How to get list item by absolute URL of the item

Pass the URL as query string named DocumentPath.

  protected void Page_Load(object sender, EventArgs e)

        {

            string documentPath = Convert.ToString(Request.QueryString["DocumentPath"]);

            if (string.IsNullOrEmpty(documentPath))

            {

            }

            else

            {

                SPSecurity.RunWithElevatedPrivileges(delegate()

                {

                    using (SPSite site = new SPSite(SPContext.Current.Site.ID))

                    {

                        using (SPWeb web = site.RootWeb)

                        {

                            SPListItem item = web.GetListItem(documentPath);

                        }

                    }

                });

            }

        }

            }

        }

How to create webpart page programatically in SharePoint 2010 (Source Code)

/// <summary>
/// Create new webpart page
/// </summary>
/// <param name="list">List to keep page</param>
/// <param name="pageTitle">Page title</param>
/// <param name="layoutTemplate">Layout template id</param>
/// <returns>Operation result</returns>
private static string CreateWebPartPage(SPList list, string pageTitle, int layoutTemplate)
{
    const string newItemTemplate = "<?xml version=\"1.0\" encoding=\"UTF-8\"?>" +
                                    "<Method ID=\"0,NewWebPage\">" +
                                    "<SetList Scope=\"Request\">{0}</SetList>" +
                                    "<SetVar Name=\"Cmd\">NewWebPage</SetVar>" +
                                    "<SetVar Name=\"ID\">New</SetVar>" +
                                    "<SetVar Name=\"Type\">WebPartPage</SetVar>" +
                                    "<SetVar Name=\"WebPartPageTemplate\">{2}</SetVar>" +
                                    "<SetVar Name=\"Overwrite\">true</SetVar>" +
                                    "<SetVar Name=\"Title\">{1}</SetVar>" +
                                    "</Method>";
    var newItemXml = string.Format(newItemTemplate, list.ID, pageTitle, layoutTemplate);
    return list.ParentWeb.ProcessBatchData(newItemXml);
}

Using:
var web = SPContext.Current.Web;
var pagesLibrary = web.GetList("Pages");
CreateWebPartPage(pagesLibrary, "MyPage", 2);
Values for LayoutTemplate parameter:

1. Full Page, Vertical
2. Header, Footer, 3 Columns
3. Header, Left Column, Body
4. Header, Right Column, Body
5. Header, Footer, 2 Columns, 4 Rows
6. Header, Footer, 4 Columns, Top Row
7. Left Column, Header, Footer, Top Row, 3 Columns
8. Right Column, Header, Footer, Top Row, 3 Columns

Code to create column in the existing document library

public static void AddCustomDocumentLibraryColumn(string documentLibraryName, string columnName,string columnDisplayName,string columnDescription, SPFieldType columnDataType)

        {

            try

            {

                SPSecurity.RunWithElevatedPrivileges(delegate()

                {

                    using (SPSite site = new SPSite(SPContext.Current.Site.ID))

                    {

                        using (SPWeb web = site.OpenWeb())

                        {

                            SPDocumentLibrary docLibrary = null;

                            if (web.TryGetDocumentLibrary(documentLibraryName, out docLibrary))

                            {

                                if (!docLibrary.Fields.ContainsField(columnName))

                                {

                                    bool allowUnsafeUpdateState = web.AllowUnsafeUpdates;

                                    web.AllowUnsafeUpdates = true;

                                    SPView defaultView = docLibrary.DefaultView;

                                  

                                    SPField libField = docLibrary.Fields.GetField( docLibrary.Fields.Add(columnName, columnDataType, true));

                                    libField.Description = columnDescription;

                                    libField.Title = columnDisplayName;

                                    libField.Required = true;

                                    libField.Update();

                                    defaultView.ViewFields.Add(libField);

                                    defaultView.Update();

                                    docLibrary.Update();

                                    web.Update();

                                    web.AllowUnsafeUpdates = allowUnsafeUpdateState;

                                }

                            }

                        }

                    }

                });

            }

            catch (Exception ex)

            {

            }

        }

TryGetDocumentLibrary is an extended function of the Web.

public static bool TryGetDocumentLibrary(this SPWeb web, string documentLibraryName,

                                                                       out SPDocumentLibrary documentLibrary)

        {

            bool isDocumentLibraryExist = false;

            web.AllowUnsafeUpdates = true;

            if (true == web.Properties.ContainsKey(documentLibraryName))

            {

                try

                {

                    documentLibrary = web.Lists[new Guid(web.Properties[documentLibraryName])]

                                                                                    as SPDocumentLibrary;

                    isDocumentLibraryExist = true;

                }

                catch

                {

                    web.Properties[documentLibraryName] = null;

                    web.Properties.Update();

                    TryGetDocumentLibrary(web, documentLibraryName, out documentLibrary);

                }

            }

            else

            {

                documentLibrary = (from SPList list in web.Lists

                                   where list.RootFolder.Name.Equals(documentLibraryName,

                                                            StringComparison.InvariantCulture)

                                   select list).FirstOrDefault() as SPDocumentLibrary;

                if (documentLibrary != null)

                {

                    web.Properties.Add(documentLibraryName, documentLibrary.ID.ToString("B"));

                    web.Properties.Update();

                    isDocumentLibraryExist = true;

                }

            }

            web.AllowUnsafeUpdates = false;

            return isDocumentLibraryExist;

        }