Sharepoint Webpart custom properties get default values on server reboot

Question

I have noticed that the custom properties of a webpart I developed return to their default values when I reboot my machine.

Is that a normal behavior? are the properties saved as far as the server is up, or there is some parameters I am missing.

namespace TestWebpart
{
    [ToolboxItemAttribute(false)]
    [XmlRoot(Namespace = "TestWebpart")]
    public class GraphWebpart : Microsoft.SharePoint.WebPartPages.WebPart
    {
        // Visual Studio might automatically update this path when you change the Visual Web Part project item.
        private const string _ascxPath = @"~/_CONTROLTEMPLATES/Test_Graph/TestWebpart/GraphWebpartUserControl.ascx";

        protected override void CreateChildControls()
        {
            ReloadElements();
        }

        protected void ReloadElements()
        {
            Controls.Clear();
            GraphWebpartUserControl control = (GraphWebpartUserControl)Page.LoadControl(_ascxPath);

            control.xmlDataUrl = XMLFileUrl;

            Controls.Add(control);
        }

        private static string _xmlFileUrl;
        [WebBrowsable(true),
        Personalizable(PersonalizationScope.Shared),
        DefaultValue(""),
        Description("xml"),
        DisplayName("xml"),
        WebDisplayName("xml")]
        public string XMLFileUrl
        {
            get { return _xmlFileUrl; }
            set { 
                _xmlFileUrl = value;
                ReloadElements();
            }
        }
}
}

Thank you.

EDIT2: Deleting static from the fields throws the flowing exception:

Web Part Error: An error occurred while setting the value of this property: TestWebpart:XMLFileUrl - Exception has been thrown by the target of an invocation.
Hide Error Details

[WebPartPageUserException: An error occurred while setting the value of this property: Blue_Graph.GraphWebpart.GraphWebpart:XMLFileUrl - Exception has been thrown by the target of an invocation.]
  at Microsoft.SharePoint.WebPartPages.BinaryWebPartDeserializer.ApplyPropertyState(Control control) 
  at Microsoft.SharePoint.WebPartPages.BinaryWebPartDeserializer.Deserialize() 
  at Microsoft.SharePoint.WebPartPages.SPWebPartManager.CreateWebPartsFromRowSetData(Boolean onlyInitializeClosedWebParts)

Answer 1

Your private _xmlFileUrl variable should not be static