423255 MBeans of SessionIdManager can leak memory on redeploy

jetty-server/src/main/java/org/eclipse/jetty/server/session/AbstractSessionManager.java

@@ -227,11 +227,25 @@ public abstract class AbstractSessionManager extends AbstractLifeCycle implement
                 _sessionIdManager=server.getSessionIdManager();
                 if (_sessionIdManager==null)
                 {
-                    _sessionIdManager=new HashSessionIdManager();
+                    //create a default SessionIdManager and set it as the shared
-                    server.setSessionIdManager(_sessionIdManager);
+                    //SessionIdManager for the Server, being careful NOT to use
+                    //the webapp context's classloader, otherwise if the context
+                    //is stopped, the classloader is leaked.
+                    ClassLoader serverLoader = server.getClass().getClassLoader();
+                    try
+                    {
+                        Thread.currentThread().setContextClassLoader(serverLoader);
+                        _sessionIdManager=new HashSessionIdManager();
+                        server.setSessionIdManager(_sessionIdManager);
+                    }
+                    finally
+                    {
+                        Thread.currentThread().setContextClassLoader(_loader);
+                    }
                 }
             }
         }
+        
         if (!_sessionIdManager.isStarted())
             _sessionIdManager.start();