }
+
+}